The Sun

Eclipses

3.10 - Understand the causes of solar and lunar eclipses

An eclipse is when the shadow of a celestial body, such as the Earth or Moon, obscures the light of another object in space.

There are two main types of eclipse:

• A solar eclipse when the Sun is obscured by the Moon
• A lunar eclipse where the Moon is obscured by the Earth's shadow

There are three main types of solar eclipse

• Total solar eclipse
• Partial solar eclipse
• Annular eclipse

There are two types of lunar eclipse

• Total lunar eclipse
• Partial lunar eclipse

Any celestial body that receives light from a source, like the Sun, will cast shadows behind it. The Moon and Earth always cast shadows behind them. We cannot see the shadow, but eclipses clearly show us evidence of these shadows.

Eclipses occur when the Sun, Moon and Earth are lined up with one another. They can occur only when the Moon is in its full moon or new Moon phase. This alignment is called 'syzygy', when three or more celestial objects align.

It is a coincidence of time and place that we are able to see total solar eclipses. The Sun is over 400 times larger than the Moon yet is over 400 times further away. The Earth observer views them at roughly the same size, making Earth ideally placed to see the features of a solar eclipse

Imagine that as the Earth orbits the Sun, it leaves a line behind. This line forms an ellipse and is called the ecliptic, which is the path the Earth takes as it goes around the Sun. All other planets (except Pluto) in the Solar System orbit the Sun at a different distance from it, but at a similar angle

Now imagine the Moon orbiting Earth and trailing a line behind it. This ellipse is at a similar angle to the ecliptic but is positioned about 5% from it

Twice an orbit the Moon's orbit intersects the ecliptic. These points of intersection are called nodes. Once going from north to south of the ecliptic (descending node), again when going from south to north of the ecliptic (ascending node). Most of the time the Moon is not aligned between Earth and Sun though. When it passes a node and is in line with both bodies an eclipse happens. All three are on the same celestial latitude. When the Moon moves between Sun and Earth, we have a solar eclipse. When the Earth is between Sun and Moon, we have a lunar eclipse

The different position of nodes and the angular difference between the ecliptic and the Moon's orbit explain why we do not have an eclipse every lunar orbit. Usually the Moon orbits so that the Moon's shadow rises above or below Earth (and so also avoids Earth's shadow).