Eclipsa

Meet Eclipsa, the Moon sliding exactly in front of the Sun.

How big something looks depends on its size divided by its distance. The Sun is about 401 times wider than the Moon, and about 389 times farther away (computed from mean values). The two ratios are so close that the Sun and Moon look almost the same size in our sky, each about half a degree across. So, when they line up, the Moon can just cover the Sun's bright disc.

Then, for a few minutes, the sky darkens and the Sun's faint outer atmosphere, the corona, becomes visible. The coincidence is temporary on astronomical timescales, because the Moon is slowly moving away from Earth.

Eclipses have been useful science. The 1919 eclipse tested general relativity, and helium was first identified in the Sun's spectrum during an 1868 eclipse.

Both distances vary along the orbits. When the Moon is farther than average, it looks slightly smaller than the Sun and leaves a ring of sunlight: an annular eclipse. And because the Moon's orbit is tilted about 5°, eclipses do not happen every month. Never look at the uneclipsed Sun without proper eye protection.