Questions and Answers About the Moon

Students have submitted lots of questions about the Moon. Click on each question to see its answer. Do you have a Moon question that isn’t already answered here? Submit it using the form at the bottom of this page.

The Moon

The Moon is made from many of the same things that we find here on Earth. Scientists studied about 800 pounds of moon rocks brought back by the Apollo astronauts. Their tests showed that the rocks from the Moon are similar to three kinds of igneous rocks that are found here on Earth:
  • Basalt                basalt
  • Anorthosites.  anorthosite
  • and Breccias   breccias
Scientists found three minerals on the Moon that are not found on the Earth. They are: Armalocolite, Tranquillityite, and Pyroxferroite. Nasa link to Lunar Samples
The Moon has less gravity because it is smaller than the Earth. Astronauts liked walking on the Moon. They were able to take giant steps because they didn't weigh as much there. If you were on the Moon you would weigh only one sixth of what you weigh here on the Earth. If you weigh 60 pounds on Earth, you would weigh only 10 pounds on the Moon. If you weigh 180 pounds on Earth, you would weigh 30 pounds on the Moon.
moon footprintTwelve men walked on the Moon from 1969 to 1972.
  • Apollo 11: Buzz Aldren and Neil Armstrong (July 1969)
  • Apollo 12: Alan Bean and Charles Conrad (November 1969)
  • Apollo 14: Edgar Mitchell and Alan Shepard (January 1971)
  • Apollo 15: James Irwin and David Scott (July 1971)
  • Apollo 16: Charles Duke and John Young (April 1972)
  • Apollo 17: Harrison Schmitt and Eugene Cernan (December 1972)
The Moon stays in orbit because of the Earth's gravity. moon orbits earth
The Moon does not have any air. That is why astronauts had to take tanks of air with them. astronaut
You can jump higher on the Moon because it has less gravity than the Earth. This means that you will weigh less on the Moon. To find out how much you what you would weigh on the moon, take what you weigh on Earth and divide it by 6.     I weigh 80 pounds on the Earth  boy      but I only weigh 13 pounds on the Moon.  boy 2
The Moon has a diameter of  2,158.8 miles.  It takes four Moons to equal the diameter of the Earth. Moon diameter Earth diameter The moon is 6,790 miles around at its middle.  If you could drive a car around the moon at 75 miles per hour, it would take you about three and a half days to make the trip. In terms of area, the Moon is about 14.6 million square miles.  This is less than the area of the Asian continent, which is about 17.2 million square miles.
Craters are bowl-shaped depressions on the Moon. They are formed when rocks from space crash onto the surface of the Moon. moon craters
The spacecraft called Clementine sent back images of what scientists believe may be water ice at both the north and south poles of the moon. But this is not the same thing as liquid water.  Here is a photo of the Moon's south pole taken by Clementine.   moon south pole
No.   The Sun is 865,000 miles in diameter.   The Moon 6,790 miles in diameter. For an easy comparison draw a circle on a paper about eight inches in diameter. That will be the Sun. Now place a dot on it. That is the Moon.   Sun moon relative size
The Moon is 384,400 (three hundred eighty forty thousand, four hundred) kilometers away from Earth. Or if you want it in miles, it is about 238,000 miles. moon distance from earth That's enough space to fit in about 30 Earths lined up side by side between our planet and the moon.
Scientists think that an object about the size of Mars crashed into the Earth three to four billion years ago.  A large piece of the Earth was broken off by the impact, and combined with pieces of the object that struck the Earth.   Together, these pieces formed our Moon.  At first it was in a molten (liquid) state, but over millions of years it cooled and formed the solid moon we see today.   moon formation Image credit:  Joe Tucciarone      
The Moon does have rocks on it.  This is basalt, one of the rocks the astronauts brought back to the Earth.   basalt     Instead of sand, the Moon has a layer of broken rock called regolith, created from millions of meteors crashing onto its surface.  This is lunar regolith dug by astronauts on the Apollo 16 mission.   lunar regolith
It takes the moon 29 and a half days to turn completely around.   But it also takes the moon that long to make one orbit around the Earth.  That means that we always see the same side of the Moon, so it doesn't appear to turn around. But you can see that the Moon really does turn around if you notice that the same side of the Moon does not always face the Sun. Look at the X on the Moon in this diagram.   You can see that the X always faces towards the Earth, but not always towards the Sun.   moon rotation
The Moon is a very dull place compared to Earth. There is no air to breathe, no running water and nothing growing on the Moon. moon landing   moon quarter view  moon massif  There are only rocks. moon walk moonscape
The diameter of the Moon is 2,160 miles (diameter is the distance from one side of a sphere to the other, going right through the center).    That's the same distance as if you drove from Salt Lake City, Utah, to New York City, New York. moon compared to united states
The Moon spins like a top (like the Earth does), but it spins much slower than the Earth.  The Earth takes only 24 hours to turn around once.  The moon takes 29 and a half days to turn around once. moon rotation
