The Moon is a familiar sight to anyone who has ever looked up at the sky at night (or even during the day). It has connotations of romance, mystery, and magic. What is often overlooked by many who take the sight of it for granted is that it is also an astronomical object, a planetary body like many others in the Solar System.
Lunar Numbers
The Moon has a radius of just under 1,100 miles, which puts it at roughly a quarter the size of Earth, its planetary sibling. Its day, the time it takes for the Moon to rotate once on its axis, is 29.5 Earth days long. This also is the time it takes for the Moon to appear to go through an entire cycle of lunar phases as it orbits the Earth.
The Moon is much less dense than Earth: despite being only a quarter of Earth's size, Earth is over 80 times more massive than the Moon. This is because the Moon is made out of lighter materials as a whole than Earth, which has a very dense core. Since the Moon has so much less mass than the Earth, it also has a great deal less gravitational force. Anyone who has ever seen video of astronauts on the Moon can see that they do not walk so much as bounce. This is because on the Moon they weighed only one-sixth as much as they weighed on Earth.
Earth and Moon: Lunar Formation and Geology
No one knows exactly how the Moon came to exist. One popular theory is that it was literally born from its sibling, the Earth. The thinking is that very early in Earth's history, when it was still fairly molten, a body roughly the size of Mars struck it and sent a lot of material from Earth's upper layers flying into the space around the young planet. This material eventually coalesced into a single body, which became Earth's Moon.
The newly formed Moon was very volcanically active and prone to lava flows. These massive flows are still visible on the Moon's surface today, as the dark patches one sees on the Moon. These surfaces are each called a "mare," Latin for sea. This is proof of how little the Moon's surface changes with time, since it has not been volcanic for at least a billion years.
Because the Moon's surface does not change very much with time, there is no geologic activity to erase the craters on its surface. As such the Moon is covered with craters of all shapes and sizes, up to the South Pole-Aitken Basin. At around 1,500 miles in diameter, the Basin is actually the largest known impact structure in the entire Solar System.
Mysteries of the Moon
The Moon holds the distinction of being the only planetary body other than Earth to host humans, with the Apollo missions of the 1960s and early 1970s. It has been such an object of fascination for mankind throughout the centuries that one might suppose it has been explored to satisfaction. This, of course, is not the case, and mankind continues to send robotic probes to explore its closest companion. The most recent of these was the LCROSS Mission, which purposely struck the Moon in early October 2009 and conclusively proved the existence of water ice on the Moon. If the existence of such a vital material has remained hidden on the Moon this long, there is no telling what other secrets Earth's younger sibling still contains.
Sources:
Spudis, Paul D. 2004. "Moon." World Book Online Reference Center. World Book, Inc. Accessed April 8, 2010.
Taylor, G. Jeffrey. July 17, 1998. "The Biggest Hole in the Solar System." Planetary Science Research Discoveries. PSRD. Hawaii.edu/July98/spa. Accessed April 8, 2010.
Talia Sepersky - I am a 2008 graduate of Wellesley College, where I earned my Bachelor's in astrophysics. A year later I earned my Master's in science ...
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