Sidereal Time

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• Sidereal Time (ST):
  1. Definition of sidereal time: The sidereal time equals the right ascension of any point on the celestial sphere crossing the meridian at a given moment.
  2. Would this definition amount to the same result? The sidereal time is the right ascension of any star when that star is at its highest in the sky.
  3. Sidereal time measures the moment when any particular region of the celestial sphere passes across one's meridian. Sidereal times are not simultaneous for different observers, but are longitude dependent. That is to say, the sky will appear the same to two different observers at two widely separated terrestrial longitudes (in two different time zones) at the same sidereal time (but not simultaneously).
     
• Find the Sidereal Time with your Star Clock
  1. Make sure you have an assembled Star Clock and know how to use it to tell local time.
  2. Find beta-Cassiopeiae and the pointer stars of the Big Dipper on the circumpolar star chart which forms the inner dial of your Star Clock.
  3. The radial lines coming out in all directions from the center of the inner dial represent equal values of right ascension.
  4. According to the definitions of ST given above, once you have found the local time with your star clock, the sidereal time would be read from the Right Ascension scale on the outside edge of the inner dial, just below the current date. 
  5. Example: The right ascension of the pointer stars of the Big Dipper is 11 hours (confirm this by inspecting your star clock). Thus, when the pointer stars of the Big Dipper are high in the sky, or crossing the meridian, the sidereal time is 11 hours.
     
• Application:
  • When there's something you've seen in the sky, tell your friends by email who live in different time zones "when" to go out and see it in sidereal time. They will have star clocks calibrated for their time zone and use them to convert sidereal time to their local time in order to know when to walk out of the house into the starry night. This is better than saying something like "go out when the pointer stars are overhead at the meridian," but amounts to the same kind of thing.
  • Each month Sky and Telescope provides the right ascension of interesting celestial objects (Moon, planets, asteroids, comets, etc.). Use your star clock to convert ST to local time so you will know when to go out and observe these heavenly wonders!

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