32
3 Astronomy Around 1875
Fig. 3.1 The coordinates of a star in the northern hemisphere. For details see the text.
Figure by the author
longitude and latitude on Earth, namely relative to two points. On Earth these
are the North Pole and the point on the equator that lies on the Greenwich
meridian. The situation is presented in Fig. 3.1. The shaded surface is that
of the Earth and the half sphere represents the sky. The intersection line
between sphere and plane is the horizon with the directions north and south
marked with ‘N’ and ‘S’. The observer is in the center of the spherical dome.
The point in the sky right above the observer is called the zenith. The circle
from south on the horizon via the zenith to north is called the meridian. The
straightforward way to represent the position of a star is as in the left-hand
part of Fig. 3.2 with the azimuth along the horizon (usually measured from
Fig. 3.2 Coordinates on the sky of a star in azimuth and altitude (left) and in right
ascension and declination (right). The symbol à indicates the vernal equinox. Figure by
the author