Solar eclipses

A solar eclipse occurs when the shadow cone of the Moon intersects the surface of the Earth and is observable by anyone within this shadow zone (see Figure 1).

Figure 1: The type of eclipse that is observed depends on the position of the Earth in the Moon's shadow.

Two conditions have to be met for a solar eclipse to occur. The first concerns the relationship between the orbits of the Earth and the Moon, which are not in the same plane, but are inclined at around 5 degrees (5° 8' 43") to each other. The Moon crosses the plane of the Earth's orbit twice in each complete lunar orbit. For an eclipse to occur the Moon must be near one of these intersection points, called nodes.

The second condition is that the Sun, the Earth and the Moon must also be lined up, corresponding to the phase of the New Moon.

Types of solar eclipses: annular, partial, total

The Moon's shadow consists of two cone-shaped areas (see Figure 1), known as the umbra (externally tangent to the Sun and Moon) and the penumbra (internally tangent to the Sun and Moon). For an observer standing between the Moon and the umbra cone summit the eclipse is total. If the observer is beyond the cone summit, the eclipse is annular (ring-like): the apparent diameter of the Moon is too small to mask the whole solar disk. For an observer standing in the penumbra, only a part of the Sun is masked: the eclipse is partial.

Figure 2: Why the eclipse goes west to east

The most favourable conditions for a total eclipse are when the Moon is at its perigee, the Earth is farthest from the Sun (around July) and when the Sun is observed near zenith. When these conditions are all met, one can have a totality duration of more than 7 minutes.

Science of Eclipses