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NASA's Space Shuttle Earth observations photography database contains over 250,000 images.

Beth Barbier

The Earth was born about 4 and a half billion years ago, at the same time the whole solar system (the Sun, Earth, and other planets) formed. An enormous cloud of gas started to get smaller and smaller as the gas particles attracted each other with gravity. Most of the gas went to the center of the solar system and formed the Sun, but several other pieces spinning about the Sun solidified into the planets, including the Earth.

Dr. Eric Christian

Current scientific thinking puts the Earth at about 4.5 billion years old.

Dr. Eric Christian

The primary separation of the elements takes place when the rock is molten. Differences in melting and condensation temperatures and different densities cause the similar molecules to clump. There are other smaller effects, such as like molecules forming crystals (they fit together better). There is also some separation (due to differences in condensation temperature) in the proto-solar-system gas cloud. This is why there are different types of meteoroids.

Dr. Eric Christian
(September 2001)

The mass of Earth is 5.98 x 10²⁴ kg, or 1.32 x 10²⁵ lb. I was able to look up the mass of Earth at the Welcome to the Planets link and then converted the kilograms to pounds.

Beth Barbier

There is an AU (Astronomical Unit), which is defined as the average distance from the Earth to the Sun, but I have never heard of AO or AOS as a unit of measurement. AOS is used as an abreviation for "Acquisition of Signal" for spacecraft telemetry, however.

Dr. Eric Christian
(October 2000)

The Earth (and other planets, and stars) is spherical because the spherical shape is the lowest energy state that a group of matter can be in. Small asteroids and moons can be non-spherical, but after they reach a certain size (when the force of their gravity can "break" the rock from which they are made), all the bumps are pulled down, and they become more spherical. There is a maximum size that mountains can attain that gets smaller as the planetoid gets more massive. So Mars can have larger mountains (Olympus Mons for example) than the Earth can, because it weighs less. As long as the maximum mountain size is small compared to the radius of the planetoid (true for objects considerably smaller than the Moon), the body will be spherical.

Dr. Eric Christian

The Earth is not a perfect sphere, so the distance to the center of the Earth varies from 6378 km (3963 miles) at the equator to 6357 km (3950 miles) at the poles.

Dr. Eric Christian

The inner core may be hotter, but it is at a much higher pressure. At a fixed temperature (say 5000 degrees), high pressure can make a liquid into a solid. So, even though the temperature increases as you move into the inner core, the pressure increases faster and wins.

Dr. Eric Christian
(March 2001)

It's a combination of radioactivity (the radioactive materials in the Earth generate heat) and the residual heat from the formation of the Earth. When all of the matter that created the Earth fell together, it picked up kinetic energy falling in. When it stopped at the proto-Earth, the kinetic energy was turned into heat. The Earth hasn't cooled yet. The Moon, being much smaller, has had time to cool and probably has a solid core.

Dr. Eric Christian

What about...?

I'm an astrophysicist, not a geologist. I got my answer from long ago geology courses and research on the web. It's a standard part of geological theory that radioactivity is part of the heating process. See, for example, this Scientific American article.

Why is there not an abundance of fission products observed in materials emitted from either volcanoes or sea-borne vents? Or why are there not more fission products as one penetrates the Earth's crust into deep mines?

There IS increased radioactivity seen in volcanoes (see, for example this news article).

I don't know about undersea vents, but I wouldn't be surprised if there was increased activity coming from them as well. Your question implies that you think there should be lots and lots of radioactive gas, but you don't need that much radioactivity in the mantle and core.

As for deep mines, local composition is far more important for how much radioactivity there is than depth.

If you want more information, I suggest you contact a real geologist or go to your local college/university library to do some research.

Dr. Eric Christian
(September 2007)

Earth's core temperature is about 6,000° C. By coincidence, this is about the same as the Sun's surface temperature (but much cooler than the Sun's core temperature, which is about 15,600,000° C). The Earth's core is cooling, but at a very slow rate. Over the past three billion years it has probably cooled by a few hundred degrees. Currently, the Earth's core temperature is not changing much because, through radioactive decay (nuclear fission - the breakup of the nuclei of heavy elements, like uranium), it is generating about as much heat as it is losing.

To answer the second part of this question, some definitions are in order. A star is a self-luminous body that shines by generating energy internally through nuclear fusion (the combining of nuclei of light elements like hydrogen and helium). The Sun is a star. A planet shines by reflected light from the Sun. The solar system has nine "major" planets (of which Earth is one) and innumerable "minor" planets (asteroids and comets of various kinds).

Star masses range from about 0.04 times, to 150 times, the mass of the Sun. The mass of the Earth is 0.000003 times that of the Sun (and the mass of Jupiter, the largest planet in the solar system, is 0.001 times that of the Sun).

Although stars lose mass as they evolve, none lose enough to wind up anywhere near the mass of even the most massive planet. So, the bottom line is: Stars do not evolve into planets.

For a great animated simulation showing the evolution of stars with masses between 0.1 and 120 times that of the Sun see the stellar evolution simulation created by Terry Herter for his Astronomy 101/103 course at Cornell University. You will need a JAVA enabled browser to view this simulation.

Dr. Ed Tedesco
(January 2005)

As you go further inside the Earth, the force you feel due to gravity lessens, assuming the Earth is has a uniform density all the way throughout. Less force means you weigh less.

The reason is that the mass attracting you is inside a sphere, and is given by M = (4/3) * pi * (radius)³ * density

The force you feel is given by F = G * M * (your mass) / (radius)²

This means the net force is F = G * (4/3) * pi * radius * density * (your mass)

(pi=3.14159 and G = Newton's gravitational constant)

So as you go further inside the Earth, the radius is decreasing, so the force you feel is decreasing. The mass above you oddly enough doesn't contribute at all to any net force on your body.

In reality, of course, the Earth is not of uniform density, and there is a slight increase in force as you go down from the surface, before it begins to decrease again. Still, you weigh less standing on the Earth's core.

As far as what happens above sea level - you must realize that what happens outside the Earth is different from what happens inside the Earth. Inside, as you go deeper and deeper, the mass attracting you is less and less (as stated). Above sea level (the surface of the Earth, specifically) as you go further and further away, the mass remains constant (obviously), but the distance gets larger and larger, which makes the force (given by F = G * M(Earth) * M(you) / r²) smaller.

Notice that the formula that applies inside the Earth is different from the one that applies outside.

Dr. Louis Barbier
(October 2003)

First, the most important thing to know is that gravity exists absolutely everywhere in the universe. Every bit of matter exerts a force on every other bit of matter. This means that you are attracted, and attract, everything in the universe! The force exerted depends on the distance of the object and the mass. The Earth exerts the most force on you because it is close (right here!) and very massive.

Forces add like vectors, so their direction is very important. If you could be at the exact center, the forces that each bit of Earth matter exerted on you would cancel out (up cancelling down, east cancelling west, etc.). This only occurs for a single point, though, and you would still feel a gravitational force on the rest of your body.

Remember, gravity is universal and exists everywhere. This is the fundamental law of physics.

Angela Richard
(March 2003)

Gravitation is attractive, never repulsive, so, in the absence of other forces there, matter will always tend to move to the center of the sphere.

