First vertical
In astronomy, the first vertical or east-west vertical is the vertical plane orthogonal to the meridian plane , i.e. the great circle on the celestial sphere that runs through the east point , zenith , west point and nadir . Its east branch has the azimuth 90 °, its west branch 270 ° if the azimuth is counted from the north.
In the first vertical, the stars in their daily movement (caused by the rotation of the earth ) reach the greatest vertical speed, which is about 10 "/ s (angular seconds per second of time) in middle latitudes .
From an observation point of view, this means that a star passage near the first vertical enables particularly sharp time measurement from zenith distances, which is a simple method for determining sidereal time in astrogeodesy . Even with a second theodolite , accuracies better than a tenth of a second (approx. 0.05 s) can be achieved. In combination with star measurements in the Greatest Digression , the effect of time errors can therefore be almost completely eliminated.
While in the first vertical the vertical star movement has its maximum, the horizontal movement in the meridian is greatest. Astronomy and geodesy make use of this when determining length and time with passage instruments.