The reason why we can observe total solar eclipses is that the moon and the sun happen to be almost exactly the same size in the sky as seen from earth. The sun is, of course, much larger, with a diameter around 400 times that of the moon – however, the moon is also much closer  – around 400 times closer to the earth than the sun is.

The moon, however, is also slowly moving away from the earth, at the very slow rate of around 4cm every year. This means that, eventually, this delicate balance in ratios between size and distance for the moon and sun will change. In around 600 million years, the earth will experience its final total solar eclipse, as the distance between the earth and the moon would increase enough to make the apparent size of the moon too small to completely cover the sun as seen from earth.

We do experience something similar to this already. The moon’s orbital distance from the earth changes, roughly between 350,000km and 400,000km away. When the distance is over around 373,000km during a solar eclipse, the moon’s apparent size in our sky is not large enough to cover the entire sun, and an annular solar eclipse occurs instead.