Tell Time Looking at the Night Sky

John Conway taught me how to tell time at night. But first I need to explain the notions of the “time in the sky” and the “time in the year.”

The clock in the sky. Look at Polaris and treat it as the center of a clock. The up direction corresponds to 12:00. Now we need to find a hand. If you find Polaris the way I do, first you locate the Big Dipper. Then you draw a line through the two stars that are furthest away from the Big Dipper’s handle. The line passes through Polaris and is your “hour” hand. Now you can read the time in the sky.

The hand of the clock in the sky makes a full rotation in approximately 24 hours. So if you stare at the sky for a long time, you can calculate the time you spent staring. Keep in mind that the hand in the sky clock is twice as slow as the hour hand, and it turns counter-clockwise. So to figure out how long you’re looking into the sky, take the sky-time when you start staring, subtract the sky-time when you stop staring and multiply the result by 2.

To calculate the absolute time, we need to adjust for the day in the year.

The clock in the year. A year has twelve months and a clock has twelve hours. How convenient. You can treat each month as one hour. In addition as a month has about 30 days and an hour has exactly 60 minutes, we should count a day as two minutes. Thus, January 25 is 1:50.

Fact: on March 7th at midnight the clock in the sky shows 12:00. March 7th corresponds to 3:15. So to calculate the solar time you need to add up the time in the sky and the time in the year and multiply it by 2. Then subtracting the result from 6:30, which is twice 3:15, you get the solar time.

You are almost ready. You might need to adjust for daylight savings time or for peculiarities of your time zone.

This time formula is not very precise. But if you are looking into the sky and you do not have your watch or cell phone with you, you probably do not need to know the time precisely.

4 Comments

  1. ano:

    Unfortunately, the first step “Look at Polaris” is already close to impossible in most urban areas — having a clear line of sight to a star so close to the horizon is rare. 🙁

  2. Austin:

    You can also tell time by the moon, if it’s visible and you know where north is. The phase of the moon indicates its times of rise and set, and its direction indicates how far it has advanced from rise (in the east) to set (in the west).

  3. Pseudonym:

    Here’s an outline of a technique which should work in the Southern Hemisphere.

    The South celestial pole lies very close to the intersection of two extremely easy-to-locate features of the Southern sky:

    1. The major axis of Crux, suitably extended.
    2. A perpendicular bisector of the line between Alpha Centauri and Beta Centauri.

    This is useful for locating the pole, but to tell time, you wouldn’t actually need to do this. All you need is a “clock hand” of the right orientation relative to the horizon, and the major axis of Crux gives you that.

    Anyone want to work out the details?

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