The Oppolzer Zenith Telescope

This approximately 2-meter-long, horizontally mounted zenith telescope is Oppolzer's own design, and its installation in an optimally suitable location was a major reason for him to prefer his professorship in Innsbruck to his appointment in Prague.

Zenith telescopes are a development of the meridian circles and, like these, are used to determine the time and star locations, but only of stars that are located near the zenith, i.e. vertically above the telescope.

This restriction to stars close to the zenith increases the measurement accuracy enormously: zenith telescopes can measure the coordinates of their locations to within decimetres. They are so precise that an annual spiral wobble of the Earth's poles by several meters could be discovered in cooperation with several participating observatories. Moreover, the North Pole drifts westwards by just over 10 meters per century, which is caused by mass displacements in the Earth's interior. The Innsbruck zenith telescope was also involved in such international research projects to measure the fluctuations in polar height.

As early as 1899, Oppolzer, then still an assistant at the German University of Prague, published his plans for this instrument with a lens diameter of 10.8 cm and a focal length of 200 cm. He had the zenith telescope built by Gustav Heyde in Dresden in 1901 and presented his own design to his colleagues at the meeting of the Astronomical Society in Göttingen the following year.

Due to Oppolzer's early death and the First World War, it was not until 1923 that Viktor Oberguggenberger made intensive use of the zenith telescope and published his polar altitude measurements. The Innsbruck zenith telescope played an important role for the last time in 1957 as part of the International Geophysical Year, and in the 1960s more modern methods took over the task of the zenith telescopes.