Meridian passage

In astronomy and astrogeodesy, the time and place in the sky of the location at which a star crosses the meridian is referred to as a meridian passage or (meridian) transit .

With constant declination , the star also reaches its greatest height above the horizon at this moment of passage . The sun , moon and planets , however, have a declination that changes over the course of the year, so that they do not always have their greatest height above the horizon at the moment of the meridian passage.

The measurement of meridian passages serves v. a. the astronomical time determination (see sidereal time ), the length determination and the determination of exact star locations . For this purpose, special instruments with a special thread network or micrometer have been developed, such as the meridian circle and the passage instrument . Newer of these instruments can also automatically register the star passages .

The achievable accuracy is called throughput error and is - depending on the instrument, measuring method and experience of the observer - on the single thread between 0.03 and 0.5 seconds. However, it can be more than halved by a thread network with several threads or an impersonal micrometer , and again by measuring over several nights.

Some methods (e.g. Circummeridian ) also use measurements just before and after the meridian transit.

Meridian passage

See also