The Moon, otherwise known as Luna, is the only natural satellite of Earth. It was created 4.6 billion years ago, and it is widely accepted that it was created when Earth collided with a planet-sized object called Theia. It’s the fifth-largest moon in our solar system and is the second brightest object in the sky (after the Sun).

The Moon Profile

orbit: 384,400 km from Earth

diameter: 3476 km

mass: 7.35e22 kg



History of The Moon

Called Luna by the Romans, Selene and Artemis by the Greeks, and many other names in other mythologies.

The Moon, of course, has been known since prehistoric times. It is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon’s phases. The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon’s orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.

Due to its size and composition, the Moon is sometimes classified as a terrestrial “planet” along with Mercury, Venus, Earth and Mars.

The Moon was first visited by the Soviet spacecraft Luna 2 in 1959. It is the only extraterrestrial body to have been visited by humans. The first landing was on July 20, 1969 (do you remember where you were?); the last was in December 1972. The Moon is also the only body from which samples have been returned to Earth. In the summer of 1994, the Moon was very extensively mapped by the little spacecraft Clementine and again in 1999 by Lunar Prospector.

The gravitational forces between the Earth and the Moon cause some interesting effects. The most obvious is the tides. The Moon’s gravitational attraction is stronger on the side of the Earth nearest to the Moon and weaker on the opposite side. Since the Earth, and particularly the oceans, is not perfectly rigid it is stretched out along the line toward the Moon. From our perspective on the Earth’s surface we see two small bulges, one in the direction of the Moon and one directly opposite. The effect is much stronger in the ocean water than in the solid crust so the water bulges are higher. And because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day giving two high tides per day. (This is a greatly simplified model; actual tides, especially near the coasts, are much more complicated.)

But the Earth is not completely fluid, either. The Earth’s rotation carries the Earth’s bulges slightly ahead of the point directly beneath the Moon. This means that the force between the Earth and the Moon is not exactly along the line between their centers producing a torque on the Earth and an accelerating force on the Moon. This causes a net transfer of rotational energy from the Earth to the Moon, slowing down the Earth’s rotation by about 1.5 milliseconds/century and raising the Moon into a higher orbit by about 3.8 centimetres per year. (The opposite effect happens to satellites with unusual orbits such as Phobos and Triton).

The asymmetric nature of this gravitational interaction is also responsible for the fact that the Moon rotates synchronously, i.e. it is locked in phase with its orbit so that the same side is always facing toward the Earth. Just as the Earth’s rotation is now being slowed by the Moon’s influence so in the distant past the Moon’s rotation was slowed by the action of the Earth, but in that case the effect was much stronger. When the Moon’s rotation rate was slowed to match its orbital period (such that the bulge always faced toward the Earth) there was no longer an off-center torque on the Moon and a stable situation was achieved. The same thing has happened to most of the other satellites in the solar system. Eventually, the Earth’s rotation will be slowed to match the Moon’s period, too, as is the case with Pluto and Charon.

Actually, the Moon appears to wobble a bit (due to its slightly non-circular orbit) so that a few degrees of the far side can be seen from time to time, but the majority of the far side (left) was completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959. (Note: there is no “dark side” of the Moon; all parts of the Moon get sunlight half the time (except for a few deep craters near the poles). Some uses of the term “dark side” in the past may have referred to the far side as “dark” in the sense of “unknown” (eg “darkest Africa”) but even that meaning is no longer valid today!)

The Moon has no atmosphere. But evidence from Clementine suggested that there may be water ice in some deep craters near the Moon’s south pole which are permanently shaded. This has now been reinforced by data from Lunar Prospector. There is apparently ice at the north pole as well.

The Moon’s crust averages 68 km thick and varies from essentially 0 under Mare Crisium to 107 km north of the crater Korolev on the lunar far side. Below the crust is a mantle and probably a small core (roughly 340 km radius and 2% of the Moon’s mass). Unlike the Earth, however, the Moon’s interior is no longer active. Curiously, the Moon’s center of mass is offset from its geometric center by about 2 km in the direction toward the Earth. Also, the crust is thinner on the near side.

There are two primary types of terrain on the Moon: the heavily cratered and very old highlands and the relatively smooth and younger maria. The maria (which comprise about 16% of the Moon’s surface) are huge impact craters that were later flooded by molten lava. Most of the surface is covered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts. For some unknown reason, the maria are concentrated on the near side.

Most of the craters on the near side are named for famous figures in the history of science such as Tycho, Copernicus, and Ptolemaeus. Features on the far side have more modern references such as Apollo, Gagarin and Korolev (with a distinctly Russian bias since the first images were obtained by Luna 3). In addition to the familiar features on the near side, the Moon also has the huge craters South Pole-Aitken on the far side which is 2250 km in diameter and 12 km deep making it the the largest impact basin in the solar system and Orientale on the western limb (as seen from Earth; in the center of the image at left) which is a splendid example of a multi-ring crater.

A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs. These provide most of our detailed knowledge of the Moon. They are particularly valuable in that they can be dated. Even today, more than 30 years after the last Moon landing, scientists still study these precious samples.

Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old. This is a fortuitous match with the oldest terrestrial rocks which are rarely more than 3 billion years old. Thus the Moon provides evidence about the early history of the Solar System not available on the Earth.

Prior to the study of the Apollo samples, there was no consensus about the origin of the Moon. There were three principal theories: co-accretion which asserted that the Moon and the Earth formed at the same time from the Solar Nebula; fission which asserted that the Moon split off of the Earth; and capture which held that the Moon formed elsewhere and was subsequently captured by the Earth. None of these work very well. But the new and detailed information from the Moon rocks led to the impact theory: that the Earth collided with a very large object (as big as Mars or more) and that the Moon formed from the ejected material. There are still details to be worked out, but the impact theory is now widely accepted.

The Moon has no global magnetic field. But some of its surface rocks exhibit remanent magnetism indicating that there may have been a global magnetic field early in the Moon’s history.

With no atmosphere and no magnetic field, the Moon’s surface is exposed directly to the solar wind. Over its 4 billion year lifetime many ions from the solar wind have become embedded in the Moon’s regolith. Thus samples of regolith returned by the Apollo missions proved valuable in studies of the solar wind.

Interesting Facts about Earth’s Moon

The Moon is estimated to be 4.5 billion years ago. We were not around to see its creation, but it was most likely made when a large object collided with Earth. This would have blasted out rocks that began to orbit around the Earth. These were then drawn together and over time melted into one another. Once they cooled down, they became the Moon.

We were not around to see its creation, but it was most likely made when a large object collided with Earth. This would have blasted out rocks that began to orbit around the Earth. These were then drawn together and over time melted into one another. Once they cooled down, they became the Moon. The Moon would have been hit by other rocks for 500 million years. These are rocks from the original collision, and this is why the Moons surface is covered in craters, over 500,000 of them!

These are rocks from the original collision, and this is why the Moons surface is covered in craters, over 500,000 of them! The Moon is very visible from Earth. With a telescope or even a very good pair of binoculars, you can see the moons surface and its craters.

With a telescope or even a very good pair of binoculars, you can see the moons surface and its craters. The Moon is tidally locked to the Earth. This means that the same side of the moon is facing the Earth at all times. Images have been captured from space that shows the opposite side to look very similar to what we can see.

This means that the same side of the moon is facing the Earth at all times. Images have been captured from space that shows the opposite side to look very similar to what we can see. There is no atmosphere on the Moon. This is why the craters are still there, and there is no rain and wind or weather activity to erode the surface as we see on Earth.

This is why the craters are still there, and there is no rain and wind or weather activity to erode the surface as we see on Earth. The Moon has seas! Not real oceans like we have on Earth but large smooth pits of lava. These appear on the moon as areas darker than the rest of the surface. Each sea has a Latin name.

These appear on the moon as areas darker than the rest of the surface. Each sea has a Latin name. The Moon goes through phases. These happen when the moon is lit differently by the Sun. The Moon appears to Earth as a small crescent, as it rotates around the Earth this expands to a full moon and back to a small crescent. This happens once every 29 days and is called a lunar month.

These happen when the moon is lit differently by the Sun. The Moon appears to Earth as a small crescent, as it rotates around the Earth this expands to a full moon and back to a small crescent. This happens once every 29 days and is called a lunar month. The Moon causes the rise and fall of the ocean’s tides on Earth. This is because the Moon has a gravitational force which causes the oceans to bulge outwards on both sides of the planet. This is the Moon pulling the water towards itself. Due to the different rotations of the Earth and the Moon, the area of the planet affected changes throughout the day. This means any given area of Earth will experience a high tide (caused by this bulge) every 12 hours and 25 minutes.

This is because the Moon has a gravitational force which causes the oceans to bulge outwards on both sides of the planet. This is the Moon pulling the water towards itself. Due to the different rotations of the Earth and the Moon, the area of the planet affected changes throughout the day. This means any given area of Earth will experience a high tide (caused by this bulge) every 12 hours and 25 minutes. The Moon is slowly moving away from Earth. Don’t worry! It is only going 3.8cm away each year, and we won’t notice a difference for some time!

Don’t worry! It is only going 3.8cm away each year, and we won’t notice a difference for some time! You weigh less on the Moon than you do on Earth. This is because the moon has a much weaker level of gravity than Earth. This is due to the satellites smaller mass. You would weigh about 16.5% of what you do on Earth.

This is because the moon has a much weaker level of gravity than Earth. This is due to the satellites smaller mass. You would weigh about 16.5% of what you do on Earth. People have walked on the Moon! But only 12 of them. All of which were males. These were all part of the Apollo missions between 1968 and 1972. Nobody has been there since but there are future plans to revisit the Moon.

All of which were males. These were all part of the Apollo missions between 1968 and 1972. Nobody has been there since but there are future plans to revisit the Moon. The Moon is the fifth largest natural satellite. It is only beaten in size by the very large Moons of Jupiter and Saturn.

More about the Moon

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