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Solar Eclipse Introduction

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A solar eclipse

Solar eclipses provide a unique method of observing the sun. The moon's shadow not only prevents light reaching the Earth but also radio waves. By recording the signal strength of solar radio noise various details can be deduced. The sun is made up of layers, the innermost part is the solar interior. The next layer is the photosphere and it radiates most of the visible light. The photosphere is only 300 to 400km thick. After the photosphere is a layer called the chromosphere (2000km thick). During an eclipse the chromosphere can be seen as a pink layer. On the outside is the corona, which has a thickness of several million kilometres.

The solar eclipse proceeds with four key stages:

At the first contact the the sun and the moon appear to touch.

Graphical representation of 1st contact
1st contact (09:57 BST).

Graphical representation of 2nd contact
2nd contact (11:11 BST).

Totality is the time of the total eclipse. It occurs between second and third contacts. The sun is hidden behind the moon, the sky turns dark. Birds will stop singing and stars will be visible.

Graphical representation of 3rd contact
3rd contact (11:13 BST).

The moon and sun then move apart until at fourth contact they only just coincide.

Graphical representation of 4th contact
4th contact (12:32 BST).

The radio sun

The sun not only emits light but also radio waves. If you were to listen to the sun with a radio the only sound you would hear would be crackly noise. The radio waves are emitted from the chromosphere and lower corona, this means that when the sun is viewed with radio equipment is appears larger than when viewed with optical equipment. With lower frequency radio waves the sun appears larger. This means the times of contact for radio equipment do not match what is seen by eye.

On Earth it seems the sun is burning smoothly, but this is not the case. Sometimes scientists can detect dark areas on the sun called sunspots. The number of sunspots changes over an 11 year cycle. The eclipse on 11th August 1999 is important because the sun is at the most active part of its cycle.

[1] Times of contact (for visible light) are given in British Summer Time (BST) and are correct for Falmouth.

Ionosphere and Radio Propagation Group,
Department of Communication Systems,
Lancaster University.

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