Telling the time using the stars

How long does it take the Earth to spin?

No doubt you will have answered 24 hours. That is actually wrong. It takes the Earth 23 hours and 56 minutes to spin on its axis. If you could see the stars all the time you would notice that they move a little bit further than 360 degrees in 24 hours. About 1 degree more.

24 hours is just the average time between mid-day (when the Sun is at its highest) and the following mid-day for all the days throughout the year.

The reason there is a difference is because, relative to the Earth, the Sun moves a little bit quicker than the stars.

• Every day it moves about an extra degree in the sky.
• Every 30 days or so it moves through one of the zodiacal constellations. What constellation was the Sun in when you were born?
• Every year it makes a complete revolution in the sky.

Why is Pisces best viewed in August / September?

Imagine we looked at the position of the stars at midnight on the same day every month throughout the year. Because the Earth takes a little less than 24 hours to rotate the every month the stars would be further anti-clockwise by about 30 degrees. After a year they would be back where they started from. I am starting this celestial clock on march 21st because on that day the celestial meridian is aligned north south. I am using the celestial meridian as the hand of my clock.

So how do we tell the time then?

Well, using the information above you should be able to calculate the position of the clock hand at midnight on any day of the year.

Number of degrees anticlockwise from north = number of days after march 21st x 365/360 degrees.

Now if you measure the actual angle of the clock hand you should be able to work out how long to go before or after midnight it is bearing in mind that 1 degree is about 4 minutes.

Example 1 - On June 8th the celestial meridian (the clock hand) is 62 degrees past north. What time is it?

Well June 8th is 79 days after march 21st. (10 + 30 + 31 + 8 = 79)

At midnight on June 8th the clock hand should be 79 x (365/360) = 80.1 degrees past north.

If it is just 62 degrees past north then it is before midnight. About 18 x 4 minutes before = 72 minutes = 1 hours and 12 minutes before.

The time is 10:48 p.m.

Example 2 - On November 2nd the celestial meridian is 118 degrees before north. What time is it?

Well November 2nd is 226 days after march 21st (10 + 30 + 31 +30 +31 +30 +31 +31 +2 = 226)

At midnight on November the 2nd the clock hand should be 226 degrees after north = 134 degrees before north.

So its after midnight. 16 x 4 minutes after = 64 minutes or 1 hour 4 minutes after.

The time is 01:04 hours