Telling Time by the Stars - Sidereal Time
Problem:
Let
the vernal equinox occur at noon solar time on March 21 of a certain
year. Estimate the sidereal time at 3:00 pm solar time on November 29
of the same year.
Solution:
Fix the earth-sun line in [inertial] space. Let the earth rotate
on its axis once a day, and let the celestial sphere rotate about the
celestial poles once a year. As viewed from the north celestial pole,
the earth will rotate counter clockwise, and the celestial sphere, clockwise.
The rotation rate of the earth is one rotation every 24 solar hours
(length of the solar day). The rotation rate of the celestial sphere
is one rotation every tropical year (365.2422 days). The relative rotation
rate of the earth and celestial sphere is
360o/day
+ 360o/(365.2422 days)
= 360o/day
+ 0.9856o/day
= 360.9856o/day
Thus,
relative to the earth, the celestial sphere completes one rotation (sidereal
day) in something less than a solar day. In fact, the sidereal day is
just
(360/360.9856)
x 24 hours = 23.9345 hours
= 23
hr 56 min 4.2 sec
i.e.,
approximately 3 min 56 sec shorter than the solar day. Another way to
show the same thing is to form the ratio of the length of the sidereal
day to that of the solar day:
(24
hr/solar day)/(23.9345 hr/sidereal day) = 1.0027 sidereal day/solar
day
The
tropical year is then
(365.2422
solar days) x 1.0027 = 366.2284 sidereal days
Now
we may solve the problem of estimating the sidereal time on 3:00 pm,
November 29. From noon on March 21 to noon on November 29 is 253 solar
days. From noon to 3:00 pm on November 29 is an additional 3/24 = 0.125
solar days. Hence, the total elapsed time from the vernal equinox to
3:00 pm on november 29 is
253.125
solar days
Now
253 solar days = 253 x 1.0027 = 253.683 sidereal days. Thus, solar noon
on November 29 is
(0.683
sidereal days) x (23.9345 hr/sidereal day) = 16.347 hr
= 16
hr 21 min.
(i.e.,
a sidereal clock at noon solar time on November 29 reads 4 hr 21 min
ahead of a solar clock).
Also,
0.125 solar days = 0.125 x 1.0027 = 0.125 sidereal days (to within the
accuracy of the calculation).
(0.125
sidereal days) x (23.9345 hr/sidereal day) = 2.992 hr
= 3
hr 00 min (to within rounding accuracy)
The
sidereal time at 3:00 pm, November 29 is, therefore
(16
hr 21 min) + (3 hr 00 min) = 19 hr 21 min
An
alternative approach would be to convert 253.125 solar days into sidereal
days in a single step:
(253.125
solar days) x 1.0027 = 253.808 sidereal days
If
all clocks start at noon on the vernal equinox, then the sidereal clock,
at 3:00 pm solar, November 29, is reading 0.81 day past sidereal noon
or
(0.808
sidereal day) x (23.9345 hr/sidereal day) = 19.339 hr
= 19
hr 20 min
Accuracy
in the last digit is due to rounding error.