Astronomy Principles and Practice Fourth Edition.pdf

Effect of precession on a star’s equatorial coordinates 143
Figure 11.8. Measurement of the position of .
Figure 11.9. The celestial sphere illustrating the precessional movement of the celestial equator.
11.9 Effect of precession on a star’s equatorial coordinates
Let us consider now the sort of changes made in the star’s coordinates by precession, taking the
precessional period to be 26 000 years. In figure 11.9, the north pole K of the ecliptic is set at the
top of the diagram, with the ecliptic as shown. This is often a convenient orientation for a diagram in
problems on precession where the north celestial pole and the celestial equator shift.
Let P 1 , P 2 , P 3 , be the positions of the north celestial pole now (2000 AD say), one-quarter and
one-half of the precessional period later. The corresponding positions of are 1 , 2 , 3 . The
various celestial equators are also inserted.
Consider a star whose present right ascension and declination are 0 h and 0 ◦ respectively. The star
is, therefore, at 1 on the celestial sphere.
By 8500 AD, the First Point of Aries and the north celestial pole have moved to positions 2 and
P 2 because of precession, and the meridian from the north celestial pole of that date through the star
will cut the equator of that date at A. The star’s equatorial coordinates are then RA = 6 h ,Dec= ε ◦ N,
since 2 A is of length 90 ◦ . That this is so is seen from the fact that 2 is the pole of the great circle
KP 2 1 .