Sidereal vs. Solar Day
- We measure civil time according to the position of the Sun in the sky.
- Our civil day is the time from Noon to Noon and is, by agreement, exactly 24 hours
- This time period is called a ``Solar Day''.
- The time from Noon to Noon is analogous to the time
from Full Moon to Full Moon -- both depend on alignment
with the Sun.
- The actual rotation period of the Earth is 23 hours 56 minutes and 4 seconds.
- This time period is called the ``Sidereal Day.''
- Sidereal means with respect to the stars (rather than
- During the course of a day the Earth travels a short distance
in its orbit around the Sun.
- After the Earth has completed one rotation
the Earth must turn for an extra 4 minutes in order to bring the Sun
back to the same point in the sky.
- The time from Noon to Noon (the Solar Day)
is longer than the true rotation period of the Earth (Sidereal Day) by
about 4 minutes as a result.
- Note that 4 minutes is 24 hours divided by 365!
- Unlike the Sun, the stars rise and set on a Sidereal schedule.
- A star or constellation will be at precisely
the same position in the sky 4 minutes earlier with each successive day.
- In the course of a month this daily 4 minute shift adds up
to a two hour difference in the rising time of a star or constellation.
- A star which is directly overhead at 10 p.m. on October 1
will be directly overhead at 8 p.m. on November 1.
Updated September 24, 2003