Dr. Eric Christian
(July 2002)

The major probe of the Earth's interior is seismic waves from earthquakes. Scientists can use an array of seismometers to track the speed of the waves as a function of depth, and from this can infer density, temperature, etc. The sharp discontinuities at the mantle-crust and mantle-core interfaces are especially noticible.

Dr. Eric Christian

These values are directly calculable and have been proven by measurements repeatedly.

Radius at poles = 6,356,800 meters
Radius at equator = 6,378,400 meters
omega (angular velocity) = .00007292115 s^-1

If you plug these numbers in you find:
gravitational acceleration at poles = 9.8322 m s^-2
no centrifugal acceleration at poles
Total acceleration at poles = 9.8332 m s^-2

gravitational acceleration at equator = 9.7805 m s^-2
centrifugal acceleration at equator = -.0002 m s^-2
Total acceleration at equator = 9.7803 m s^-2

So it is obvious that the oblateness of the Earth is 250 times more important than the centrifugal acceleration.

Dr. Eric Christian

"Up" and "down" are different depending upon where you are. They come from gravity, which is the force that holds everyone and everything onto the Earth. "Down" points in the direction of gravity, which is toward the center of the Earth, and "up" is in the opposite direction. If you look at a globe, no matter where a person is standing, "up" is the direction away from the center of the Earth and points to the sky.

Dr. Eric Christian
(May 2011)

We do not stay on the Earth because it is spinning, but because of the force of gravity.

I am not aware of any planets that do not spin.

Dr. Louis Barbier
(November 2001)

You already have half the answer! The speed of the Earth's rotation can be thought of in two ways - the angular speed (which you've calculated) and the linear speed (of a point on the surface).

Express the angular speed (traditionally referred to by the Greek letter omega) in radians/sec:

omega = 360 degrees / 24 hours = 2 * pi radians / (24 * 60 * 60) second
(pi = 3.14159.....)
= 7.272 x 10^-5 radians/sec = 0.00007272 radians/second

Now to convert the angular speed to the linear speed of a point on the Earth's surface, multiply omega by the radius of the Earth, R.

To put it in familiar units, let's express R in miles: R = 3822 miles (roughly). So the linear velocity on the surface V is:

V = omega * R = 0.00007272 * 3822 = 0.278 miles/second
or
V = about 1000 miles/hour (Surprisingly fast!)

Dr. Louis Barbier

The Earth's spin is slowing down by about 1.5 - 2 milliseconds per century, and that angular momentum is moving into the Moon's orbit, which is getting larger. The reason for this, and the reason a figure skater can only spin for so long, is friction. In the case of the skater, it's air resistance and friction with the ice. In the case of the Earth, it's the friction due to tides moving around the Earth.

Dr. Eric Christian
(June 2000)

It takes nearly four minutes for a star at the celestial equator to move one degree (about a finger's width held out at arm's length). Stars nearer to the poles will move even less (Polaris doesn't appear to move at all in the northern hemisphere). So in five seconds you won't get much of a streak. With longer exposures you could maybe do it, but you'll need to accurately measure the length of the track and know how far the star is from the pole (its celestial latitude) to know the total length of the circle it would make if you could photograph it for a full day.

An easier way is to measure transit times. Measure the time that a star passes the same point (goes behind the roof of a building or crosses an overhead wire) on successive days and divide the time interval by the 360 degrees the star has traveled. If you miss a day or more because of clouds, it still works if you divide by 360 times the number of days. You have to keep your eyes in exactly the same spot for this to work, so a mounted telescope (that doesn't move) or just a cardboard tube will help.

Dr. Eric Christian
(April 2000)

The Earth's orbit varies from 0.983 AU out to 1.067 AU. We are actually closer to the Sun in the winter than in the summer. The orbit crosses the 1 AU point two times a year - in spring and fall.

Dr. Louis Barbier
(November 2002)

Your question provides the opportunity to elaborate on the connection between Kepler's Laws of planetary motion and Newton's Law of Gravitation.

The simple answer to your question is that there is nothing special at the other focal point. Here's the explanation.

In the late 16^th century, Tycho Brahe made the most accurate observations of the positions of the planets at that time, with his state-of-the-art observatory on the Danish island of Hven. In his attempt to model and explain Brahe's observations, Johnannes Kepler devised his laws of planetary motion. The first two are relevant here.

Kepler's first law states that all planets move on ellipses, with the Sun in one of its two foci, as you mentioned, and this still holds. Looking at this statement as a purely mathematical description, it is not obvious whether the Sun should be in one or the other focus, or whether there is any special significance to the second focus. That is where a physical argument and/or an observation would have to come in, which would then define the correct initial condition to determine the correct focus.

Brahe's observations also provided the handle to connect Kepler's laws with Newton's Law of Gravitation. The Sun is the source of the gravitation that pulls radially on the planets in our solar system and keeps them in orbit. It also slows them down on their way out to the furthest part of their orbits, and speeds them up on their way back in. If, in fact, there were another equal source of gravitation located in the other focus of the ellipse, there would be no reason for a planet to move faster near the one focus and slower near the other focus, as was observed.

Let us explore this with an illustrative fictitious experiment, diagrammed in the figure below.

Shown are three cases with different initial speed (arrows)
and the respective elliptical orbits in color. In all cases,
the satellite is closest to the planet and Focus 1 when it is moving
with its fastest speed.

We will launch a satelllite from a high tower on a planet that does not rotate (to make things simpler). This figure is obviously not to scale, but to orient you, in the diagram, the tower comes out of the planet on the top here, and the satellite launch goes straight to the right from its top.

When the satellite is launched from the tower at its slowest speed, its orbit looks like the red (innermost) ellipse. It is so slow that after launch it starts to fall toward the planet - and thus speeds up and falls into this type of orbit. As explained, the satellite is fastest where it is closest to the planet: on the side of the orbit opposite the launch tower. We'll call the planet's center Focus 1 (black "plus sign"). Focus 2, for this orbit with the slowest speed, is at the red plus sign (which would not actually be on the tower if the diagram were to scale).

Launching the satellite at a higher speed pushes the far side of the orbit further and further out. In the black (middle) orbit example, the speed is such that the ellipse turns into a circle. The planet is now in the center of the circle, and a circle is a special case of an ellipse. Because it's a circle, the locations of both foci are the same, at the black plus sign.

Finally, in the green (outermost) orbit case, the launch speed is fast enough for the satellite to continue to increase its distance from the planet. It slows down as it reaches the maximum distance possible with this speed - on the opposite side from the launch point. Again Focus 1 (in the planet center) is close to the point in the orbit where the satellite is at its fastest speed, and Focus 2 for the green orbit (green plus sign) is on the side where the speed is slowest.

Kepler's second law, derived solely from observations at the time of its discovery, states that on a planet's orbit around the Sun, the planet-Sun line sweeps out equal areas during equal time periods (fractions of a planet's orbital period). This quantifies the speed of an orbiting planet based on its distance from the Sun, and is in accordance with actual observations. A planet reaches its fastest speed at perihelion, its closest location to the Sun. This observational fact also provides a basis for the location of the Sun as one of the two foci. Again, it has to be the one on the side where the planet moves fastest.

This web page at the University of Kentucky may be helpful.

Dr. Eberhard Moebius
(October 2007)

Of course, this can't happen, but if it did, everything not attached would go flying off to the east, parallel to the surface of the Earth. The speed would depend upon your latitude. Only the people at the poles would be safe. You wouldn't go flying off into space because the 1000 mph maximum (at the equator) isn't enough to overcome gravity, which would still be present. If you survived, the resulting six month day and six month night would probably take care of you pretty quick.

