~600 Million

Last Total Solar Eclipse

One of the simplest experiments on the surface of the Moon by the Apollo astronauts was to deploy a set of mirrors. The mirrors were pointed back at the Earth, so that astronomers could shine high-powered lasers at the Moon and measure precisely how long it took the laser light to reflect back to the Earth. Knowing the travel time and the speed of light, the distance to the Moon can be measured to better than 0.04 inches (1 millimeter). Over time, these kinds of measurements have revealed that the Moon is slowly spiraling away from the Earth, at a rate of about 1.5 inches (3.8 centimeters) per year.

Why is the Moon receding from the Earth? The answer is the conservation of angular momentum. The Moon’s gravity raises tidal “bulges” in the Earth’s oceans that produce a drag force on the Earth’s crust that is (gradually) slowing down the spin of our planet. Earth’s loss of angular momentum is the Moon’s gain. As the Moon speeds up, it moves farther away, according to Kepler’s Laws of Planetary Motion. The fact that the Moon is moving away today means it was much closer very long ago when it formed. Indeed, the Moon may once have appeared as much as fifteen times bigger than it does today in our night sky!

One implication of the Moon’s spiraling-out motion is that it is slowly (very slowly) getting smaller in the sky as seen from the Earth. Today, the apparent angular size of the Moon in the sky varies (depending on where the Moon is in its slightly elliptical orbit) from just a little less than the apparent angular size of the Sun to just a little more—when total solar eclipses are possible. As the Moon gets smaller in the sky over time, however, eventually it won’t be able to completely cover the Sun’s disk. Some day, about 600 million years from now, our far-distant descendants (whoever or whatever they are), will gather to watch the last total solar eclipse visible from the Earth’s surface. From then on, all similar events will be annular solar eclipses, displaying only a glorious annulus or ring of sunlight around the dark face of the Moon.

In the far future, the Moon will be too far away from the Earth to completely block the disk of the Sun, and all solar eclipses will be “annular” eclipses like this one, from May 20, 2012.