The Sonnenbau

The Sonnenbau was constructed in the 1940s, especially for solar observations.

Two boxes with movable mirrors (coelostats) were installed on the rails on the south side of the building. These coelostats redirected the sunlight into the building. After the war, this construction was used for some time to measure the weak global magnetic field of the Sun.

During the war, observations of the Sun were of major importance because of the impact of solar activity on radio communication on Earth. The measurements helped predict radio frequency interference, which was particularly significant for the high frequency radio communication on submarines. Besides the solar observatory in Bergedorf, at that time a number of this type of observatories were built in Germany. They were mostly located at higher altitude, in places with better observing conditions (such as Göttingen, Schauinsland, Zugspitze, Wendelstein). Some of these stations still exist.

 In 2010, the Sonnenbau was reconstructed and since has been used as a seminar room and computer lab.

 

Fig. 1: The Sonnenbau with its coelostats ready for operation. After 1955.

Fig. 2: This close-up picture of a sunspot shows a dark area in the centre (umbra) and a surrounding peripheral area (penumbra). Outside the penumbra, granules are recognisable. (Royal Swedish Academy of Sciences)

 

The Sun

The Sun is at a distance of approximately 150 million km from Earth. This distance is called an “astronomical unit”. Because of the immense pressure (250 billon bar) and temperature (more than 15 million degrees Celsius) in the centre of the Sun, hydrogen atoms can merge (nuclear fusion) and produce energy. Energy is transported to the outer layers by radiation and the movement of hot gas bubbles (convection), leading to its emission. A honeycomb pattern (granulation) arises on the surface of the Sun, created by neighbouring ascending and descending gas bubbles. Sometimes, the Sun shows spots. At these spots, the magnetic field forestalls the convection. This is why the spots are cooler and appear darker. Whilst each individual spot varies in shape and size on a timescale of days, the average number of spots goes through a cycle of eleven years. Some spots can become many times larger than the Earth. The Sun is about five billons years old and has enough fuel for another five billion years.

 

Fig 3: This picture taken by SOHO (ESA/NASA) shows the Sun during an active period. Several groups of dark sunspots are easily recognisable.