Dr. Eric Christian
(August 2000)

Yes, your logic is correct. If the Earth stopped rotating on its axis everything on Earth, away from the poles, would appear to weigh more due to the absence of centrifugal force.

Mathematically, F = mg, where g is the acceleration due to gravity at the Earth's surface (9.80 kg/m²), m is your mass, and F is the force (in this case, weight) resulting from mass m in gravitational field g. If you are very far from any massive body then g ~ 0 and so you are weightless (F = m x 0 = 0).

However, for most places on a rotating Earth, we feel a centrifugal force that slightly decreases our weight. This is because for circular motion, the force (F_c) due to acceleration (a) is given by F_c = ma = m(v²/R) Cos(Theta), where v is the speed of the object (you, in this case), R is the radius of the circle it is moving in, and Theta is the angle between the rotation axis and your position on the Earth's surface. For the case of the Earth, R is its radius and Theta = 0 deg at the equator and 90 deg at the poles. So, since Cos (0) = 1 and Cos (90) = 0, the force on you, in a direction away from the rotation axis, is m(v²/R) at the equator and zero at either pole.

Numerically, this means that if your mass is 70 kg, the force of gravity acting on you is F = mg = 70 x 9.8 = 686 (kg/m)² (a unit called a "Newton"), or 154 pounds.

If you are standing on the equator, the centrifugal force acting on you is F_c = m(v²/R) Cos(Theta) = 70(463²/ 6371000) Cos (0) = 2.4 Newtons = 0.54 pounds.

So, you would weigh about a half-pound less at the equator than you would at the North or South Pole. (For the purists, the explanation above assumes the Earth is a sphere of uniform density and neglects relativistic forces.)

Dr. Ed Tedesco
(January 2006)

The mass of all the people on Earth is miniscule compared to the mass of the Earth, so neither of these actions would have any effect on Earth's motion.

Dr. Louis Barbier
(March 2002)

Check out the Windows to the Universe, the Starchild Web site, the Science@NASA site, or the Bad Astronomy site, which talk about the Earth, seasons, etc. Check Imagine the Universe! for information on the "equation of time" -- the asymmetrical change in the amount of daylight between sunrise and sunset through the year.

Dr. Louis Barbier and Beth Barbier

I know that as we approach the summer solstice, we gain hours of daylight. Do we gain the same amount of daylight each day as we approach the longest day of the year?

No, the day/night dividing line moves in a curve, not in a linear fashion. It's due to the interaction of several three-dimensional vectors, and so there are sines and cosines involved (and actually the product of sines and cosines). For a simple proof, near the poles, the gain of daylight goes to zero before the summer solstice is reached, because the day is already 24 hours long.

Dr. Eric Christian
(November 2000)

You can balance an egg any day of the year. The equinox is not special. See The Straight Dope web site, for example. This is a common misconception.

Dr. Eric Christian
(September 2000)

The thing that you've missed is that a "day" is not the length of time that the Earth takes to rotate through 360 degrees. Instead, the day is defined as the time it takes for the Sun to move from zenith to zenith. Because the Earth has travelled almost a degree through its orbit, it actually has rotated almost 361 degrees in 24 hours. Those extra degrees add up over a half year to keep the day synchonized. But the constellations do shift, so that what you see during the summer is overhead during the day in the winter and vice versa. The astronomical term for the time it takes the Earth to rotate 360 degrees is "sidereal day", which is 23 hours 56 minutes 4.09 seconds long.

Dr. Eric Christian
(September 2000)

The reason is that the axis of the Earth's rotation is tilted relative to the plane of the Earth's orbit around the Sun. So the circle on the Earth where the Sun is directly overhead moves north and south over the year, from the equator to the Tropic of Cancer, back to the equator, then to the Tropic of Capricorn and back to the equator. This causes sunrise and sunset to move north and south over the year as well. This effect also causes the seasons and the shortening and lengthening of the day.

Dr. Eric Christian

For more information on Sun and Moon rise and set times, Moon phases, eclipses, seasons, positions of solar system objects, common astronomical phenomena, calendars and time, and related topics, check out the Astronomical Applications Department of the U.S. Naval Observatory.

Beth Barbier
(April 2000)

The Sun gives off all colors of light, but blue light is bounced around the atmosphere a lot more than red light is (it's called scattering). The sky is blue because of the blue light bouncing around "lights up" other parts of the sky.

Dr. Eric Christian and Beth Barbier
(April 2000)

The Sun is always a little redder because of the scattering, but at sunrise and sunset the light has to pass through more atmosphere and loses much more blue light, so appears much redder.

Dr. Eric Christian

See NASA GSFC Science Question of the Week for more information.

Beth Barbier
(September 2003)

Hail can be formed by rain rising and freezing, but snow is directly formed in its solid, crystalline form.

Dr. Eric Christian
(November 2001)

Both the Earth moving (spinning) and gravity affect the wind. But the primary cause of the wind is temperature differences, not the moving of the Earth or gravity.

Dr. Eric Christian
(March 2003)

Though not our area of expertise, this is an area of current scientific study. Check out the European Science Agency's (ESA) article from September 29, 2000, Climate change: New impressions from space.

Beth Barbier

There would be an increase in temperature if the Earth to Sun distance became much smaller, but 1 meter is insignificant. For example, the Earth is actually closer to the Sun during the northern hemisphere winter.

Angela Richard
(February 2003)

The Earth's orbit changes on several time scales, each of which affect the intensity of radiation we observe at Earth.

The first change is the winter and summer solstice, where the Sun-Earth distance varies between 91,400,000 miles and 94,400,000 miles.

Now on longer time scales, the actual shape of the Earth's orbit changes every 100,000 years, vacillating between more circular and more elliptical. In this case, when the Earth is closest to the Sun it actually receives 20-30% more sunlight.

In addition, the Earth wobbles on its axis every 26,000 years, changing the time at which winter and summer occur.

Finally, the tilt of the Earth varies every 40,000 years by about 2 arc degrees, which affects the temperature difference between winter and summer.

These changes are termed the "Milankovitch theory" after the geophysicist who first proposed it and are believed to operate together to produce dramatic temperature variations on Earth. You might want to read more about the Milankovitch theory and solar radiation variations at the U.S. Naval Observatory Astronomical Applications Department and the Encyclopedia of the Atmospheric Environment.

Dr. Georgia de Nolfo
(June 2003)

A sidereal day is the length of time it takes the Earth to rotate 360 degrees. Since the Earth is revolving around the Sun, it actually has to rotate almost one degree (360 degrees/365.25 days) further until the Sun is in the same place in the sky, which is the definition of a day that everyone is used to. So a sidereal day is a little shorter, but the stars return to the same positions every sidereal day, so sidereal time is used sometimes in astronomy.

Dr. Eric Christian

If a sidereal day is 23 hours and 56 minutes, what happens to the remaining 4 minutes?

There is a good explanation of sidereal time on the Imagine the Universe website.

Beth Barbier
(March 2005)

I assume you want to know what would happen if the axis of the Earth moved (it is already turning on an axis that is vertical at the poles). The position of the equator is determined by the spin axis of the Earth, so if the new axis were at the right point (0 degrees latitude, 90 degrees longitude) then the old Prime Meridian would become the new equator (or at least half of it, the equator is a circle, the prime meridan is a semi-circle).

