Astronomy 104: Lecture 2 (Slide 11)

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Its about time

The Meridian is used for telling Time.

The most sensible astronomical time is Sidereal Time. One Sidereal Day is the time between two successive meridian transits of a star.

1 Sidereal Day = 23^h 56^m 4.091^s

If you set your alarm clock by sidereal time, in 6 months, you would be showing up to work 12 hours early!

The reason: the Sun (what everyone else uses to tell time) moves about 1° per day.

However, the the apparent solar day it not 24 hours long either!

This is a subtle effect caused by two processes.

1) the Earth doesn't move at a constant rate around the Sun (we'll learn about this when we study elliptical orbits)
2) the tilt of the Earth's axis (which is ???) means some of the Sun's motion is North-South

Therefore we define the mean (average) solar day which is 24^h exactly.

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