A total solar eclipse occurs when the Moon moves between the Sun and the Earth, fully blocking our view of the Sun. This happens because of a heavenly coincidence: the Sun and the Moon, when viewed from the Earth, appear to be the same size. The Sun may seem at first to be the larger but this is because it is so bright. If you stick your thumb and forefinger out to gauge their apparent (or angular) sizes you will find they are the same.
In reality the Sun’s diameter is 400 times that of the Moon. But, with perfect compensation, it sits 400 times further away. This symmetry has not always existed but scientists have calculated that things will remain this way for another 650 million years, after which the Moon will drift too far from the Earth for it ever again to be able to cover the Sun completely.
During the year the Earth takes to complete its journey around the Sun, the Moon speeds around the Earth about 13 times. Once in each of these cycles (known as synodic months) the Moon passes between the Sun and us and we see a new Moon. We do not see a monthly solar eclipse, though, because the Moon’s orbit is at a slight angle to that of the Earth – so the Sun and Moon ‘miss’ each other as they pass across our skies.
To understand the orbits relevant to an eclipse it is easier to consider a model in which the solar system is inverted, with the Earth at its centre. This model, still in use for some purposes today, represents not the reality of planetary orbits but what we actually see when we look at the skies. In this model, the Sun’s apparent path across the sky is known as the ecliptic.
As viewed from the Earth the Moon circles us every 27.5 days, and the eclipse year of 346.62 days is the time for the Sun (as seen from the Earth) to complete one revolution with respect to the same lunar node. The Moon catches up with the Sun every 29.5 days and would pass between it and Earth, causing a solar eclipse, if it weren’t for those inclined orbits (see diagram above). But the two orbital paths do, of course, intersect at two points. These points – or nodes – represent two periods each year during which the Moon can indeed pass in front of the Sun. When the Moon passes exactly in front of the Sun we either see a total or annular eclipse; these are called ‘central’ eclipses because the Moon passes centrally across the Sun.
If the Sun, creeping along its yearly path, happens to be almost exactly at one of the two intersections when the Moon is racing round on its monthly cycle, there will be a central solar eclipse. If the Sun is a little further from the intersection there is still the chance that the Moon will at least overlap with it, creating a partial solar eclipse.
In fact, the Sun moves so slowly that it is impossible for it to get past either intersection without the Moon catching up with it at least once, and therefore guaranteeing a solar eclipse of some sort. Sometimes the Moon manages to travel all the way round and catch up with the Sun before it has fully left the intersection – causing two partial eclipses a month apart. Whatever happens during these eclipse 'seasons’, there will always be at least two solar eclipses of some sort each year. Those are the basics behind a solar eclipse. In practice, however, there are various distortions and complexities that give each eclipse its signature. For example, both the Sun and the Moon appear to follow elliptical paths around the Earth rather than circular ones, with the result that their distances from our planet vary. If a solar eclipse occurs when the Moon is at its furthest from the Earth and the Sun at its closest, a rim of Sun remains around the black disc of the Moon – and we see an annular eclipse.