But the Prime Meridan is just a reference line picked by man, and could have been put anywhere. It goes through Greenwich, England, but the French were pushing hard for a French origin. If the axis moved, the new Prime Meridan could be put anywhere. Luckily, conservation of angular momentum insures that this drastic change in the Earth's axis is not going to happen.

Dr. Eric Christian

The statement that it is the Sun's and Moon's gravity that causes tides is not incorrect, just simplified. More accurately, it is the gradient of the gravity that causes it. What happens is that the water on the side closest to the Moon is closer to the Moon than the center of gravity of the Earth and so the Moon exerts more pull on that water and it bulges out (high tide). On the other side of the Earth, the Moon is pulling more on the center of mass of the Earth than on the water and so the Moon pulls the Earth out from under the water a little bit, causing another bulge or high tide. That is why there are two high tides per day. Hope this helps.

Dr. Eric Christian

Your question is well beyond our area of expertise or interest, and we cannot answer it. I did find a bit of information on a NOAA website that may get you started, at the Center for Operational Oceanographic Products and Services FAQ.

Beth Barbier
(July 2003)

Contributed by Konstantin Parchevsky, HEPL, Stanford University (May 2007):

The Moon's gravity causes the appearance of two "bumps" of water on the Earth -- one on the Moon's side and one on the opposite side. Theoretically, in the ocean, neglecting the viscosity of water, the highest spot of the "bump" should be just below the Moon.

In reality, near the coast there is a time lag between the moment when the Moon passes the meridian (the so-called culmination) and the moment of the highest tide. This time lag is called the "lunitidal interval". It depends on the viscosity of water and the shape of the coastline and the seabed, and it has to be taken from observations. If you want your watch to predict the tides correctly, you have to input the lunitidal interval. For some cities, you can take it from the table at the end of the watch's manual.

Unfortunately, there were no data for Half Moon Bay, CA (near me). So I had to calculate the lunitidal interval by myself. This is a good time to remember a saying: "If you are stuck, read the manual!" Fortunately, there was enough information in the watch manual to do this: "When setting the lunitidal interval for this watch, use the time difference between the Moon's transit over the meridian until high tide."

That's it! Just find the time difference between the moments of culmination of the Moon and the next high tide! First you have to calculate culmination of the Moon. Google gave me a lot of links for the phrase "Moon calculator". The second link is exactly what we are looking for. Just enter the date (such as 9 May 2007) and your position (Half Moon Bay, CA) and press 'Enter'. What we need here is the Moon transit (6:59 a.m.). Then we have to find the time of the next high tide in Half Moon Bay.

This time, Google "tides Half Moon Bay". The first link [similar sites can be found -- ed.] gives us a graphic representation of the tides and the moment of the closest high tide (6:14 p.m.). Time difference is 11h 15m. This is the sought-after lunitidal interval. And it works! Now my watch predicts the tides correctly.

For the record, I am an astrophysicist (solar physicist). I work at Stanford University and perform numerical simulations of propagation of the acoustic waves in the Sun.

You might want to check out the Views of the Solar System page on meteoroids and meteorites.

Beth Barbier

Your question about an asteroid impacting Earth is well covered by our sister site, Imagine the Universe!, in Is Earth in danger of being hit by an asteroid?.

Beth Barbier
(March 2002)

I think this will answer your questions well: Caveat Impactor

(Don't believe everything you hear on Fox News!)

Beth Barbier
(July 2002)

Yes, but unless the meteorite is really enormous (big enough to destroy life on Earth), the Earth's gravity changes by an amount that is too small to measure. The Earth is picking up several tons per day of dust and other stuff, not including the very uncommon meteorite.

Dr. Eric Christian
(November 2000)

The cusps are due to the interaction of the Earth's magnetic field and the magnetic field imbedded in the solar wind. They move around as the Earth moves around the Sun and as the tilt of the Earth's axis moves. They also move in position, as well as distance from the Earth, as the magnetic field of the Sun changes. So they don't point at any fixed direction.

Dr. Eric Christian
(November 2000)

Your question has a tremendous amount of latitude in it, both literally and figuratively. The magnitude of the Earth's magnetic field within the magnetosphere varies greatly with location within the magnetosphere. It is not a simple question to answer.

The magnetosphere is formed when the flow of the solar wind impacts the Earth's magnetic field (a dipole field), compressing it, causing magnetic reconnection, causing complex currents to flow, etc. The dipole field changes greatly under the influence of the solar wind.

Try to get a copy of "Physics of the Magnetopause" published by the AGU. That will help.

The magnetic field in the solar wind near Earth is about 5 nT, or 5 x 10(-5) Gauss. The magnetic field on the surface of the Earth is about 0.5 Gauss - big difference.

If you were to imagine a spacecraft passing from the solar wind and into the magnetosphere along the Earth-Sun line (at the sub- solar point) you would see the following:

The 5 nT field changes by a factor of 4 as you pass through the Earth's bow shock. This is a compression wave (discontinuity) that results from a supersonic flow striking the Earth's magnetic field and being stopped).

The magnetic field then climbs by another factor of 4 as the spacecraft passes through the magnetosheath, a turbulent region of flow behind the shock and still upstream of the magnetosphere.

The spacecraft then encounters a region where the flow stops moving toward Earth and flows exclusively around the obstacle that is the Earth's magnetosphere. This is the magnetopause. Behind the magnetopause the magnetic field is about 40 nT. So the compression of the flow has picked up almost a factor of 10 in intensity of the magnetic field (and the plasma density with it).

From this point on, it depends greatly on where you go. If you go Earthward, it is to leading order a dipole field (somewhat squashed, but the scaling is like that). If you go behind the planet, all kinds of things are happening and location is everything.

I really can't go into much more detail than that without a more specific question, and to be honest I mostly work outside the magnetosphere, so my knowledge is limited. But basically if you go from the magnetopause to the Earth's surface, you see a magnetic field that grows ever stronger as you decend within the dipole.

Dr. Charles Smith
(October 2003)

The Sun shifts it's magnetic field every 11 years, and it has already happened for this solar cycle. The Earth's magnetic field flip is much more erratic and has happened approximately 25 times in the last 5 million years. It's been about 740,000 years since the last flip, however, so we're long overdue. There is evidence that we may be heading towards a reversal (the dipole magnetic field is weakening and the higher order terms are increasing), but we can't predict when it would happen. Depending upon how quickly the field reversal happens, it could cause problems for things like electric power lines and oil pipelines, and if the field goes to near zero, it might cause a higher background radiation at the ground, but there is no evidence that previous reversals have had any major biological effect. The forces due to the interaction of the solar and terrestrial magnetic fields are only very small perturbations.
(October 2000)

How do the magnetic poles reverse? Is it for the same reason that the Sun's poles reverse?

Both the Sun and the Earth are electromagnets, not permanent magnets (despite the Earth's core being made up of iron and nickel). So it is electric currents moving through the plasma of the Sun and the molten rock of the Earth's interior that generate the magnetic fields. These currents have instabilities that build up until the field reverses to relieve stress. The fact that the magnetic pole moves is tied in with the same instabilities, but the flip probably happens pretty quickly on Earth (less than ~ 1000 years is all they can tell).

