Ok, so you’ve got the sun, the moon and the earth…
Very difficult to see these in the correct proportions, because the earth and moon are so tiny compared to the sun, which is so very far away. Note how far the earth and moon are from each other as well.
Anyway, an eclipse happens when the moon gets in the way of the sun’s light, as seen from the earth. What a very long way to throw a shadow!
In the diagram above, if you were standing on the earth in the place where the red lines touch it and you were looking towards the sun, you wouldn’t be able to see it because the moon would be in the way:
The disc of the moon as seen from earth strangely happens to be exactly the right size to exactly block out the disc of the sun. The sun is very much bigger but because it’s so far away it seems smaller. (Stars of course are suns as well, but they are ever so far away, so they look really tiny.)
If you weren’t standing on the earth where those red lines converge, you wouldn’t see the eclipse- the shadow of the moon is only cast on a small area, and this area moves around the surface of the earth in a very long-term pattern, which can be described thus:
Each eclipse belongs to a family of eclipses.
Each family will have an eclipse somewhere on the earth every 18.03 years.
The place where this eclipse occurs on the surface of the earth will move down in latitude a bit and round the earth by 120 degrees longitude (that’s a third of the way round the whole 360 degrees).
Each family starts with a partial eclipse at one of the poles and spirals round the earth having an eclipse in another place every 18.03 years until it reaches the equator, where the eclipses will be total, and then the pattern carries on round the other side of the earth, spiralling round until it reaches the other pole.
The eclipses also occur in a particular pattern around the zodiac. Every eclipse pattern starts in a particular place in the zodiac. Each new eclipse (18.03 years later, remember) occurs 10 degrees further round the zodiac. So halfway through the pattern’s cycle – that is, after 650 years – the eclipses have gone once round the zodiac.
They go twice round the zodiac during the whole cycle.
If you put the zodiac around the equator and looked at the earth,
you could see all the eclipses in the pattern would look a bit like this:
There are around 70 to 72 eclipses in each family of eclipses
There are about 42 of these families all at different points of their cycle at any time, half going from North to South, and half from South to North.
© Sareoso April 2001
There’s more information about the Saros series on Wikipedia.