Celestial Equator

Each day and night as the fixed stars appear to rotate around the earth like a giant celestial sphere, the circumpolar stars (above the horizon in the north) turn in circles around the north celestial pole (Polaris, the north star). Perpendicular to the north celestial pole is the celestial equator, which lies directly above any observer located on the equator of the Earth.

In their daily or diurnal motion, stars located on the celestial equator rise due east and set due west as seen from anywhere on Earth. Example: Mintaka, the top star of Orion's belt.

You can trace the course of the celestial equator even if there are no stars visible (see below). However, in late autumn it runs eastward from the setting Summer Triangle through Pisces, Cetus, Orion, Monoceros, and Hydra. In late spring....

Observe on a model celestial sphere:

  • Does the celestial equator always intersect the horizon at due east and due west?
  • Does the celestial equator lie in a plane that is always perpendicular to the axis of the earth and the north celestial pole?
  • What is the altitude of the celestial equator on the meridian as seen from Shawnee?
  • From a constant terrestrial latitude, is there any variation in the altitude of the celestial equator on the meridian depending on the time of night or the season of the year?
  • Does the angle between the celestial equator and the horizon remain the same throughout the year?
  • For a review of the celestial equator, and where it is located relative to the horizon, see diurnal motion.

Equatorial Coordinates

  • Equatorial coordinates are used most often by modern astronomers to specify the location of any star, comet, deep sky object, etc. Tables of interesting objects and their equatorial coordinates may be found in the current issue of Sky and Telescope.
  • What are the units of measurement by which the celestial equator is marked off?
    • Right Ascension (RA)
    • Declination (d)
    • Sidereal Time (ST; used to know when to look for comets, planets, constellations, deep sky objects, etc.)
    • Given the average declination of a constellation, how could you tell if it is visible from your location? (Answer)