The Moon does not have any light of its own. We see the Moon because it reflects light from the Sun and even the Earth.  How much light the Moon reflects depends on where it is in its orbit. A crescent moon will not reflect as much light toward the Earth as a quarter or gibbous moon. During a full Moon we get the most light reflected back to the Earth.  Albedo (al-BEE-doh) is a term that refers to the amount of light that is reflected by a planet, moon, or other celestial object.   The Moon's albedo changes as it orbits the Earth, but the average albedo for the Moon is .12, which means it reflects about 12% of the sunlight that strikes it.   moon phases Image credit:  NASA/Genna Duberstein
The surface of the Moon is mostly gray.   It consists of rocks and regolith, which is rocks that have been crushed and broken to powder over millions of years.   moon surface
The Moon looks white because the rocks on its surface reflect the light from the Sun. full moon    
When we see the Moon depends on where the Moon is in its orbit around the Earth.
  • The waxing crescent rises shortly after sunrise and follows the Sun across the sky.  Because it is so close to waxing crescent moonthe Sun, we can't see it until after the sun sets.  It is only visible for a short time before it dips below the horizon.
  • The 1st Quarter moon rises in the late morning or early afternoon. You can see this phase first quarter moonof the moon in the afternoon and evening.
  • The Waxing Gibbous moon rises in the late afternoon just before the sun sets and can waxing gibbous moon be seen until a few hours before sunrise.
  • The Full Moon rises when the Sun sets. It is visible all night long. It sets in the morning full moonwhen the Sun rises.
  • The Waning Gibbous moon rises later in the evening and can be seen until the earlywaning gibbous moon morning hours just following sunrise.
  • The Third Quarter moon rises after midnight and can be seen till just before noon.   third quarter moon
  • The Waning Crescent rises a few hours before sunrise and can be seen until a few waning crescent moonhours before the sun sets.
  • The New moon rises and sets with the Sun. For about two to three days we will not benew moon able to see the moon in the day or the night.
If you mean floating in the air, the answer is no.   Your body weighs less on the Moon because the Moon has less gravity -- but it would still weigh enough that you wouldn't float like a balloon.  So even though you can jump higher on the moon, you will still come back down.astronaut If you mean floating in water, the answer is still no because the Moon has no water to float in that we know of.  But IF there was a pool or lake on the moon, you could float and swim in it and those things would feel about the same as they do on Earth.  This is because how you float in water is not determined by gravity.  What counts in floating are the density of your body and the density of the water, which would not change on the moon.
Our Moon is not a planet. A planet is something that goes around the Sun or another star. A moon is something that goes around a planet. Our Moon circles around the planet Earth. moon orbits earth Our solar system has only eight planets but it has over 150 moons. For more information about the number of moons in our solar system click here.
The Moon, just like the Earth, receives light from the Sun.  And also like the Earth, only half of the Moon at a time is in the light.  The Moon rotates, so every part of the Moon has night and day -- but the Moon rotates much more slowly than the Earth, so its days and nights are longer.    On the Moon, one day lasts for fourteen-and-a-half Earth days, followed by a night that is also  fourteen-and-a-half Earth days long. earth moon day night
Just like the Earth, the temperature of the moon changes. During the day, the temperature can get as hot as 253 degrees Fahrenheit.  During the night it can be as cold as 380 degrees below zero. That means the Moon can be hotter and colder than anywhere here on Earth. moon rotation
Yes, there is light on the Moon. Some of the light comes from the Sun and sometimes the Moon gets light reflected from the Earth.  In the picture below, you can see both kinds of light.  On the left, the light from the Earth dimly lights up most of the Moon.  Along the the right edge of the Moon where it is very bright is the light from the Sun. earthshine
The moon doesn't have any life on it because it doesn't have what living things need.  Most living things need air, water, and a source of food.  The Moon has none of these things.  The Moon can also be too hot and too cold for many living things. barren moon
Saturn VRight now we do not have a way to get to the Moon.  Over 45 years ago men traveled to the Moon in space capsules powered by Saturn V rockets. These giant rockets were used only 11 times, on Apollo missions 8 through 17 and for the Skylab Orbital Workshop.  However, this form of technology is no longer being used. We're not sure when it will happen, but NASA has a plan to get us back to the Moon.   For more information about the future of Moon exploration, click HERE.
moon roverThere are no people on the Moon today. This is a picture of one of the last people who went to the Moon, on the Apollo 17 mission in December of 1972.   The vehicle he is driving is called a Lunar Roving Vehicle, or LRV.    
moon phasesThe moon does not grow and shrink. The Moon looks like it changes because as it orbits the Earth, we see different amounts of sunlight on it.
The Moon is not a star.  The Moon is more like a planet because it does not make its own light.   However, it cannot be a planet either because it orbits the Earth.  Planets, by definition, do not orbit other planets. The important thing to remember about a star is that it makes its own light. The Moon has no light of its own.
man in the moon Image Credit & Copyright: Dani Caxete

