Astro Bob: Tell Time with the Big Dipper

Not only does the Big Dipper make a pretty good clock, using it to tell time is a fun way to understand how stars move in the sky.

Big Dipper with airglow at nadir Sept 16 2020 S.jpg
The Big Dipper reaches its lowest elevation above the northern horizon โ€” called nadir โ€” in early November around 8-9 p.m. The pale green and pink stripes in the photo are airglow, caused by air molecules that absorb the sun's ultraviolet (UV) light during the day and re-emit it in hues of green, red and blue at night. Airglow happens all around the Earth all the time and causes the night sky to never be completely dark. (Bob King)

Our beloved dog Sammy died two years ago, and our home has been without a pet until this spring when my older daughter and her black Shar-Pei "Sunny" moved in after coronavirus shutdowns . We love him and his endearing hippopotamus-like muzzle. Whenever Sunny shakes his head it sounds like blade slap from a helicopter lifting off. It's best to back away at these moments or risk getting "frothed." Like all dogs he enjoys a nightly walk.

Each evening we leave the driveway and turn north where trees now frame a slice of the northern sky that includes the Pointers, the two stars at the end of the Big Dipper's bucket. The Pointers always point to Polaris, the North Star. Just draw a line through them and extend it upward until you come to the next bright star โ€” that's Polaris.

The Dipper dips lowest above the northern horizon in early November around 8-9 p.m. local time. From the far southern U.S. it's hidden from view below the horizon, but skywatchers north of about latitude 40ยฐ can still see it low in the northern sky. Trees often hide the figure from where I live, but if I can find an open view to the north it's easy to see. To my eye it looks spread out and relaxed as if settling in for a winter's nap.

Big Dipper clock time Nov 2020 variation.jpg
You can visualize the North Star at the center of a clock face with the line connecting it to the Dipper's Pointer Stars as an hour hand. This clock is slightly different from the familiar one. Instead of clockwise, the hour hand moves counterclockwise and the hours are labeled from 0 to 6 to 18 and back to 24, also represented by 0. (Bob King)


No matter when you look at the Big Dipper you can draw a line from the Pointers to Polaris and visualize it as the hour hand of a celestial clock with Polaris at the center. The hour is measured by extending a line through the Pointers to the outer circle.

There are a couple of differences between our sky clock and a real one. First, it's a 24-hour clock with midnight or the "0 hour" at the top, 6 = 6 a.m. on the left, 12 noon on the bottom and 18 hours = 6 p.m. on the right. Second, this clock runs backwards! As the Earth turns, the Big Dipper describes a counterclockwise circle around the North Star every 24 hours, equal to one rotation of the Earth.

The next clear night, go out, face north and locate the Pointers and Polaris. To find the time, use this simple formula: Time = the Dipper clock reading minus 2x the number of months after March 6. In our example, the time is 12 hours. Rounding up, it's been 8 months since March 6. Let's do the calculation: 12 - 2x8 = 12-16 = -4. The minus sign means before midnight. Subtracting 4 hours from midnight gives 8 p.m., a good approximation of the correct time. If the answer is positive the time is after midnight.

Dipper clock example panel S.jpg
Here are several more examples of the the Big Dipper clock. (Bob King)

Nov. 1 is not quite 9 months from March 6. For greater accuracy we'll convert the time between Nov. 1 and 6 into a fraction and subtract it from 8 months. 5 days = about 0.2 month. Subtracting 0.2 from 8 to get 7.8. Redoing our calculation, 12 - 2x7.8 = -3.6 which is 3.6 hours before midnight or 8:24 p.m. Quite accurate!

If fractions befuddle your brain you can stick with whole numbers. Or just whip out your mobile phone and use the calculator.

Fortunately, we just trashed Daylight Saving Time, so it's not a factor. But when it returns in April remember to add an hour. You estimate will also be affected to some degree by where you live in your local time zone โ€” a little earlier than true clock time if you live in the western end and a little later if you live closer to the eastern edge.


After a few tries you'll soon be able to factor in that difference and fine-tune the time. Leave a comment and let us know how well your new clock works. Finally โ€” a clock that never needs winding or batteries. Nature's always ahead of the curve.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at .

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