Time from the Stars

Two successive passages of a star over your position is one sidereal day. The sidereal day is 23 hours, 56 minutes 4.1 seconds of a mean solar day, which is usually rounded to 23 hours, 56 minutes and explains why the stars appear to rise about four minutes earlier each night (see Figure 10.4).

As night falls, find a star or constellation rising on the eastern horizon. It is not necessary to identify it. Like the sun it will travel across the sky to set in the west, and just as you can tell the time of day from the position of the sun in the sky, you can use a star to tell you time of night.

To estimate the actual time, use the stars in the Big Dipper and Cassiopeia. A line joining the stars called Ruchbah (Delta Cassiopeia) and Mizar (Zeta Ursa Major),

10.4 The Sidereal Day

shown in Figure 10.5, almost passes through the Pole Star and forms the hand of a 24-hour star clock. The tip of the hand is Ruchbah and when it is at the twelve o'clock position it is 0120 hours. Twelve hours later it points towards the six o'clock position and it is 1320 hours. Unlike its earthbound mechanical relatives the hand of this clock runs backwards.

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