People began naming the features of the Moon around 1609, when Galileo first looked through his home-made telescope.  It is a very human action to look at something strange and try to see familiar things in it.  That has led many people to look at the Moon and imagine they can see a face in it.   In the photo above, the features that create that effect have been highlighted to make them show up more clearly.   These features are not really a face, of course ... they are geological formations that just happen to look like a face.    Here is a list of some of geology features on the Moon:

  • Mare = lava plain
  • Mons = mountains
  • Montes = mountain range
  • Palus = dark plain
  • Rima = crack, rille
  • Rupes = cliff
  • Sinus = bay
  • Vallis = valley
Craters are formed when meteors strike the Moon.  The meteor initially penetrates the surface faster than its energy can be released, boring a hole into the surface. The energy is released from underground with a tremendous explosive force, and the result is very similar to an underground nuclear explosion. The energy is primarily released from a point below the surface, so the crater formed tends to be circular, even if the meteor hits the surface at an angle. Only in a very few cases where the meteor hit the surface at a very low angle and bounced back off the surface are oval craters thought to have formed.  
The Moon has 1/6th the gravity of Earth.   Another way to say that is the Moon has 17% of Earth's gravity.   That means objects on the Moon weigh 1/6th of their weight on Earth.  So if you weighed 100 lbs on Earth, you would only weigh 16.67 lbs on the Moon.  Although humans have not yet visited Mars, we know that the gravity there is about 38% of Earth's gravity.  But the gravity you would experience traveling through space to visit either the Moon or Mars is so small, we call it zero gravity. gravity chart
Our moon has been called simply "the Moon" for centuries -- although it does have many other names in different cultures.   The French call it Lune, in Mexico it is called Luna, and in Germany it is Mond.  If you lived in ancient Greece, you may have called it Selene or Artemis.   Because there were people long before there were written records, we do not know what the earliest people called the moon.  Our English word, Moon, comes from similar words that meant "month" or "monthly" in far more ancient languages, probably because the lunar cycle is about one month long.   The first moons other than ours weren't discovered until the 1600's, when Galileo saw satellites around Jupiter.  Since people only knew about one natural satellite up to that point, these satellites around Jupiter were named "moons" because they orbited their planet in the same way that the Moon orbits our planet Earth.  Later, these natural satellites of other planets were given names, at least in part to help distinguish them from our Moon, but our own moon has continued to be called simply the "Moon". Earth's Moon
If you were on the Moon, you would see the Earth in the sky, similar to the way you see the Moon in the sky when you are on Earth ... but with some important differences.  The Earth is larger than the Moon, so it would look bigger, and it would look mostly blue. earth from moon Unlike the Moon, the Earth will not "move across the sky"; it pretty much "stays put" in one location. That is not to say, however, that the appearance of the Earth does not change. Read on!

Our Moon spins on its axis so that as it orbits the Earth, it always presents the same face to the Earth. As a result, when viewed from the Moon, the Earth will always remain in about the same spot in the sky all the time! (This may be easier to see if you set up two balls (using a light as the Sun) and make a model of the situation; place yourself on the Moon ball and you'll see what the Earth then looks like at any point in your orbit.)

