Prime vertical

As a first vertical or east-west vertical the vertical plane is in astronomy called orthogonal to the meridional plane, ie on the celestial sphere that great circle which passes through the east point, zenith, and nadir West Point. Its eastern branch, the azimuth 90 °, its western branch 270 ° when the azimuth from the north is counted.

In the First Vertical reach the stars in their daily ( caused by the Earth's rotation ) movement, the largest vertical speed, " is in mid-latitudes about 10 per second of time.

Observation technology, this means that a star passage near the First verticals enables a particularly sharp timing of zenith distances, which is a simple method to determine the sidereal time in the Astro geodesy. Even with a Sekundentheodolit can be precisions better than a tenth of a second (about 0.05 s ) reach. In combination with star measurements in the greatest digression can therefore be the effect of time errors almost completely eliminate.

During the First Vertical vertical motion star has its maximum, the horizontal motion in the meridian is at its greatest. Of these, make astronomy and geodesy in the length or timing with passages instruments use.