Astronomy Principles and Practice Fourth Edition.pdf

60 The celestial sphere: coordinate systems
Figure 8.1. Definitions related to the observer’s position on the Earth.
Hence, NOP 2 = φ,sinceZON is a right angle. That is,
The altitude of the pole is the latitude of the observer.
The horizontal system of coordinates has the observer at its origin so that it is a strictly local or
topocentric system.
The observer’s celestial sphere is shown in figure 8.2 where Z is the zenith, O the observer, P
is the north celestial pole and OX the instantaneous direction of a heavenly body. The great circle
through Z and P cuts the horizon NESAW at the north (N) and south (S) points. Another great circle
WZE at right angles to the great circle NPZS cuts the horizon in the west (W) and east (E) points.
The arcs ZN, ZW, ZA, etc, are called verticals. The points N, E, S and W are the cardinal points.
It is to be noted that west is always on the left hand of an observer facing north. The verticals through
east and west are called prime verticals; ZE is the prime vertical east, ZW is the prime vertical
west.
The two numbers that specify the position of X in this system are the azimuth, A,andthealtitude,
a. Azimuth is defined in a number of ways and care must be taken to find out which convention is
followed in any particular use of this system.
For example, the azimuth may be defined as the angle between the vertical through the south point
and the vertical through the object X, measured westwards along the horizon from 0 ◦ to 360 ◦ ,orthe
angle between the vertical through the north point and the vertical through the object X, measured