Dr. Eric Christian

As far as I know, there is no significant change of Earth's gravitation with time, because this is only a function of the Earth's size and mass, both of which are not changing significantly over time. There is only one regular change of the gravitational acceleration over time at a fixed location, and that has to do with the reason for the tides. The gravitational forces of the Moon and the Sun at a specific location change with their relative position in the sky. The effect is of the order of a few 100,000ths of the gravity at the Earth's surface. However, I am not the instant expert on precision gravity related questions.

As far as variations in the Earth's magnetic field at the surface are concerned, two different causes must be distinguished:

1. The natural magnetic field of the Earth changes over time due to changes in the strength and configuration of its core, which are driven by the hot liquid interior of the Earth.
2. The magnetic field changes because of gigantic currents that flow around the Earth in the magnetosphere. Depending on the space weather conditions, the currents in the magnetosphere change over time, and so does the Earth's magnetic field.

Neither type of change is just a change in the strength of the field. The direction of the magnetic field also changes. Therefore, the knowledge of magnetic field strength over time at only one location would not be sufficient to track changes in the Earth's magnetic field.

A detailed introduction into the Earth's magnetic field and related changes, with links to databases, may be found at the National Geophysical Data Center maintained by NOAA. On this page, General Information provides a very good introduction. Magnetic Declination On-Line allows computation of the magnetic declination (the direction a compass needle will point) for any location on Earth and for any date from 1900 to today (and even into the near future). The World Magnetic Model provides maps with magnetic parameters for specified locations or the entire globe.

Dr. Eberhard Moebius
(January 2003)

Earth's atmosphere reaches over 560 kilometers (348 miles) from the surface of the Earth. You can read more about it at the NASA homepage.

Beth Barbier
(March 2000)

The question of whether light/photons reach the Earth's surface depends on several factors. Recall that light can get both absorbed and scattered by molecules in the atmosphere. The fate of a given photon as it enters the atmosphere depends on the chemical nature of our atmosphere. The Earth's atmosphere is primarily nitrogen and oxygen.

The basic idea to keep in mind is that the atoms and molecules have discrete energy levels which are known to be quantized (from quantum mechanics). Photons also have quantized energy. Because atoms or molecules require discrete energy boosts in order to be excited (i.e. cause an electron to transition from a low energy level to a higher one), the arriving photon, in order to get absorbed, must carry at least the difference in energy between the low and high energy states of the atom. If the photon is of even greater energy, it may also get absorbed by kicking out the electron altogether (with the electron departing with some extra kinetic energy).

So, photons will get absorbed depending on the chemical/atomic structure of the molecules in the atmosphere and on the energy of the incident photons. For instance, there are two compounds responsible for absorption in our atmosphere in particular: oxygen (O₂) and water vapor (H₂O). Molecular oxygen and ozone are the strongest absorbers in the ultraviolet and water vapor, while methane, nitrous oxide, ozone, and carbon dioxide absorb in the infrared. X-rays can be absorbed in the atmosphere by individual nitrogen or oxygen atoms since they generally have enough energy to kick out an electron (and the chances of seeing an individual atom in a thick atmosphere such as Earth's are pretty high!).

For more on how radiation interacts with matter, see the Interaction of Radiation with Matter at HyperPhysics.

Dr. Georgia de Nolfo
(December 2004)

I'm sure that this number (5 ft/year) is WAY too large. That would put the Earth 1000 miles larger a million years ago. 5 feet per million years is probably more like it. The effect on the atmosphere would be small, because it is the total amount of mass of the Earth that provides the gravity that holds the atmosphere in place, and that is only increasing (about 1 ton per hour from micrometeorites). Over the billions of years that the Earth has been around, it has lost its atmospheric hydrogen and some of its helium just by random diffusion.

Dr. Eric Christian

The National Space Science Data Center (NSSDC) at NASA GSFC has a page on the Moon with the distance from Earth listed as 384,467 km. It would be standard practice to give center-to-center distances.

What is the distance between the surface of the Earth and the surface of the Moon?

With the center-to-center distance above and the diameters of the Earth and the Moon, you could figure out the distance from surface to surface. From The Nine Planets, the diameter of the Earth is 12,756 km, and the diameter of the Moon is 3,476 km. Just subtract half of the diameters (= the radius), from the center-to-center distance:

• Radius of Earth: 6,378 km
• Radius of the Moon: 1,738 km
• Surface-to-surface distance: 376,351 km

Beth Barbier
(April 2000)

The mountains on the Moon and on Earth, for the most part, formed in completely different ways. Because the Moon is so much smaller, it cooled most of the way through, and so is mostly rocky now, although it was molten at one time. Most lunar mountains are caused by impact craters. On the Earth, the upper rocky crust floats on the molten mantle, and mountains are caused by plate tectonics, when enormous plates of the crust bump up against each other. The deepest craters on the Moon are about the same size as Mt. Everest, however.

Dr. Eric Christian
(October 2000)

You are asking two related questions, so let me tackle them in sequence. First, you asked why the Moon is covered with impact craters from meteors and asteroids, while the Earth seems to be much luckier in that respect. You answered part of that question yourself: the Earth has an atmosphere, which protects its surface from meteors up to a moderate size. These meteors evaporate upon entry into the atmosphere and can be seen as "shooting stars." Larger meteors, though, reach the ground. There are quite a few famous meteor craters on Earth, for example, the Meteor Crater near Flagstaff, Arizona, or the Noerdlinger Ries, an almost perfectly circular valley in Germany. Admittedly, these are fewer craters than are found on the Moon, and most of the Earth's craters are much younger. In fact, the Earth was pelted with many impacts at the same time when the Moon was hit so badly, up to about 3.5 billion years ago.

Why don't we find these craters anymore on Earth? The answer to this part of the question is that the Earth is geologically active and has a severe climate, which leads to hefty erosion by water, ice and wind over the long time scale of billions of years. In other words, the Earth's surface has been completely reshaped several times over by the ongoing plate tectonics, which moves the continents around and builds up mountain ranges. On the other hand, all these formations, including impact craters, are subject to severe weathering already on the scale of hundreds of millions of years.

Let me come back to your second question: Why does the Earth have an atmosphere and not the Moon? The answer lies in the fact that the Moon features so much less mass than the Earth; it has only about 1/80 the mass of the Earth. As a consequence the strength of gravity on the Moon's surface is about 1/6 that on Earth, and the escape speed, i.e. the minimum speed that an object must have to escape the gravitational grip of the Moon, is much less (about 1/5) than that for Earth. (For the return trip from the Moon the Apollo lander had to achieve only a much lower speed than the Saturn rocket when leaving the Earth.) Therefore, atoms and molecules of a gas are much more likely to escape the Moon than the Earth. Being at the same distance from the Sun, the average temperatures on Earth and the Moon are very similar (not the extremes because of the differences in the atmosphere). Therefore, the average energy of the atoms and molecules in a gas, or their average speed, is the same on Earth and on the Moon. As a consequence the Moon has lost all of its gases in the distant past. To play with the idea of keeping or losing an atmosphere you may want to try out an in-class activity from my general astronomy class. You will find it at
http://www-ssg.sr.unh.edu/406/Activities/Activity6_2004.pdf
and the solutions are available at
http://www-ssg.sr.unh.edu/406/Activities/Activity6_2004_sol.pdf.

