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When the Sun, Moon and Earth come into a straight line an ECLIPSE occurs. This does not happen very often because the Moon's orbit is inclined to that of the Earth. A TOTAL eclipse occurs when all the Moon or Sun is blotted out. A PARTIAL eclipse is when only part of the Sun or Moon is covered.

Lunar eclipse

When the Earth comes between the Sun and the Moon we get a 'Lunar eclipse' or eclipse of the Moon (Figure 1).

The Moon passes into the shadow of the Earth, it ever completely disappears because a small amount of the Sun's light is refracted by the Earth's atmosphere and falls on the Moon during the eclipse. This light gives the Moon a reddish appearance.

You can see form Figure 1 that at the distance of the Moon the Earth's shadow is much bigger than the Moon and so the Moon takes some time to pass through it.

Solar eclipse

Its much more impressive when the Moon comes between the Sun and the Earth. The shadow of the Moon falls on part of the Earth and this is a Solar eclipse or eclipse of the Sun (Figure 2(a)). When seen from the Earth the Noon and Sun look almost exactly the same size and so in a total eclipse the Moon just covers the Sun.

schoolphysics solar eclipse animation

To see an animation of a solar eclipse click on the animation link.

Figure 2(b) shows the view of the Sun from the Earth during the eclipse. Diagram (c) shows the total eclipse. The shadow of the Moon falls on the Earth and because the Moon and Earth are moving compared with the Sun this shadow moves across the Earth's surface during an eclipse.

Eclipses do not occur as often as you might expect. This is due to the tilt of the Moon’s orbit around then Earth compared with the plane of the Earth’s orbit around the Sun. An eclipse, of the Sun or the Moon, will only occur if all three bodies – Sun, Earth and Moon are in a line. You can see this from the following diagrams.

In the first one (Figure 3(a)) there will be no eclipse but in the second one (figure 3(b)) the three bodies are all in a line and a person standing on the Earth at A will see a total eclipse of the Sun.

(Note – the sizes of the orbits and the Sun, Moon and Earth are NOT to scale)

© Keith Gibbs 2020