In anticipation of AstroNotes articles planned for the near future, a short primer on certain fundamental aspects of the celestial sphere and its associated geometry is in order. While astronomers proper and longtime astronomy faculty will already be familiar with this topic, newer faculty and perhaps those from other disciplines, but teaching astronomy for the first time, may find it helpful. The geometry of planes, spheres, and great circles figures heavily in celestial geometry.

These are usually included in any good introductory astronomy or astrophysics textbook such as
Bradley W.
Dale A.
Introduction to Modern Astrophysics
, 2nd ed. (
Cambridge University Press
) or Karttunen et al., Fundamental Astronomy, 6th ed. (Springer, 2017).
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