how does a solar eclipse occur?

The Earth, Moon and Sun are not to scale in this diagram. The Moon is actually 400 times smaller than the Sun, but is also 400 times closer to the Earth than the Sun. This would be impossible to draw!

Here’s a movie showing the solar eclipse shadow. It is cast by the Moon onto the surface of the Earth.

As the diagram above shows, there is a large area surrounding the dark central shadow, the umbra, which is in partial shadow. This area of partial shadow is known as the penumbra and is much larger (about 7,000 km across) than the umbra. If you were standing in the penumbra you'd only see part of the Sun covered by the Moon. What you see depends upon where you are standing when the solar eclipse happens. This diagram gives examples of this:

From these diagrams you can see that during a total solar eclipse not all the Earth is plunged into darkness. The umbra, the full shadow, is really quite small, and is never more than 268 km across. This means that the chance of any one place on Earth falling under the umbra is also small. Even though a total eclipse occurs somewhere every year or two, it isn't likely to be where you live. A bit like the lottery - someone, somewhere usually wins the jackpot, but it isn't someone you know!