An eclipse is the obscuring of the face of the Sun or the Moon. There are various types of eclipse though. Both lunar and solar eclipses can be ‘total’ or ‘partial’ and solar eclipses can also be ‘annular’.
A solar eclipse occurs when the Moon passes directly between the Earth and the Sun. An observer sees the Moon as a black disc, as the side towards Earth is facing away from the Sun and cannot ‘shine’. The Moon blots out the face of the Sun and casts its shadow onto a narrow region of the Earth. This is the area of ‘totality’ and it races across the Earth’s surface due to the planet’s rotation. The area of totality can be up to 169 miles wide, usually less and in any one spot totality lasts for anything between a few seconds up to 7 minutes 31 seconds.
The only reason solar eclipses are possible, given that the Sun’s diameter is 400 times greater than the Moon’s, is that, by coincidence, the Sun is also 400 times more distant from Earth. Thus the two objects appear the same size to an Earth-bound observer.
As the Moon orbits the Earth every month, why is there not a solar eclipse every month? The reason lies in the fact that the plane of the Moon’s orbit lies inclined at an angle of 5 degrees to the plane of the Earth’s orbit around the Sun. In most months the Moon simply misses the exact position in the heavens needed to cast a shadow on the Earth. When the Moon only partly obscures the Sun a ‘partial’ eclipse occurs and there is no area of totality. Partial eclipses happen much more frequently than total eclipses.
‘Annular’ solar eclipses occur when the Moon is in position for a total eclipse but is at its furthest from Earth. The Moon’s orbit around Earth is not an exact circle. At its furthest, the Moon appears slightly smaller in the sky and as it crosses the face of the Sun a ‘ring’ of the Sun can be seen around the circumference of the Moon.
Lunar eclipses occur when the Earth passes between the Moon and the Sun and blocks the Sun’s rays. The Earth then casts its shadow across the face of the Moon. These events last for much longer than solar eclipses, and can take up to three hours. They also can be seen across a much wider area, in fact from any point on Earth where the Moon is visible above the horizon. Again, a partial eclipse happens when Earth’s shadow falls only partly on the Moon. The Moon’s 5 degree tilt as it orbits,
relative to the Earth’s orbit, means that lunar eclipses do not occur each month, but they are more frequent than solar eclipses.
As the Moon is moving further away from the Earth very slightly each year, in a billion years or so it will be too small to cause solar eclipses!