The Moon is receding from Earth at a speed of 3.8 centimeters per year, which equals its distance of 38 billion centimeters per ten billion years. Ten billion years is roughly twice the age of the Solar system and comparable to the age of the Universe.
By coincidence, the mean angular diameter of the Moon, 0.515 degrees, is nearly identical to the mean angular diameter of the Sun, 0.533 degrees. From our vantage point on Earth, this allows us to witness a full eclipse of the Sun by the Moon. But as the Moon is receding, its angular size will shrink and become significantly smaller than that of the Sun within hundreds of millions of years. Altogether, there are two coincidences at play here: the near equality of the lunar and solar sizes in our sky and the fact that we live on Earth while this coincidence is happening.
But there is more to make us feel lucky. The mean density of the Sun is 1.41 grams per cubic centimeter, comparable to the mean density of the Moon – 3.34 grams per cubic centimeter. Again, this a pure coincidence because the Moon is a cold solid rock whereas the Sun is mostly made of a hot hydrogen gas.
Or, is it a coincidence?
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