As the Earth revolves around the Sun, and the Moon revolves around the Earth, it happens sometimes that all three fall into alignment. When this happens at New Moon, the Moon covers up the Sun in our skies, and we have a Solar Eclipse. It is an interesting coincidence that both the Moon and the Sun appear to be the same size in the sky, since both their actual sizes and their distances from earth are radically different, but this is what makes a Solar Eclipse such a spectacular sight. The Moon glides slowly through the daylit sky, invisible, until it touches the Sun. A small notch appears in the Sun’s edge, which grows as the Moon progresses. The day gets darker, birds fall asleep and the world goes quiet, until with startling suddenness, the Sun is completely blotted out. We look up to where the Sun used to be and see only a black disk, with a rim of ghostly fire (The corona, the Sun’s outer atmosphere which extends millions of kilometers into space), and a few bright stars. Then, just as quickly, the process reverses as the Moon moves on and once more reveals the Sun. There are one or two Solar eclipses most years, and savvy travel agents organise package deals for people to travel from all over the world to see them.
But not all solar eclipses are the same. No orbits are perfectly circular, so the distance from Earth to Sun, and from Earth to Moon varies from moment to moment. This obviously means that the size of both Sun and Moon vary in the sky, although the difference is small enough to not normally be noticeable to the naked eye.
But things are different at eclipse time. When the Moon appears large and the Sun small, totality (The time when the Sun is completely blotted out) can last for many minutes, but when the Moon is at it’s furthest and the Sun at it’s closes, then the Sun is never completely covered at all. This is called an Annular Eclipse (from the Latin word for Ring), and looks like a bright thin ring of sunlight, as shown on the right. While the world still darkens, the thing ring of sun which remains visible is still bright enough to hide the stars and damage your eyes if you should look up without protection.
Solar eclipses are very predictable. Even without knowledge of the structure of the Solar System, the ancients noticed a definite pattern in the dates and locations of eclipses, and were able to predict them accurately and far in advance. A full description wouldn’t fit on this page, but from that pattern we find that there are between two and five eclipses every year. The area from where the total eclipse is visible is quite small, forming a track no more than a few hundred kilometers wide, though, so any one place on earth has a total eclipse a little less than once every four hundred years.
One final word, though. We take the sun for granted, forgetting just how much energy and radiation we receive from it daily. While our skins are built to cope with a certain amount of solar radiation, our eyes are not. Staring at the sun is a reliable way to damage your eyes, and even partially blind yourself. Remember that the retina has no pain sensors, and damage can take several hours to become obvious, so you can quite easily hurt yourself without even realising it. Please don’t look at the sun without proper filtering (eclipse viewers are available everywhere near an eclipse for a very small price) under any circumstances. For more information on how to view the Sun safely, click my article on Looking at the Sun