Dr. Eberhard Moebius
(April 2003)

How big are those craters on the Moon? Aren't they gigantic? How could we lose or ignore such huge structures on Earth?

There are huge structures on Earth, too, like in the Hudson Bay, but most of them have been wiped out by geology. 70% of the Earth's surface is ocean floor. And the ocean floor is being completely renewed on a time scale of 300 - 500 million years. It is as if we push old sheet metal into a melting oven and out comes completely new sheet metal on the other end.

Dr. Eberhard Moebius
(April 2003)

Because the Moon is in orbit around the Sun (as well as in orbit around the Earth). Another way of looking at it is that the center of gravity of the Earth-Moon system is revolving around the Sun, and the Earth and the Moon are revolving around their common center of gravity. The Sun is pulling on the Moon, but the Earth is "falling" right along with it, so they stay together.

Dr. Eric Christian

You might want to look at the Sky and Telescope eclipse page. It addresses this question.

Beth Barbier
(March 2000)

It's possible that you did see a "moon dog". Check out the Imagine the Universe! site's answer on this subject.

Beth Barbier
(February 2001)

And why can't we assume that the boundaries of atoms are set by heavenly motion? Is it possible that what we call "the strong nuclear force" is nothing more than the result of small entities racing in acute paths, keeping pace with larger bodies?

Although the Earth's position in space IS determined by the sum of various motions (Earth around Sun, Sun around Galaxy, Galaxy in supercluster, etc.), there is NO evidence that this has anything to do with atoms. Why should the boundaries of atoms be set by heavenly motion?

The theory of General Relativity fits the motions of the Earth extremely well, and Quantum Chromodynamics explains ALL observations of the strong nuclear force. Your hypothesis would have the strong force be different at different places and velocities. The evidence (stars, etc. seem to "burn" the same throughout the Universe) is against you.

Dr. Eric Christian

The average radius of the Earth's orbit around the Sun is 93 million miles, so the distance is

It is impossible for there to be another planet on the opposite side of the Sun from the Earth, in the same orbit. We would have seen the gravitational effect of this planet on other planets, and, because the Earth's orbit is an ellipse, not a circle, we probably would have seen the planet as well.

Dr. Eric Christian

The equation for gravitation acceleration is G x M / r², where G is the gravitational constant (6.67 x 10^-11 m³kg^-1 sec^-2).

The mass of the Sun is 2 x 10³⁰ kg, and the distance is 1.5 x 10¹¹ meters.

The Moon is 7.3 x 10²² kg, and the distance is 3.8 x 10⁸ m.

You then find that the acceleration from the Sun is .0059 m/s² (meters per second squared) and from the Moon is .000034 m/s² or 176 times smaller.

Dr. Eric Christian

As Isaac Newton figured out, the force of gravity is universal for every object that has mass. And as you pointed out correctly, the Sun's gravity is much more powerful than Earth's, and that is because of its much larger mass.

However, the effects of gravity on other objects also scale inversely with the square of their distance from each other. (By the way, distance counts between the mass centers of the objects under consideration.) If an object is 10 times as far away, gravity drops by a factor of 100.

Therefore, the motion of objects in the vicinity of the Earth's surface is almost completely determined by Earth's gravity. The Earth itself, of course, is held in orbit by the Sun's gravity. We have to and we do consider the Sun.s gravity (or better, its variation with distance across the Earth) when it comes to the tides. The tides are mainly caused by the Moon, but the Sun produces half the effect of the Moon. Therefore, we have very strong tides at new moon and full moon (the Sun pulls on the water in the same direction or opposite to the Moon, and their effects add up), but very weak tides at half moon (the Sun pulls at right angle relative to the Moon, and their effects partially cancel). For additional explanations concerning the tides see our answer about the tides.

The Earth keeps objects within its own gravitational force up to a distance of 1.5 million km (almost 1 million miles). Beyond that, objects enter the region where the Sun dominates. This distance is about 1% the distance of the Earth from the Sun, which demonstrates how much stronger the Sun is.

Dr. Eberhard Moebius
(August 2004)

Also, if I were to backtrack the 1 millisecond per 50 years that the earth's rotation is slowing down (if this rate were constant), the earth couldn't be 4.5 billion years old. Obviously the rate isn't constant, but I don't understand why.

There is a pretty complicated tidal interaction between the earth and the moon, but what it boils down to is that angular momentum (which is conserved in a system) is being transferred from the earth's spinning to the moon's orbiting. This causes the day to get longer and the moon to orbit farther away. The transfer happens in that direction because the earth is spinning faster than the moon revolves around the earth, and the earth's spinning pulls the tides so that the one facing the moon actually precedes the time when the moon is overhead. That speeds up the moon (which gets a larger orbit) and slows down the earth.

Your problem with the age of the earth comes about because you're thinking of things upside down. In the definition of angular momentum, which is what is really changing, time (the length of the day) is in the denominator. So 4.5 billion years ago, the day wasn't less than zero (which is what you get if you take the current rate of change of the day), but the angular momentum was more than twice what it is now (which is what you get if you convert the rate of change of the day to a rate of change of the earth's angular momentum). So the day was less than 12 hours long (not zero).

There are added complications in the fact that the moon was closer then, but that's the gist of it.

Dr. Eric Christian
(July 2010)

Gravitational force, F, is defined as:

F = G * M1 * M2/(r*r)
G is the gravitational constant
M1 and M2 are the masses (of the Earth and the Moon in this case)
r is the distance between them

This depends upon the product of the two masses and so is exactly the same for both.

What is really happening is that both the Earth and the Moon are orbiting the center of mass of the two bodies. Because the Earth is much heavier than the Moon, that center of mass is actually inside the surface of the Earth (it's about 3000 miles from the center).

Dr. Eric Christian
(November 2007)

This is beyond our area of expertise, but you can find answers to some questions on this page, and you can find a lot more information on the Moon at The Nine Planets site.

Beth Barbier
(March 2001)

Well, I'm no expert on motions of the Earth, but the Moon does add drag to the Earth's rotation in the form of tides, both oceanic and internal. This added drag tends to stabilize the rotation. It is also gradually slowing down the rotation of the Earth, which gradually lengthens Earth days.

Dr. Eric Christian and Beth Barbier

Because of the rotation of the Earth, there will be a moonrise and moonset about once a day. Because of the orbit of the Moon around the Earth, the time of moonrise and moonset will move relative to sunrise and sunset. At the time of the new moon (when solar eclipses can happen), moonrise and sunrise are at about the same time and place, as are moonset and sunset. Fourteen days later at the full moon the moon rises at sunset and sets at sunrise. Things aren't exact because the orbit of the Moon is slightly tilted relative to the Sun.

Dr. Eric Christian
(March 2001)

When we look at the Moon, we are seeing sunlight that is reflected off the Moon's surface and bounced toward the Earth. When the Moon is on the side Earth away from the Sun, the light is reflected back up to Earth's night side, which is in shadow. Then we see the Moon at night. It looks bright because we see it against a dark sky; there aren't any other strong sources of light from that direction except for the distant stars. It also looks generally full and round because light from the whole face of the Moon can get reflected back to Earth.

