THIS SUNDAY, June 21st, at 6am local time, people in central Africa will be treated to one of Earth’s greatest astronomical spectacles: a solar eclipse. The rising sun will dim as a black disc creeps across it, blotting out more and more of its light.

The temperature will fall; the sky will darken. A minute and a half later, it will pass, and the sun will return to its original state. This spectacle will be visible at various times throughout the day across central Africa, Arabia, northern India, southern China and Taiwan (see map), making it visible near big population centres home to more than 1bn people. The next eclipse near so many people (also over India and China) will not occur until 2064.

The temperature will fall; the sky will darken. A minute and a half later, it will pass, and the sun will return to its original state. This spectacle will be visible at various times throughout the day across central Africa, Arabia, northern India, southern China and Taiwan (see map), making it visible near big population centres home to more than 1bn people. The next eclipse near so many people (also over India and China) will not occur until 2064.

A solar eclipse occurs when the Moon moves between the sun and the Earth, casting a shadow over the Earth’s surface. Although the sun is 400 times larger than the Moon, it is also nearly 400 times as far away from the Earth, which means the two spheres appear about the same size in the sky.

In a total eclipse, the corona, the cloud of hot gas that forms the sun’s outer atmosphere, can still be seen as a bright halo around the moon. In an annular eclipse, like the one that will take place this weekend, the Moon is too far away from Earth to completely cover the sun, leaving a “ring of fire” around it. Sunday’s eclipse will be particularly spectacular. At the point of greatest eclipse, just a tiny sliver (under 0.2%) of the sun’s disc will be visible from Earth.