If the moon is moving away from the earth, will we stop getting eclipses?

I already knew that the moon is gradually moving away from the earth, and that eventually it will be far enough away that a total eclipse of the sun will no longer be possible. Only as North America experiences mass hysteria over the total eclipse later today did it occur to me to question just when that might happen.

Total eclipses occur when the moon moves directly between the earth and the sun so as to block out the sun completely. By sheer coincidence - a quirk of the age we happen to live in - the moon is currently just the right distance away from the Earth that its shadow blocks out the entire sun almost exactly, which is why, within the "path of totality" of an eclipse event, we can only see the corona surrounding the sun, and not the sun itself.

However, that was not always the case. Back in prehistory, the moon was much closer to the earth and, by the same token, in the future it will be further away. In fact, the moon is moving away from the earth at a rate of about 3.8cm (1½ inches) a year. The Moon exerts a tidal or gravitational force on the earth (you could think of it as a kind of friction), which has the effect of gradually slowing the earth's spin (a day on earth used to be much shorter than the 24 hours we have today). Due to the law of conservation of angular momentum, this slowed spin also has the effect of making the moon spiral further away from the earth.

So, at some point in the future, the moon will be far enough away that it's shadow won't block the whole of the sun, and eclipses will be "annular" rather than full, with the moon's shadow blocking just the centre of the sun, as we see it, leaving a ring of sun around a black shadow.  In fact, we already get annular eclipses now: because the moon's orbit around the earth is elliptical not circular, sometimes it is further away than other times. About half of the eclipses we see are currently annular, because only about half of the time is the moon close enough to the earth to block out the sun completely. Today is one of those times.

But as the moon moves away from the earth, there will come a time when, even at its closest, the moon will not appear big enough to completely block the sun. There will be still be eclipses when the three bodies line up, but they will all be annular. No more full eclipses. But, don't worry, best calculations suggest that it will be about 650 million years before that point is reached. In the meantime, enjoy!