The Moon moves around the Earth roughly once every 28 days. When it is on the side closer to the Sun and at least a little to one side of the Earth-Sun line, light can still reflect from the Moon and reach the Earth. In this case, the light will be reflected onto the sunward side of the Earth and we will see the Moon in the daytime. Of course only light striking the "edge" of the Moon will get reflected toward the Earth so we see only a crescent shape. The Moon seems to look dimmer in the daytime because your eye has to pick out the light from it against the wash of light that comes directly from the Sun and is spread around by the atmosphere.

Another way to think about it is to remember that the Moon is a sphere, like the Earth, and the sunward half of that sphere is always lit by the Sun. Whatever part of that lit half of the Moon can be observed from Earth is what we see. If the Moon is in a position to be observed from the sunward side of Earth, we see it in the daytime. If the Moon is on the side of Earth away from the Sun, we will see it in the night. The Moon seems to change shape during the month because we are able to see only part of its sunlit side depending on how the Moon is lined up with the Earth and Sun.

There are two special points in the Moon's orbit around the Earth when it is very nearly lined up with the Earth-Sun line. If it is on the side away from the Sun, we see the entire lit half of the Moon, we call that a full moon. Sometimes the Moon actually crosses directly into the Earth's shadow, that's called a lunar eclipse. If the Moon is lined up on the sunward side of the Earth, we can see only unlit side. This is called a new moon. Once in a while the Moon's shadow falls on the Earth. This is a solar eclipse.

Dr. Jeff George
(March 2003)

Half the Moon is in direct sunlight nearly all the time. That is why we can see the Moon. The phases are because we see only the sunlit part, and only during a full moon does the sunlit half fully face the Earth. The only times the Moon doesn't get direct sunlight is during lunar eclipses, when the Earth's shadow blocks the Sun.

The temperature changes drastically depending upon whether that portion of the Moon has sunlight (day) or not (night).

Dr. Eric Christian
(September 2007)

The basic reason is because the Moon is so far away, but it also has to do with how your eyes and mind work together (it is what is called an optical illusion). If you look at two things (say a building and a tree) that are different distances from you and then walk sideways, you will see that the two objects shift, but that the closer one shifts to the side more than the one that is farther. The Moon is so far away that it does not appear to shift at all. The human mind interprets this in a funny way. It thinks that the Moon is closer than it really is, but that it is moving sideways at the same speed as you are. So you think that the Moon is following you.

Dr. Eric Christian
(September 2002)

In order for it to maintain its location, the Moon would have to be moved about four times farther away from the Earth, to the L1 libration point. Currently two spacecraft, ACE and SOHO are located near the L1 point, and always stay between the Earth and the Sun. The problem with the Moon at L1 is that, being farther away, the Moon would appear much smaller and not block the entire Sun (although maybe that's not really a problem - the Earth would get colder, but would still get some Sun).

Dr. Eric Christian

The classic "Earthrise" photo was taken by Apollo 8, which was in orbit around the moon. You are correct, except for regions near the edge of the earthward side of the moon (there is some tilt and wobble). Most of the moon has the Earth in the same place in the sky.

Dr. Eric Christian

(August 2010)

The Earth and the Moon revolve around the center of mass (CM) of the Earth/Moon system, which in turn is revolving around the CM of the solar system, which in turn is revolving around the CM of the Galaxy...

The Earth is most of the mass of the Earth/Moon system, so the CM is closer to the Earth, and the Sun is most of the mass of the solar system, so the CM of the solar system is actually inside the Sun. But from far away, the paths of the Earth and Moon WOULD look like braided strings around the Sun.

A separate motion of the Moon is its rotation (spinning). Since it spins with the same period as its revolution around the Earth/Moon CM, one face always points towards the Earth (not the Sun, all parts of the Moon get day and night).

Dr. Eric Christian

I can find no news on new intense lunar quakes, so I find this story very hard to beleive. There is no way the Moon, with it's solid core, could shake itself apart. There just isn't enough energy. There are moonquakes, but they are small. They can be caused by tidal forces (with the Earth), temperature fluctuations between day and night, contraction of the Moon's core (by gradual cooling), or meteorite impact. But I fully expect the Moon to be there in one piece 6 months or 6 million years from now.

Dr. Eric Christian
(May 2000)

First, let me state for the record, that planetary objects and satellites do not, as a rule, blow up. If the Moon did, you could expect large chunks of it to hit the Earth at pretty high speeds. Very messy. After a while, the survivors might see a very pretty ring of debris about the Earth.

Dr. Eric Christian
(September 2000)

Your question is beyond our area of expertise, but you might want to check out the Astronomy Society of the Pacific web page on this subject.

Beth Barbier
(April 2000)

The Moon is never lit only on the top or bottom. You can read more about this on Wikipedia.

Dr. Louis Barbier
(April 2008)

The answer is easy, and it's also easy to actually show the students the answer:

1. Take three students and make them the Sun, Earth, and Moon. Stand the Sun up in front of the classroom facing the Earth and the Moon.
2. The Earth should face the Sun -- and then the Earth's front is lit by the Sun (day), and its back is dark (night).
3. Put the Moon behind the Earth, also facing the Sun. Again, the Moon's front is sunlit, and its back is dark. Now only people on the night side of the Earth can see the Moon, but all they see is the front (sunlit) part of the Moon. This is a full moon, and everyone on Earth sees it (when they're on the correct side of the Earth).
4. If you move the Moon to the Earth's right side, still facing the Sun, everyone on the right side of the Earth sees half of the Moon's front (sunlit) and half of their back (dark). This is the third quarter moon. And again, everyone sees the same Moon.
5. Stand the Moon directly in front of the Earth, and all you can see is the Moon's back, which is the new moon. Solar eclipses can only happen during a new moon.
6. If you put the Moon ahead and to the left of the Earth, people on the Earth can see mostly back, but a little bit of the front. This is the waxing crescent.

So the phase of the Moon is set by the position of the Moon relative to the Earth and the Sun, not by where you stand on the Earth.

Dr. Eric Christian
(February 2010)

One good activity involves having students play the parts of the Earth and the Moon. There's a description on this Astronomical Society of the Pacific site.

Beth Barbier
(January 2005)

The Moon rotates on it's axis and revolves around the Earth in 27 1/3 days. The time from one full moon to the next is 29 1/2 days. The reason they are not the same is that during the 27 1/3 days that it take the Moon to go around the Earth, the Earth has moved relative to the Sun, and it takes another 2 days or so before the Moon is directly on the other side of the Sun (which is what gives a full moon). It is similar to the reason why the Earth rotates on its axis once every 23 hours 56 minutes and 4.1 seconds, but its 24 hours from noon to noon.

Dr. Eric Christian
(September 2001)

One good activity involves having students play the parts of the Earth and the Moon. There's a description on this Astronomical Society of the Pacific site: The Moon: It's Just a Phase It's Going Through...

Beth Barbier
(January 2005)

Most of the light from the Moon is direct reflection of sunlight. A lunar eclipse happens because the Earth's shadow crosses the Moon, preventing the Sun from directly illuminating the Moon. You can see some "earthlight", reflection of sunlight that hits the Earth, then hits the Moon and bounces back to the Earth. When there is a crescent moon, you can frequently (if the sky is dark) still see the "unlit" portion of the Moon. That light (much dimmer than normal moonlight) is from the Earth.

Dr. Eric Christian
(November 2000)

Shadows are caused by sunlight, and since the Sun shines on the Moon, there must be shadows. In fact you can determine the height of objects on the Moon by the length of their shadows (just like on Earth).

