Telling Time by Sun and Stars

by John P. Pratt
©1998 by John P. Pratt. All rights Reserved.

Astronomy Home Page

Sundials and Zone Time

So what ever happened to sundials? We used to have sundials on buildings and in parks. Where is a sundial anywhere in the city now? I think they're gone because of daylight savings time! Someone would have to changed the numbers on a sundial twice a year. So all we need is a sundial with a moveable wheel to fix that.

But it is useful to know how to tell time by the sun anyway. For me, one very practical use is to know when I won't get sunburned. The most harmful time for sunburn is within about three hours of when the sun is highest (due south in the northern hemisphere). So just when is the sun highest?

We speak of "high noon" when the sun is supposed to be highest, but it doesn't work that way in real life for two reasons. First, in the summer, we add an hour for daylight savings. Secondly, here near Salt Lake City, we are about half an hour west of the center of our time zone (which is close to Denver) so we have to add another half hour because we set our clocks by Mountain Standard Time, not Salt Lake Local Time. That means that here, the sun is due South at 1:30 p.m. in the summer. If you are local here, memorize that! If not, find out your time correction and remember it. Then if you are on a summer hike, and it is 1:30 p.m., you will know the shadows will point due North.

The Star Clock

You can use the pointers of the Big Dipper as a star clock to tell the time. If you lived in the middle ages, you might have carried a little wheel called a "nocturnal" to do this. Because you are able to do simple math in your head, you don't even need the little wheel, but you need to practice doing it about once a month so you don't forget how. Just follow these five easy steps to read the time to within half an hour.

  1. Read the Clock. Find the Big Dipper in the Northern sky. Remember that it is pretty low to the horizon in the summer, but the pointers are nearly always visible, unless there are mountains due north. Imagine one big hour-hand on a clock, which is centered on the north star (to which the two pointer stars "point." Read the time to the nearest quarter hour as if it were a normal clock. In the illustration, it is about 1:45, or 1 3/4 hours.

  2. Add one hour for every month after March 7. Do this to the nearest quarter month. The star clock will read 12:00 at midnight on March 7, so memorize March 7, no matter where you are. If today is April 9, then it is about 1 month after March 7, so add 1 hour, getting 2 3/4 hours.

  3. Double the time (because it is really a 24-hour clock), getting 5 1/2 hours in this example.

  4. Subtract from 24 (or 48 if necessary). We subtract because the clock is going backwards, that is, counterclockwise. That gives us 18 1/2 hours, or 18:30 on a 24-hour clock. So it is 6:30 p.m. local time.

  5. Correct for Zone Time. Because you've already memorized that when the sun says noon that we call it 1:30 during daylight savings, and because you remember that daylight savings begins the first Sunday in April, you add 1 1/2 hours. That means your watch would say it is about 8:00 p.m.

If you just practice this a few times, you'll remember it and you'll be surprised just how accurately you can tell time by the stars and amaze your friends at summer campouts.