The Meridian Circle

Until the middle of the 19th century, astronomers knew little about the nature of stars and planets, but they could measure their locations in the sky. Over the millennia, the instruments used for this purpose became increasingly accurate and the beautiful Innsbruck meridian circle with a lens diameter of 10.8 cm and a focal length of 115 cm represents a showpiece in this technology. It was built around 1860 by the Viennese company Starke and was kept in the private observatory of Oppolzer's father, himself a well-known astronomer, until he moved to Innsbruck.

Astronomical instruments of a much simpler design were already being used in ancient times. The reason for this is that the observation of celestial events is necessary in order to create calendars and these are - whether for the Mayans, Egyptians or Babylonians - a foundation of every advanced civilization. As early as the third millennium BC, the Egyptians introduced a 365-day calendar and the Babylonians the seven-day week.

The meridian circle works as follows: One has to wait until the star whose exact location is to be measured passes through the instrument's crosshairs in its southernmost position - i.e. in the meridian – and one has to take note of the exact time when this happens. The horizontal position of the star can now be calculated from this time and the coordinates of the location of the meridian circle. The vertical position can be read directly from the scale engraved in the brass circle using one of the four measuring magnifiers.

However, even more can be done with such instruments: If we know the location of the star that is currently in its southernmost position, the exact time can be calculated from this and from the known position of the meridian circle. These exact times measured at the meridian circle were forwarded to various people throughout Innsbruck in the 1910s as part of a so-called time service.
For thousands of years, astronomers have not only been the guardians of the calendar, but also masters of timekeeping.