What do you call it when the Moon blocks the Sun?

A solar eclipse, and it happens at all because of an extraordinary coincidence: the Sun is about 400 times wider than the Moon and about 400 times further away, so the two appear almost exactly the same size in our sky. No other planet in the Solar System gets a total eclipse this precisely fitted.

It is also temporary. The Moon is receding about 3.8 centimetres a year, and in something like 600 million years it will be too far away to cover the Sun at all - total eclipses will end, leaving only annular ones with a ring of light around the edge. We happen to be alive during the window in which the fit is close.

Totality is worth distinguishing from a partial eclipse, which is a different experience rather than a lesser one. Only when the last sliver goes does the corona become visible, the temperature drop sharply, and animals begin behaving as though night has fallen. That corona is how eclipses earned their scientific keep: in 1919 Arthur Eddington photographed stars near the eclipsed Sun and found their apparent positions shifted, confirming that mass bends light and making Einstein famous overnight.