I say that it is about the same because there are some differences. For example, there are slightly different apparent sizes of the Earth due to the fact that the orbit of the Moon is not a perfect circle; sometimes the Earth is closer (and appears larger) and sometimes it is farther away (and appears smaller) in each orbit. Also, because the orbit of the Moon is tilted about 5 degrees with respect to the Earth's equator, from the Moon there will be locations where the Earth will slowly rise and set during the lunar month as seen from the surface. The Moon undergoes a motion called "libration" which causes it to rock slightly back and forth relative to a line connecting the centers of the Earth and the Moon. This libration effect, as seen from the Moon, will cause the Earth to move slightly back and forth in the sky relative to a fixed point above the lunar horizon.

It is also important to note that the Earth will go through a complete set of phases each lunar month, with a "Full Earth" happening when it is "New Moon", and a "New Earth" happening during "Full Moon" (in other words, they will appear to be in exactly opposite phases).

In summary, while it generally remains in the same location, the Earth does not remain perfectly stationary in the lunar sky from every point on the Moon, but moves in a rather complicated way depending on your location on the lunar sphere!

(This information courtesy of's Starchild page.)

Since the Moon does not have a magnetic field (like the Earth does), a compass on the Moon would not work -- there are no magnetic north or south poles. compass
First, let's clear up a common misunderstanding.  There is no "dark side of the Moon" (except for the Pink Floyd album, but that's a whole different topic).  When people say "the dark side of the Moon," they are usually referring to the side of the Moon which we can't see from Earth.  It is more accurate to call this the "far side of the Moon," because it's not any darker than any other part of the Moon.

All surfaces of the Moon are exposed to the Sun at some point during a month.  During the sunlit days on the far side of the Moon you could see the surface of the Moon very well and, since the Moon has no atmosphere, you could also see the Sun and stars in a black sky. The trick to seeing the stars, though, would be to shield your eyes from all reflections and glare off the surface (and, of course, from the bright light of the Sun). Without blocking off all this extra light, the light from the stars would be overwhelmed by the glare and our eyes are not sufficiently sensitive to be able to see the much, much fainter light from the stars.

moon phases

However, when the Moon is full, its far side is turned completely away from the sun, so during a full moon, the far side of the Moon really would be dark.  On those days, the surface of the Moon would be extremely hard to see, but the stars would appear very much brighter than they do from here on Earth.

If you mean radio waves from the Earth, they would not make it to the far side of the Moon because the body of the Moon would block those waves.  However, the far side of the Moon is an excellent place for astronomers to study "radio astronomy" because all the other radio waves from all over the universe would make it to the far side of the Moon. And, they would have no interference from the radio waves produced on Earth.
This is a good question, but I'm not sure what you mean by it. Obviously, we need water to survive, but we also have to have a way to get that water inside us. If the container of water is sitting on the surface of the Moon and you are in a spacesuit and have no way of getting that water inside your spacesuit without opening up your spacesuit and exposing yourself to the vacuum of space, that container of water will do you no good. However, if the container of water is inside your spacecraft or lunar habitat or whatever (assuming your spacecraft/lunar habitat is pressurized and contains an Earth-like atmosphere), you can certainly drink that water.  
The Moon and the Sun combine to cause tides in the Earth's oceans. The Earth is wrapped in a layer of water, with continents and islands poking up through it.  If there were no other heavenly bodies exerting gravity on the Earth, we might expect that layer of water to be evenly distributed.  But the Moon, and to a smaller extent the Sun, both tug on that layer of water with their gravity.  When the Earth, Moon and Sun are in a straight line (at new moon and full moon times), the pull is strongest and we experience Spring Tides, when high tides are highest and low tides are lowest.  When the moon is at right angles to the line formed by the Earth and the Sun, the Moon's and Sun's gravities partially cancel each other out, and we have Neap Tides, which are more moderate. tides moon sun
Yes, you can see sunrises and sunsets from the Moon, but they would look very different than they do from the Earth.  For starters, one day on the Moon is almost 28 Earth days long, so it would take 14 days to get from sunrise to sunset, and another 14 days from sunset to the next sunrise.