Dr. Louis Barbier
(April 2001)

I'm not certain what you're referring to here. If you mean that pictures on the Moon show a black sky, you should know that there isn't an atmosphere on the Moon to reflect sunlight, so its sky always looks black. The surface of the Moon, however, does reflect sunlight, so the surface facing the Sun is well-lit, and that side is experiencing daytime. (Think about the definition of day and night on Earth.)

Beth Barbier
(March 2000)

Water was found to exist as very small, very spread out ice crystals in craters on the north and south poles of the Moon. Craters are the result of ancient meteorites that hit the Moon's surface. The amount of ice is very uncertain, but estimates are 10 - 300 million tons spread over the polar regions.

Dr. Eric Christian

The short answer is "no". Not directly at least. Meteoroids impact the Moon (the Earth, and other planets) because the trajectory of their orbits and that of the Moon intersect. Of course, gravity is responsible for the shape of the orbits themselves, so one might indirectly say gravity is responsible. But not in the way I think you are asking.

Imagine, for example, when you fire a bullet from a gun at a target. While it is true that there is a gravitational attraction between the bullet and the target, the bullet would still hit the target if there were no gravity. Its trajectory will intersect the target location.

Meteoroids are of two distinct classes. Some revolve around the Sun (like the planets do) and have orbits of small eccentricity (nearly circular) and those orbits are near the plane in which the planets orbit. The more common type is associated with debris streaming off comets as they approach the Sun. This debris ends up scattered along the (highly eccentric) orbit of the comet. As the Earth's orbit passes through these debris "clouds" we have meteor showers. Since the Moon is moving along with the Earth, it will often experience many of these meteoroids too, which will strike the Moon's surface.

Dr. Louis Barbier
(April 2003)

The first attempt to measure the amount of meteoritic dust falling onto Earth was made by Hans Pettersson in the 1950s. His measurement came up with a maximum infall rate across the entire Earth (what scientists call an upper limit) of about 15 million tons per year. His sample was contaminated by volcanic dust, etc., and the real number (measured out in space) is only about 20,000 - 40,000 tons per year.

Creationists ignore the new measurements, and the fact that Pettersson's value was an upper limit, and misinform the public. Interestingly enough, there are plenty of places on the Moon where the dust is more than 100 feet deep, but most of the dust is from meteoric impacts on the Moon itself throwing up debris. NASA was NOT expecting a deep layer of dust where Apollo landed (in the highlands). Remember, we had already landed unmanned probes on the Moon before Apollo (Surveyor).

For more information, you can check the Talk.Origins archive.

Dr. Eric Christian
(November 2000)

The mass of the Moon is less than 1/80 (0.0123) that of the Earth, and its diameter is a little more than a quarter (0.273). Gravitational acceleration is proportional to M / (R * R), so for the Moon (0.0123) / (.273 * .273) = 0.165 or about 1/6 that of the Earth.

Dr. Eric Christian
(May 2000)

The gravity on the Moon is about 1/6th that of Earth, which comes out to about 5.3 feet per second per second acceleration (32 * 0.165 = 5.28 feet/sec/sec). To get something off the surface of the Moon you'll need energy equal to the mass of the object times this acceleration times the height you want to achieve (M x G x H). A balloon gets its lift for the same reason a boat floats, it weighs less than what it's displacing. If there is no air to displace, you get no lift.

Drs. Eric Christian and Louis Barbier

The magnetic field of the Earth only extends about a quarter of the way to the Moon. So the compass wouldn't point at the Earth, and since the Moon has nearly no magnetic field, it wouldn't be much good at all.

Dr. Eric Christian
(September 2001)

Definitely, a magnet would stick to an iron bar on the Moon. In fact, it will stick to an iron bar any place in the universe, provided the temperature is not excessively high. The effects of magnetism, such as a magnet and iron sticking together, are based on the presence of a magnet and have nothing to do with the question whether the environment is within a larger magnetic field or not. If astronauts bring a permanent magnet to the Moon they will observe that it will attract any piece made of iron.

They only question may be whether the magnet keeps its magnetic quality or not. If a magnet is heated to more than about 700 degrees centigrade, it will lose its magnetism and behave like an ordinary hunk of iron. However, on the Moon it is not hot enough that this would happen. So the magnet will still work.

Now to the second question: will the magnet induce magnetism on the iron? Yes, it will. Again, any permanent magnet will induce magnetism on a piece of iron on the Moon. The only difference between the Earth and the Moon is that the Earth's magnetic field can induce magnetism on a piece of iron by itself, similar to what a refrigerator magnet can do. This can't happen on the Moon, but any magnet still can induce magnetism there.

Dr. Eberhard Moebius
(August 2003)

Nothing in the flashlight (basically just batteries, wires, switch, and bulb) requires air, so it would work fine.

Dr. Eric Christian
(April 2002)

There is no air on the Moon. NASA embedded stiff wire in the American flag so that it wouldn't just hang straight down, and adjusted the wire so that the flag appeared to be waving. They thought (probably rightly) that a flag as flat as a board wouldn't look right.

Dr. Eric Christian
(November 2000)

Parachutes use air resistance to slow down a descent. Since there is no air on the Moon, they would be completely ineffective. The parachute would fall at the same rate as a rock.

Dr. Eric Christian
(July 2002)

Actually, both arguments have some element of truth. You do need oxygen to ignite the firearm and space is for all intents and purposes a vacuum (with no available oxygen to take part in combustion). Now engineers have been able to get around this difficulty over the years with rocket fuel by mixing both a fuel and an oxidizer together in the rocket's combustion chamber. For example in solid rocket boosters (SRB) the typical mixture consists of an ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent). You can read more about SRBs on the Kennedy Space Center site.

Now as it turns out, guns are also self-igniting systems, where the mixture of fuel and oxidizer is usually located inside the bullet itself. So indeed, guns can be fired in space, although you won't hear the typical accompanying "boom"! Of course, a firearm could indeed be used as a method for propulsion, although there may be more efficient ways to acquire propulsion in space. See this site on rocket propulsion for more details on propulsion in space.

Dr. Georgia de Nolfo
(April 2003)

The StarChild website has such a test: Problems in Space.

Beth Barbier
(November 2004)

The Moon will not leave the Earth's gravity, even though the orbit of the Moon is increasing slightly. The Earth's rotation is slowing down (due to "tidal braking"), and to conserve angular momentum the Moon is accelerating. The Moon's orbit increases by about 3 cm/year.

The Earth and the Moon eventually will be "locked" together with each only having one side constantly facing the other. (Right now the same side of the Moon faces the Earth, but all sides of the Earth see the Moon. In the future this will not be true!) Life on Earth will be quite different then, but this won't occur for billions of years yet. When it does occur, the Moon's orbit will be 50% larger than it is now, and a month will be about 50 days.

Dr. Louis Barbier

The Great Wall can barely be seen from the Shuttle, so it would not be possible to see it from the Moon with the naked eye.

Beth Barbier

The Moon was called Selene or Artemis by the Greeks and Luna by the Romans. I'm sure other cultures also had names for the Moon. But in English, Moon (from Mona and Moone in Old and Middle English) was used before anyone had any idea that the other planets had moons. So it was more a case that the specific name for the Moon was extended to mean small bodies revolving around planets elsewhere. The Moon's name is the Moon.

Dr. Eric Christian
(June 2002)