Although the sun is only an insignificant star in our galaxy, it is the dominant object in our solar system and essential for life on Earth. A variation of only a few percent of its energy output would cause disastrous climate changes on Earth. Therefore, it is very important to observe and study the Sun and try to understand how it works.


Solar activity and the sunspot cycle


In all human cultures the Sun played a major role and was thus observed by astronomers. Sunspots were observed already more than 2000 years ago by Chinese and Greek scientists. In the 19th century it became clear that the number of sunspots was related to the solar activity and undergoing a cyclic change. Since then sunspots are counted systematically by various observatories around the globe and define cycle of solar activity.

With the CESAR solar observatory, taking pictures every week, you can participate in the world wide campaign of sunspot counting.

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Sun in H-Alpha


Rotation of the Sun and the Chromosphere


Sunspots cannot only be used to measure the activity of the Sun but also its rotation. Since the 17th century the moving of sunspots across the solar disc was observed by astronomers and used to measure the equatorial rotation of the sun. These observations also led to the discovery that the Sun rotation was slower at higher latitudes than on the solar equator (= differential rotation of the Sun).

With the CESAR solar telescope, taking pictures every week, we can determine the solar differential rotation.

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Sun rotation (SOHO)




With the formulation of Kepler's  third law at the beginning of the 17th century it became possible for astronomers to determine the distances between Earth, Venus and the Sun by employing the parallax method to precise observations of Venus moving across the solar disc. Since that time every "transit" of Venus except the last two in 2004 and 2012 was used to determine the distance of the Sun-Earth.

Although now superseded by modern measurement techniques, this old method is still a very instructive way of understanding the orbital movements of the inner planets in our solar system.

The images taken during the two expeditions to Svalbard and Australia to observe the transit of Venus on 2012-06-06 can be used to determine the distances of Earth, Venus and the Sun.

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Moon, Earth and Sun