The colors of sunrise and sunset would also be very different on the Moon.  On Earth, the sun's light is scattered by our atmosphere, which gives us the colors of sunrise and sunset, and causes the gradual lightening at sunrise and darkening at sunset.  The atmosphere is also what makes Earth's sky look blue during the day.  The Moon has no atmosphere, so there would be no colors.  The Sun would appear as a plain bright white dot in a black sky.  The surface of the Moon would suddenly be lighted at the moment the Sun becomes visible, and would plunge back into darkness at the moment it sinks out of sight -- there would be no gradual dawn or dusk.

When the Sun, Earth, and Moon are in a perfectly straight line, the Earth does block the Sun's light from reaching the Moon.  When that happens, it's called a lunar eclipse, and the Moon darkens to a deep reddish-orange color (sometimes called a Blood Moon).   Even in a full lunar eclipse, the Moon remains visible because some light still reaches it,  just like when you cast your shadow on the ground, the ground gets darker but doesn't completely disappear.   But the Earth, Moon, and Sun are usually NOT in a perfectly straight line.  The Moon's orbit around the Earth is tilted about 5 degrees from the Earth's orbit around the Sun, so the Moon is usually above or below the shadow that the Earth casts. orbital planes  
Apollo 11 was the first mission to land humans on the Moon and return them to Earth.  It launched on July 16, 1969, and went into orbit around the Moon 75 hours and 50 minutes (3 days, 3 hours, 50 minutes) later.   They traveled at speeds a little over 24,000 miles per hour. moon landing plaque Today's technology is basically the same as 1969's.   We still use chemical propulsion systems to power our rockets, just as they did then.  The only way to travel faster with that system is to burn a lot more fuel, and fuel is heavy.  The more fuel a rocket carries, the harder it is to launch.   NASA is exploring two other propulsion systems that might allow us to travel faster in space.  One is Nuclear Thermal Propulsion, which could lead to engines 100 times more powerful than the ones we have now.  But it would only be used in outer space, meaning we would still need to use chemical propulsion rockets to launch from the Earth.  The other is called VASIMR (Variable-Specific-Impulse Magnetoplasma Rocket).  It would use hydrogen as a fuel.  Because hydrogen is plentiful throughout the universe, it might be able to launch carrying only enough fuel to reach its destination, then collect more hydrogen there for the return trip.  However, both these systems will require years of study and development and it will be a long time before either one is ready to use.
There are no active volcanoes on the Moon now, but there is evidence of volcanic activity in the past.  The side of the Moon which faces toward Earth is covered with formations called "mare"  (pronounced MAH-ray), which is Latin for seas.  They have nothing to do with water, though.  The mare are smooth plains of basaltic rock.  Scientists believe they were formed by flowing lava which cooled and hardened.  However, unlike Earth, there are no volcanic cones built up in the middle of these lava flows.  There are several ideas about why this is true.  Some scientists think the lava on the moon came out of long cracks called fissures rather than from a central point.  Others suggest that the Moon's smaller gravity (1/6th that of the Earth) allowed the lava to spread out more evenly without building up cones.   The majority of volcanic activity on the Moon took place between 3 billion and 4 billion years ago.

This "false color image" taken by the Galileo orbiter uses added colors to show geological features of the Moon.  Blue to orange shades indicate volcanic lava flows.   The band of gray along the right side of the image shows you what the Moon actually looks like without any special color enhancements. false color moon image By NASA/JPL -

