Solar eclipse

So much depends on the Sun. Could anything ever block its light from us?

Mercury and Venus, the two planets between the Earth and the Sun, look tiny next to the Sun and never block all of it. But the Moon can. The Moon is the Earth's natural satellite: it goes round the Earth while the Earth goes round the Sun.

Sometimes the Moon comes between the Sun and the Earth and blocks the Sun's light from reaching us. This is a solar eclipse (in Hindi, सूर्य ग्रहण).

But the Moon is far smaller than the Sun. How can it hide it? Activity 12.4: ask a friend to stand about 5 metres away; their head is the Sun. Close one eye and hold up your thumb. You can cover their whole head with it, though your thumb is much smaller.

How big something looks — its apparent size — depends on two things: its real size and how far away it is. Your thumb is tiny but very close, so it looks as big as your friend's head.

The Moon is much smaller than the Sun, but also much closer to us. The two happen to look almost the same size in our sky, so the Moon can cover the whole Sun.

Mercury and Venus are much bigger than the Moon, but much farther away, so they look far smaller than the Sun. When Venus passes between the Sun and the Earth, it shows only as a tiny black dot crossing the Sun's bright face — a rare event called a Transit of Venus.

During a solar eclipse, the Moon's shadow falls on a small area of the Earth. People inside it see the Sun completely covered: a total solar eclipse, and it goes dark in the daytime. People where only part of the Sun is covered see a partial solar eclipse.

Because the Earth spins and the Moon keeps moving in its orbit, the Moon's shadow slides across the Earth, so total darkness in any one place lasts only a few minutes. Then the eclipse becomes partial again, and daylight returns.