Eclipses

Total Eclipse - Credit NASA Solar eclipse at totality. ©NASA.

By 2009 Royal Society Kohn Award winner Dr. Lucie Green, UCL Department of Space and Climate Physics

Why do eclipses occur?

Eclipses occur when the moon blocks out the sun’s light and a shadow is cast on the earth. The sun is 400 times bigger than the moon but it is also 400 times further away from us. This means that the sun and the moon have the same size in the sky. At least twice a year, the moon moves in between the earth and sun, covering the sun either partially or totally.

Why do we study them?

An eclipse is a beautiful sight and the only time when the sun’s outer atmosphere, the corona, can be seen and studied from earth. The corona - which is millions of Celsius hotter than the sun itself - is the source of a magnetized hot gas that streams out into the solar system affecting the earth and other planets. This solar wind causes the Northern Lights - the Aurora Borealis.

How do we predict them?

Eclipses can be predicted by using detailed knowledge of the moon’s orbit around the earth and the earth’s orbit around the sun, thanks to the work of German astronomer and mathematician Johannes Kepler (1571-1630). Incredibly, the Ancient Greeks developed a device (100-150BC), known as the Antikythera Mechanism, that could accurately predict the location of the Moon and Sun.

What is the most interesting thing about eclipses?

The distance to the moon can be measured by bouncing a laser off a special mirror left on the moon's surface by the Apollo astronauts. The moon is moving away from the earth at about 4cm per year, so its size in the sky is getting smaller. In 500 million years time the moon will be so small in the sky that we will have no more total solar eclipses.