M/sˆ2 is a shorthand way of saying "meters per second squared" --  the unit used to express acceleration due to gravity.  If falling objects fell at a steady rate, we could express their speed in a simpler way, as "meters per second."   But objects don't behave that way when gravity acts on them.  Instead, the further they fall, the faster they go. On the Moon, a falling object travels 1.67 meters during the first second it falls. If it continues to fall, during the next second, it will travel 3.34 meters (1.67 x 2). If it falls for a third second, during that second it will travel 5.01 meters (1.67 x 3). (For purposes of comparison, the rate of acceleration on Earth is about 9.8 m/sˆ2, so objects on Earth fall roughly 5 times faster than objects on the Moon.) On Earth, there's a limit to how much a falling object can accelerate, because of the resistance of the air the object falls through. This maximum rate of acceleration is called terminal velocity. But on the Moon, there is no atmosphere to provide resistance, so a falling object will continue to accelerate until it reaches the surface.
We know of 12 aliens who visited the Moon between 1969 and 1972. They were human astronauts who were part of NASA's moon landing program. They are native to the Earth, so if we look from a Moon-centered point of view, they would be aliens! But there is no evidence that shows that any OTHER aliens have ever visited the Moon. moon landing
Scientists have recently been applying new techniques in seismology (the study of earthquakes and seismic waves that move through the Earth) to the data that was collected by the Apollo program in the late 1960's and early 1970's. This has allowed them to speculate in detail about what the interior of the moon might be like. They believe is has a solid core of mostly iron, surrounded by a liquid iron outer core. Next is the mantle, composed of rocks and minerals with the heavier ones generally toward the center and the lighter ones toward the Moon's surface. Finally comes the crust or outer layer, composed of rocks like basalt that are relatively light (for rocks).lunar core
First, this is a highly unlikely event, so don't stay awake at night worrying about it! But if it did happen, you can bet it would be pretty bad. About 50,000 years ago, a meteorite 160 feet across struck the Earth. It left a hole 3,900 feet across and 560 feet deep (Meteor Crater in Arizona). The Moon's diameter is 11,400,000 feet ... so you can imagine how much destruction it would cause.
Meteor Crater

