10. Solar history (a brief guide)

Observations of the Sun started a long time ago, because the Chinese, who were well advanced, noticed that on some occasions the Sun had black spots on its face. They did not know what they were, and judged them to be ‘birds crossing the face of the Sun’ or gave other explanations. The Chinese did not have any optical instruments except their eyes, but they could see the Sun when it was low in the sky and clouds reduced the light. (Please do not follow this example.)

The real revelation came from the telescope, and the astronomer Galileo Galilei used it not just on the Sun, but used it to discover a host of other objects. He observed sunspots, the name for the dark areas that crossed the face. His observations proved that they were part of the Sun and not going around it like a planet.

During the nineteenth century, a German amateur astronomer, Heinrich Schwabe, observed the Sun regularly for around 20 years. When he published these observations, he suggested that the solar activity varied. Sunspots came and went in periods of 7 to 17 years, the average being around 11 years. This was the discovery of the sunspot cycle. It seems he was trying to find a new planet within the orbit of Mercury. He did not find it, but instead discovered the solar cycle! After hearing about Schwabe, Rudolf Wolf, the director of Bern Observatory, became interested and after reading Schwabe’s works, and doing further study, made an arbitrary counting system, called the Universal Sunspot Numbers, which evolved to the Relative Sunspot Number.

English astronomer, Richard Carrington, studied the work of Schwabe and decided that there was evidence to suggest that the sunspots at the start of a sunspot cycle appear at higher solar latitudes and then progressively appear at lower latitudes towards the end of the sunspot cycle. Carrington also discovered that sunspots near at the solar equator travel faster than those lying nearer the solar poles, giving further evidence that the Sun has a gaseous make-up.

We see the Sun in white light, which is a mixture of colours from red to violet, and by splitting up the Sun’s light using an instrument called a spectroscope, we can see all of these colours. The German physicist, Fraunhofer, split up the Sun’s light and noticed dark lines within the solar spectrum. These dark lines correspond to various chemical elements within the Sun.

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