You can tell the approximate time at night by using Polaris and the Big Dipper as a giant clock in the sky. Polaris remains almost fixed in the northern sky, while the Big Dipper appears to rotate around it as Earth turns. By looking at the Dipper’s position, you can estimate how much time has passed and, with a correction for the date, roughly what time it is. The first step is finding Polaris.
How to Find Polaris Using the Big Dipper
Polaris, also known as the North Star, lies very close to the north celestial pole and is therefore an excellent fixed reference point for our star clock. Fortunately, you don’t need to know the constellations well to find it.
Start with the Big Dipper, the familiar seven-star pattern within Ursa Major. The two stars forming the outer edge of its bowl are Merak and Dubhe, commonly known as the Pointer Stars. Draw an imaginary line from Merak through Dubhe and continue it across the sky for about five times the distance between those two stars. This leads you to Polaris, which marks the end of the Little Dipper’s handle in Ursa Minor. Once you’ve found Polaris, you have the center of your celestial clock.
How the Big Dipper Works Like a Clock
Imagine Polaris at the center of a giant 24-hour clock. Now imagine a line extending outward from Polaris toward the Big Dipper. As the Dipper appears to circle Polaris during the night, this line acts rather like the hand of a clock. The principle is simple. A complete circle contains 360 degrees and corresponds approximately to one day, so the Big Dipper appears to move around Polaris by roughly: 360° ÷ 24 = 15° per hour
That means a quarter-turn around Polaris represents roughly six hours, half a turn about 12 hours, and a complete turn about 24 hours.
The motion is only apparent. The Big Dipper isn’t actually orbiting Polaris. Its movement across the sky is primarily the result of Earth rotating on its axis.
The Easy Way: Measure How Much Time Has Passed
If you only want to know how much time has passed, you don’t need to make any correction for the date. Note the position of the Big Dipper relative to Polaris when you begin observing. As the night progresses, the Dipper appears to move counterclockwise around Polaris. Because it covers roughly 15 degrees each hour, you can use the change in angle as a rough measure of elapsed time.
- 15° — about 1 hour
- 30° — about 2 hours
- 60° — about 4 hours
- 90° — about 6 hours
You don’t need to measure these angles precisely. A quarter of the way around Polaris is 90 degrees, so smaller movements can be estimated by eye. This method works particularly well for following the passage of time during a single night, because you’re simply comparing the Big Dipper’s new position with the position you observed earlier.
How to Estimate the Actual Time
Telling the actual time is a little more involved because the Big Dipper isn’t in the same position at a given clock time throughout the year. A traditional reference date for the Big Dipper star clock is March 8. On this date, imagine Polaris at the center of a 24-hour clock, with the Big Dipper occupying approximately these positions:
- Midnight: Big Dipper above Polaris
- 6 AM: Big Dipper to the left of Polaris
- Noon: Big Dipper below Polaris
- 6 PM: Big Dipper to the right of Polaris
Of course, you won’t normally see the Big Dipper at noon, but including the daytime positions completes the 24-hour clock and makes the pattern easier to understand. There is one important complication. These reference positions gradually shift with the date, so seeing the Big Dipper directly above Polaris does not always mean it is midnight.
Why the Star Clock Changes With the Date
Earth is not only rotating on its axis but also moving along its orbit around the Sun. As a result, the stars return to approximately the same position in the sky about 3 minutes 56 seconds earlier each night compared with our ordinary solar clock. For practical star-clock use, round this to about 4 minutes per day. That works out to roughly 30 minutes per week or two hours per month.
That’s why the March 8 positions on the diagram are only a reference. For a rough estimate, subtract about two hours for each month after March 8 from the time indicated by the reference clock.
A Simple Star-Clock Example
Suppose you look north and find the Big Dipper directly above Polaris. On the March 8 reference clock, that position represents approximately midnight. If you see the Dipper in the same position around April 8, one month later, it reaches that position roughly two hours earlier. Subtract two hours from the March reference: Midnight − 2 hours = about 10 PM See the same arrangement around May 8, and the seasonal shift is approximately four hours: Midnight − 4 hours = about 8 PM
You don’t need to calculate the correction to the minute. This is a naked-eye method for estimating time, so the simple two-hours-per-month rule is usually the most useful one to remember.
How Accurate Is Telling Time by the Stars?
The Big Dipper star clock is best treated as an approximate natural clock, not a replacement for a watch. Its accuracy depends on how well you judge the Dipper’s position, how precisely you allow for the date, and your location within your time zone. There is also a difference between astronomical time and the civil time displayed on your watch. If daylight-saving time is in effect, for example, you’ll need to allow for the additional hour.
For measuring how much time has passed during a single night, the method is simpler because no seasonal correction is needed. Find Polaris, watch the Big Dipper turn around it, and you have a natural clock moving across the northern sky.