How to tell the time at night in the Northern Hemisphere by using the Big Dipper or Plow and the Pole Star


 

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Ken Ward's Astrology Pages

Astrology: Telling the Time at Night from the Stars.

clear.gif (807 bytes) While the people in Scandinavia can tell the time from the sun all day and all night, in other parts of the world, we cannot see the sun at night. However, there are various ways of telling the time from the stars.

One way to do this is to use the Pole Star and the Big Dipper or the Plow. It is sometimes said that the Pole Star is the brightest star in the sky, however, this isn't true. It is sometimes easier to find the Big Dipper and then to find the Pole Star from the Big Dipper.

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You can find the Pole Star by drawing an imaginary line through the two stars that make up the end of the "bowl" in the Big Dipper (or the end of the plow) and follow it for about 5 times the apparent distance between these two stars. You will be led to the Pole Star. The names of the two stars are Arabic names: Dubhe and Merak.

The point is that this line will rotate like the hands of a big 24 hour clock and from the angle of this clock hand, you can determine the time. If you imagine the first time reading as a reading on a normal clock and then, say an hour or two later estimate the time on this clock, you can estimate the difference in time between these two readings. Because this is a 24 hour clock, you will have to double the estimated time to get the real difference in hours. It is better to imagine a normal clock, because imagining a 24 hour clock is difficult when we are used to the normal 12 hour clock.

You can easily create some things to help you tell time from the Big Dipper Clock. You could, however, tell the time without any tools at all. The first thing you need to know is that the clock will show 12 oclock on March 7th, so for every month thereafter you add one hour to the apparent time. You do this because the Big Dipper Clock keeps Star (or Sidereal) time. The star days are 4 minutes longer than our days, so sidereal time is about 2 hours per month faster than our regular time.

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You can estimate the time from scratch by estimating normal time and adding one hour for each month after March. You then double the result to get the time from the stars. To get normal clock time, you will need to know what time the sun is at its highest. This will vary with localities and may involve Summer Time corrections in civil time. So in some areas the sun is at its highest at 1 pm in Summer, in others, the time may be different.

When you estimate time in this way, you will be able to tell the time within, say an hour. If you use simple equipment with which to read the Star Time, you can be much more accurate. The Ancient Egyptions measured time very accurately and they could determine sunrise within a few seconds.

The important point in this page is that there is a big clock (or even several) in the sky with which you can determine Star Time. Civil or clock time is always related to this Star Clock. While some elements of time are arbitrary (for example, the 24 hour day and the 60 minute hour), the length of the day is real in terms of the movements of the stars and the movements of the sun (or apparent movements of these!)

While our normal clocks and civil time relate to Solar Time, the big clock in the sky works on Sidereal Time.