- Sidereal Time (ST):
- 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.
- 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.
- 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
- Find the Sidereal Time with your
- Make sure you have an assembled Star Clock and know how to
use it to tell local time.
and the pointer stars of the
Big Dipper on the
circumpolar star chart which forms the inner dial of your Star
- The radial lines coming out in all directions from the
center of the inner dial represent equal values of right
- 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
- 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
- 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!