Earth and Space

Solar eclipses

Physics Narrative for 11-14 Supporting Physics Teaching

The Moon blocking the Sun

A total eclipse of the Sun, a solar eclipse, is caused by the Moon passing between the Sun and the Earth.

It is important to recognise that this diagram is not drawn to scale. While the proportions of the Earth and Moon are correct, the distance between the Earth and the Moon is far too small, as is the size of the Sun. If it was drawn fully to scale, the Sun would be nearly 4 metre to the left and 70 times as big.

As depicted, the Moon is in the New Moon position, but solar eclipses do not occur every month. This is because the orbit of the Moon (around the Earth) is tilted by about five degrees with respect to the Earth's orbit (around the Sun), so that the Moon usually passes slightly above or below the line between the Sun and the Earth.

The path of the umbra part of the shadow, cast by the Moon across the Earth, is known as the path of totality. It is only about 50–200 kilometre wide.

It is quite remarkable that total solar eclipses even occur at all.

The Sun and the Moon appear the same size, allowing you to see an eclipse

Solar eclipses do occur because the Sun and the Moon appear from Earth to be about the same size in the sky. The Sun, whose diameter is 400 times that of the Moon, happens to be about 400 times as far away from the Earth. This condition permits the Moon to just cover up the Sun.

In fact, if the Moon's diameter (3480 km) were just 230 km smaller, it would not be large enough to completely cover the Sun. In this case, a total solar eclipse would never happen anywhere on Earth!

In addition to the total eclipse, there are partial or annular eclipses in which the Moon does not quite cover the Sun.

This video clip shows a total eclipse. These are remarkable events lasting anywhere between 30 and 120 second. When totality is reached it is possible to see the Solar Corona – the outer part of the Sun's atmosphere.

The outer parts of the corona break away and move outwards from the Sun, forming a high-speed flow of particles known as the solar wind.

download clip to view

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