Mike Harlow
An eclipse of the Sun occurs when the Moon comes between the Sun and the Earth casting a shadow on the Earth's surface; something that can only happen at New Moon. Given that New Moon happens once a month, why can't we see an eclipse every month? Well, there are two main reasons. Firstly, by a remarkable, some think spooky coincidence, the apparent sizes of the Sun and Moon are very similar as seen from the Earth. The result of this is that the Moon's shadow is only a few tens to hundreds of kilometres wide which limits the region of the Earth that experiences an eclipse. Secondly, the Moon's orbit is slightly tilted with respect to the Earth's orbit. This means that the Sun, Moon and Earth rarely line up exactly at New Moon, so an eclipse doesn't happen every month.
Another complication is that the point where the Moon's orbit intersects the Earth's orbit moves (the point of intersection is termed a node). So if an eclipse does happen one month, by the time the Moon has made another orbit, one month later, the node has drifted and another eclipse isn't possible. Six months later however, the Moon has moved to the other intersection point on the other side of the Earth's orbit and an eclipse can happen. For example, just six months prior to the infamous 11 August 1999 eclipse invisible from Cornwall, there was a solar eclipse (not total) visible from Australia on 16 February 1999. This motion of the nodes of the Moon's orbit takes just over 18 years (18 years 11 days and 8 hours to be precise!) to make one complete rotation and gives rise to families of eclipses. Again using the example of the Cornwall eclipse, the next one in the series is on 21st August 2017 which conveniently tracks across the USA from Oregon to Virginia and lasts 2 minutes and 40 seconds. A family of eclipses like this lasts for about 3000 years but the fact that eclipses happen more frequently than every 18 years shows that many eclipse families are running in parallel at the same time.
As mentioned above, the Moon's orbit is tilted and moving with respect to the Earth's orbit making eclipses rare events. But there is a further twist to the story: the Moon's orbit is elliptical (as is the Earth's). This has a big influence on the appearance of an eclipse and its duration. When the Moon is close to the Earth and the Sun far away, the eclipse is long, the longest possible being over 7.5 minutes of totality. At the other extreme however, something interesting happens. When the Moon is far away and the Sun close, the Moon actually appears smaller than the Sun so the eclipse isn't total at all, it's 'annular'. As the name suggests, a ring of the bright photosphere remains when the Moon is centred on the Sun. Because of the relative sizes an annular eclipse can last longer than a total eclipse, the longest annular being about 12 minutes. The pattern of eclipses every six months is usually such that a total is followed by an annular. The Australian eclipse of February 1999, mentioned above, was annular.