The Moon and Earth have been dancing around each other since the Moon was first formed 4.5 billion years ago, likely after a collision between the planet Theia and Earth.
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However, the orbits of these two bodies have changed over time. The Moon is constantly moving away from us, suggesting that one day in a few centuries from now, humanity will look up and see the last total eclipse .
We have a surprisingly accurate idea of how fast the moon is currently moving away from us, thanks to the Lunar Laser Ranging Experiment. The Apollo in the 1960s and 1970s placed reflectors on the surface of the moon, making a significant achievement. By using a laser aimed at a reflector and measuring the time it took for it to return to Earth, scientists were able to estimate the distance between the two points to be about 3 centimeters. Based on our repeated measurements, we observe that the moon is moving away from us by about 3.8 centimeters per year.
This wasn't always the case. If we look back at the current recession rate and plot it backwards, we see that the Moon collided with the Earth about 1.5 billion years ago. This is strange, given that the Moon is 3 billion years older than that.
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Instead, we can observe other evidence, such as layers left behind in rocks and corals, which can give us estimates of the length of Earth's days and the distance to the Moon at various points in history. We can also predict what will happen in the future, such as whether we will never see a total solar eclipse again.
“Over time, the number and frequency of total solar eclipses will decrease,” said lunar scientist at NASA’s Goddard Space Flight Center Richard Vondrak in 2017. “About 600 million years from now, Earth will experience the beauty and drama of a total solar eclipse for the last time.”
The fact that the Moon is currently eclipsing the entire Sun is a happy coincidence. The Sun and Moon appear to be about the same size in the sky. This is because the Sun is about 400 times farther from Earth than the Moon and has a diameter about 400 times larger. Four billion years ago, before the Moon drifted into its current orbit, it had a circumference about three times larger than it does today.
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As the Moon continues to drift, it appears smaller and smaller in sky , but it will never leave orbit around us. The Sun will grow into a red giant and swallow the Earth before we leave the Moon's influence. We will travel together.
