Historical Studies of the Moon

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Every civilization has studied the Moon. Not only is the Moon the brightest object in the night sky, its location and appearance changes rapidly from night to night in a predictable way, making it easy and appealing to track. And although it appears to be quite far away, it seems to have a strong, mysterious effect on the Earth, in the form of daily tides. Ancient people watched the Moon each night and learned to predict its phases and movements, even if they didn’t understand the reasons for those motions.
The first person to correctly explain the phases of the Moon is lost in history. By the time Pythagoras wrote in 600 B.C., the ancient Greeks knew that the Moon is spherical and that it revolves around the Earth. The Greeks understood how that motion causes the monthly changes in the Moon’s appearance. In fact, they even had pretty good measurements of the Moon’s relative size and distance from the Earth.

Eclipses, though more intermittent than the monthly cycle, were also predictable by the ancients. As early as 1000 B.C., several cultures were predicting the occurrence of lunar and solar eclipses. The ancient Greeks, again, were among the first to understand what was happening during an eclipse, and they used that understanding to learn more about the natural world. In 270 B.C., Aristarchus used a clever method to measure the size of the Moon. If you assume the Earth’s shadow is about the same size as the Earth (not a bad assumption, since the source of light – the Sun – is so far away), and measure the time the Moon spends traveling through the Earth’s shadow during a lunar eclipse, you can calculate the relative sizes of the Earth and Moon. According to this method, Aristarchus found that the Moon’s diameter is about a third of the Earth’s diameter. This is remarkably close to the modern measurement of 0.272 Earth’s diameter. Once the size of the Moon and its angular size in the sky are known, the small angle formula can be used to measure the distance from the Earth to the Moon.

The Moon is the only celestial body with features that are visible to the human eye. What we call “the man in the moon” was a source of discord among ancient scientists. Some observers suggested that the markings were shadows of mountains and valleys. But this would have meant a heavenly object was “imperfect” by being non-spherical. In Aristotle’s view, the heavens were the source of everything perfect. So Aristotle and others of his school tried to find an explanation of the markings which still permitted the moon to be a “perfect” sphere. What if the Moon was so smooth that it was actually reflecting terrestrial features, like a giant mirror? But in that case, the features on the Moon would change as it revolves around the Earth, and they do not. The favored explanation was that variations in density caused the Moon to appear non-uniform, even though it was perfectly spherical. Eventually, telescopes provided a better picture of the Moon. The mountains, valleys, and craters revealed by telescopes finally made this argument obsolete.

Despite this early controversy about the features on the Moon, no detailed drawings of those features were made until the advent of the telescope. The first to observe the Moon with a telescope and record what he saw was Thomas Harriot, in 1609. He beat Galileo by a few months, but he did not publish his work, so Galileo’s drawings were more widely recognized. Many other mapmakers of the Moon followed, including Johannes Hevelius in 1647. He was the first to show the surface of the Moon as if all the features were lit from one side (which is impossible to see at one time, since the Moon is spherical). He also started the practice of showing every part of the Moon that can be seen from the Earth – which is slightly more than a single hemisphere. This is because small irregularities in the Moon’s orbit make different areas visible to the Earth at different times.

In 1651, a Jesuit astronomer named Giovanni Battista Riccioli published a system of names for features on the Moon, which we still use today. The smooth, dark areas that make up the man in the Moon, he incorrectly called “maria,” or seas. Thus we get the “Sea of Tranquility,” and others. Not until centuries later, when humans actually traveled to the Moon, did we learn the true origins of those “seas.”