The Moon is the Earth’s nearest celestial neighbor, and the only astronomical object visited by human beings. Because it is easy to see in the sky, the Moon has been known since the dawn of humanity. But for centuries many misconceptions about it persisted in people’s minds. For example, although they were able to calculate its diameter and distance from the Earth, the ancient Greeks believed the Moon was a perfectly smooth sphere. Even today, many people believe the full Moon can affect people’s behavior, a discredited idea that lives on in such words as lunatic and lunacy.
The Moon orbits the Earth at an average distance of 384,400 kilometers. The orbit is almost a perfect circle. At its closest approach to the Earth, the Moon is 363,000 km away. At its greatest distance, it is 406,000 km away. The Moon’s orbital period, the time needed to complete one full orbit, is 27.3 days. The Moon’s rotational period, the time needed to complete one full rotation on its axis, is also 27.3 days. Therefore, from the Earth we always see only one side of the Moon’s surface. Because of its slow rotation period, daytime on the Moon lasts about 3,200 hours (about 14 Earth days). When the Sun goes below the Moon’s horizon, the lunar night lasts just as long.
When we compare the Moon to other moons in the solar system, we find that is very large compared to its parent planet (the Earth). In fact, if humans lived on Mars, we would probably have classified the Earth and Moon as a double planet. If the Moon were directly orbiting the Sun instead of orbiting the Earth, it would be considered a planet in its own right.
The Earth is 83 times more massive than the Moon, and the Earth’s radius (the distance from the center of the Earth to a point on the surface) is 3.7 times greater than the Moon’s radius. Because of these differences in mass and radius, the strength of gravity is much weaker on the surface of the Moon than on the surface of the Earth. A person who weighs 100 pounds on the Earth weighs only 17 pounds on the Moon. In other words, gravity on the Moon is one-sixth as strong as it is on the Earth. Because the Moon’s gravity is so much weaker than the Earth’s, the Moon has no atmosphere like the Earth.
You can make a scaled model of the Earth-Moon system using a basketball to represent the Earth and a baseball to represent the Moon. The ratio of the diameters is nearly the same the as ratio for the real Earth and Moon. To get the distance correct, you must place the baseball about 30 feet away from the basketball (about 30 times the basketball’s diameter). When you set up your model, it may seem the Moon is too far away, but remember that the Moon is a huge object compared to human beings. So when it appears in the sky, it seems to be closer to the Earth than it really is.
Since the Moon has no atmosphere, conditions on its surface are very different than those on the Earth’s surface. In particular, the temperature on the Moon’s surface varies through a huge range, from -240 degrees F during the lunar night to +240 degrees F, hot enough to boil water, during the lunar day.
The Moon is only object in space whose surface features can be easily seen with the unaided eye. With a good pair of binoculars or a good telescope, it’s possible to see craters and mountains on the lunar surface. In fact, the first person to observe the lunar surface with a telescope, Italian scientist Galileo Galilei, surprised the scientific community in 1609 when he reported that the Moon’s surface was not perfectly smooth—this was the common belief at the time.
There are three types of features on the Moon’s surface. First, there are thousands of craters formed from the impact of meteors against the lunar surface. The smallest craters are only a few millimeters in diameter, while the largest are nearly 200 kilometers from edge to edge. There is no geological activity on the Moon’s surface, no earthquakes (or “moonquakes”) and no volcanoes. There is also no weather—no wind or rain—since there is no atmosphere. Therefore, when a crater is formed on the lunar surface, it lasts essentially forever, unless it’s obliterated by a later meteor impact. Craters formed on the Earth are wiped away by the combined effects of geological activity and weather. Only the most recent craters are still visible on the Earth’s surface.
Another type of important feature on the lunar surface is the maria, the Latin word for “seas.” These are large darkly-covered regions that to many people appear to be large seas or oceans. But there is no liquid water anywhere on the Moon. (There may be small amounts of ice at the Moon’s north and south poles.) One of the maria, the Sea of Tranquility, is where Neil Armstrong and Buzz Aldrin first set foot on the Moon. The Moon’s maria can be seen without a telescope. It turns out there are no maria on the Moon’s far side, the side that always faces away from the Earth.
In addition to craters and maria, the lunar surface includes bright regions that resemble dry-land continents on the Earth. These regions have much higher elevation than the maria (several kilometers higher), so they are called highlands. Most of the Moon’s craters are in the highland regions. The highlands are much brighter than the maria because they are composed of different kinds of rocks.
Moon rocks from the maria brought to the Earth by the Apollo astronauts are much younger than rocks from the Moon’s highland regions. (The age of a rock is the length of time since it was last in a molten state.) The age difference is as much as one billion years. Because of this fact, and because there are many fewer craters in the maria compared to the highlands, astronomers believe the maria are relativly new features, probably formed by lava from beneath the Moon’s surface that smoothed away huge sections of its surface. The lava was probably released by particularly violent meteor impacts that fractured the Moon’s surface and allowed the lava to pour out.
Where did the Moon come from? Before scientists examined the rocks brought back by Apollo astronauts, there were three ideas: (1) the Moon formed elsewhere in the solar system and wandered close enough to the Earth to be captured into orbit by the Earth’s gravity, (2) the Earth and Moon formed simultaneously sort of like planetary twins, (3) in the remote past the Earth was spinning so fast some of its material was flung into space and formed the Moon. But the lunar rocks brought back by the Apollo astronauts did not support any of these ideas.
Today, astronomers believe the Moon may have formed about 4.5 billion years ago when a planet approximately the size of Mars slammed into a very young Earth. The impact was so violent that a huge amount of debris was flung into space, where it formed the Moon. Detailed computer simulations have demonstrated that such a scenerio is possible, and the condition of the lunar rocks brought to Earth is consistent with this hypothesis.