EclipsesDemonstrate an understanding that ... they do not occur every new and full Moon
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 four types of solar eclipse
- Total solar eclipse
- Partial solar eclipse
- Annular eclipse
- Hybrid 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.
It is a mere coincidence of time and place that we are able to see 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 trail 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).
When the Moon is on one of these nodes, the Moon, Earth and Sun are all 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).
* A hybrid eclipse is when a solar eclipse is annular when seen at some places on the line of totality and total in other places. They are caused because of the curvature of the Earth. At sunrise and sunset the cone has further to travel and is annular. At the peak of the eclipse it is sufficient to cause a total eclipse.
Other Types of Eclipses
An annular eclipse is one where the focus of the Umbra is above the Earth's surface. The Moon will appear too small to cover the Sun.
A hybrid eclipse is one where it is total in some places and annular in others.