Meteor Crater

There are two major differences that would happen if there were no Moon, and they have to do with light and tides. First, it would be much darker at night. Sunlight strikes the Moon and is reflected onto the Earth. With no Moon, we would not get that reflected light, so every night would be as dark as it is during the new moon phase. It wouldn’t make as big a difference to us in the modern world, because we have many artificial light sources that help us find our way at night. But in ancient times, or even today in places where streetlights, headlights, and other outdoor lights are not common, the night would be truly, deeply dark. On the positive side, it would make for great stargazing, because the Moon’s reflected light would never wash out our view of the stars. Second, if there were no moon, there would be no tides in our oceans. The Moon’s gravity is what causes tides (for details about this, see Question 41). So with no Moon, the oceans would be more like salty lakes … and surfers would be out of luck. Finally, if there were no Moon, we would lose a source of inspiration and wonder. As long as people have been on Earth, they have wondered about the Moon, made up stories about it, and written songs and poems inspired by it. Curiosity about the Moon was a driving force behind our space program. So our culture would be very different if we didn’t have a Moon to speculate about, dream about, and develop scientific advances so we could travel there.
Footprints stay on the Moon because there is nothing to disturb them. The Moon has no atmosphere, so there is no wind to bury them in dust. There is no rain or running water to wash them away. The Moon's surface has not had any volcanic activity for millions of years, and there is no tectonic activity (moving plates of ground, fault lines that can shift, etc), so there are no forces to swallow them up in an earthquake or bury them under other rocks. And there are no plants on the Moon to grow over them or erode them. The only things that could make a footprint on the Moon disappear are getting hit by one of the meteors and asteroids that regularly crash into the Moon, or being landed on by a spacecraft or disturbed by humans who may visit the Moon in the future. moon footprint
No. Scientists believe that when the Moon was created (see Question 12), it was between 10 and 20 times closer to the Earth than it is now. Currently the Moon's average distance from Earth is 238,855 miles ... and it moves about 1.5 inches further away each year. At that rate, we won't lose track of it for a long, long time. earth moon   The NASA/NOAA DSCOVR spacecraft captured this view of the moon crossing Earth's face in July 2016. Image by NASA/NOAA.
There are over 200 moons in our solar system. Earth has 1, Mars has 2, Jupiter has 79, Saturn has 82, Uranus has 27, and Neptune has 14. Even some of the dwarf planets have moons: Pluto has 5, Haumea has 2, and Eris and Makemake have 1 each. At least one asteroid, Ida, has a tiny moon of its own Mercury and Venus are the only planets in our solar system without moons. Larger moons (like Earth's Moon) are spherical because they have enough mass for gravity to pull them into a round shape. Many smaller moons are odd-shaped and lumpy. Moons come in a wide variety of sizes. The largest in our solar system are Jupiter’s moon Ganymede and Saturn’s moon Titan, each of which is larger than the planet Mercury. Jupiter’s moon Callisto has a diameter that’s only 58 kilometers less than Mercury. The next two biggest moons after those are Jupiter’s Io and Earth’s Moon. The smallest moon is Deimos, a moon of Mars that is only 7 miles in diameter. Some of the moons in our solar system show promise of liquid water. So far, science has identified the possibility of subsurface seas of water on Europa and Ganymede (Jupiter) and Enceladus and Mimas (Saturn). Earth’s Moon has so far shown only a few small pockets of surface water ice. At least one moon, Jupiter’s Io, has extensive volcanic activity. Earth’s Moon shows evidence of long past volcanic activity, but the most recent lava flows there probably occurred around 100 million years ago. Titan (Saturn) is the only moon, and in fact the only place in the solar system besides Earth, where liquid rains from the skies and collects into lakes, rivers, and seas on the planet’s surface. But the liquid on Titan, where the surface temperature averages -179 °C or -290 °F, isn’t water – it’s liquid methane. Titan is also the only moon in our solar system that has a thick atmosphere, although it’s composed mostly of nitrogen, so humans couldn’t breathe it. The many moons of our solar system differ in size, shape, and composition, but all of them are unable to support life as we know it. Their temperatures are too cold, their atmospheres are thin or non-existent, and most of them lack liquid water.
The half of the Moon that is turned toward the Sun is lighted. The half turned away from the Sun is in darkness. But the amount of the lighted side that we can see from Earth changes as the Moon orbits the Earth. That’s what causes us to see phases of the Moon (like new, crescent, quarter, and full). In the diagram below, imagine that the arrows between the Moons are connected to form a circle. From the Earth, we can see only what is inside the circle -- the part of the Moon that's outside the circle is turned away from us. when the Moon is directly between the Earth and the Sun (position 1), the side that is lighted is turned completely away from us, and all we see is a faint dark outline – the new Moon. As the Moon moves counterclockwise around the Earth, our view changes. At position 2, we can see a small slice of the lighted side – the crescent Moon. At position 3, we see half of the lighted side, which is one quarter of the whole Moon. At position 5, we see the entire lighted half – the full Moon. Then the process reverses and we see less and less of the Moon as it completes its orbit (positions 6, 7, & 8).
A lunar eclipse happens when the Moon passes through the Earth’s shadow. The Moon does not completely disappear – it just gets darker. This is just like when your shadow falls across something on the ground. That item doesn’t disappear, but it does get darker. Lunar eclipses can only happen during a full moon – the time when the Moon, Earth, and Sun are in a straight line, with the Earth in the middle, as shown in the diagram below. But why don’t we have a lunar eclipse every time there is a full moon? We would have that, IF the Moon’s orbit around the Earth was parallel to the Earth’s orbit around the sun. But it’s not. The Moon’s orbit is tilted by about 5°. This means that during most full moons, the Moon passes just above or just below the Earth’s shadow.  There are at least 2 lunar eclipses per year, and there can be as many as 5.
Scientists have found evidence of caves called lunar lava tubes. (This evidence is based on photographs taken from orbit; astronauts have not visited these sites in person yet.) The tubes were probably formed billions of years ago when the Moon had active volcanoes. They are passages that lava flowed through. In some places, the ground above such caves has collapsed down into the tubes, leaving a visible hole called a skylight. One of these skylights is called the Marius Hills Hole (MHH), first observed in 2009. It is estimated to be almost 100 meters deep and several hundred meters wide. Scientists believe that the lava tubes it connects to may be large enough to serve as a shelter for an underground lunar colony someday. A colony in an underground location would be sheltered from solar radiation and would experience less extreme temperature swings than one on the surface. The picture below shows the Marius Hills area. The inset in the lower left corner is a view of the Marius Hills Hole from above. Image Credit: NASALunar Orbiter 2Inset: Lunar Reconnaissance Orbiter
To understand how the Moon (and other celestial objects) spins, we have to go back billions of years, when our solar system was just a mass of gas and dust particles. Scientists believe that some outside force, such as the shock wave from a nearby supernova, hit that mass and caused it to shrink in on itself. As it shrank, it also began to spin faster and faster, similar to the way ice skaters spin faster when they pull their arms in close to them. As they were spinning, the gas and dust particles clumped together to form planets, moons, and other objects. Since space is mostly a vacuum, there is little friction to slow the spin down, and the objects that formed continued to spin. They do slow down slightly because the gravity exerted by one body on another (for example, by the Earth on the Moon and vice versa) creates a bit of friction, but it's a very slow process. The Earth's rotation slows by 2.3 milliseconds per century.

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