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What is the simplest way to approximate time after the sun has set? Perhaps using the moon's height and phase? This would make the most sense to me, but what could you do in the case of a new moon?

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The moon is not synchronized with the sun. You would have to calculate its position and have an extensive reference chart to make any sense of it. The moon is not a reliable source of time. –  Shawn Feb 15 '12 at 1:45
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The way you've asked the question, there's really no way to answer. There is no easy way to tell time at night without a watch, as Shawn explains. I've edited it to remove "straight forward" and make it more answerable. –  Russell Steen Feb 15 '12 at 2:04
    
Simpler than a watch? –  furtive Feb 16 '12 at 4:58
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3 Answers

up vote 11 down vote accepted

Another way to find the time is to use well known stars. In the northern hemisphere, you can use the Pole Star and the Big Dipper to tell the time fairly accurately. A good explanation of the procedure can be found here. Here is a abbreviated quote from that site :

  • Find the Big Dipper in the Northern sky. 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.
  • 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.

  • Double the time (because it is really a 24-hour clock).

  • Subtract from 24 (or 48 if necessary). We subtract because the clock is going backwards, that is, counterclockwise.

  • Correct for Zone Time, because you've already memorized that when the sun says noon that we call it (for example) 1:30 during daylight savings.

The website also gives an example, so you can work trough it too make sure you understand all the steps. You do need to remember of few things, but once you get the hang of it should be pretty straightforward.

I looked online and apparently for the southern hemisphere there is a similar method using the Southern Cross.

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Wow, this is really cool, and gets a more accurate time reading that from the moon. –  Kent Fisher Feb 15 '12 at 22:21
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This can be quite useful even without the elaborate procedure: 1. Notice the orientation of the big dipper at some reference time; 2. Remember that it takes 24 hours to go through a full rotation. –  nibot Feb 16 '12 at 20:59
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If you know the stars, look at the constellation in the Zodiac. For example, according to astrology, now the sun is in Aquarius. In reality, it is one constellation backwards, so it is in Capricorn. Then, look at the starred sky - which constellations of the Zodiac can you still see? E.g. if you see (left to right), Leo, Cancer, Gemini, Taurus and horizon, then you know that there is still Aries, Pisces, Aquarius below and the sun is now "at the end" of Capricorn. So actually if you count cca 24/12 = 2 hours per Zodiac sign, then it is cca 6 hours after sunset. This is simple, logical, doesn't require moon, nor any "calibration".

But if you look at the starred sky every night, it is much easier not only to orientate yourself, but also to guess time. E.g. in winter the Orion dominates the evening sky. So if you see it leaning steeply to his right side and low above horizon, you plus-minus know where the west lies, and also it indicates a coming midnight. And if you see the Lion leaning his head just above horizon, you know that you really spend too much time in the pub - the new day will start soon. :) Of course this more or less is valid now, in winter - the situation changes over year, so you must regularly calibrate your star clock. :-)

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I do not feel that the answer, as is, is usable. However this is a VERY long and complex topic I understand. Is there any link you can provide which will give (in addition to your answer) the full details of using the night sky for time and navigation? –  Russell Steen Feb 26 '12 at 5:46
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An astronomer with a protractor and pocket-calculator can figure almost anything. Unfortunately, though, astronomers don't fit very well in survival kits (and grumble when you try to stuff them in there). So normal people are handicapped, for sure, but we can still say some basic things.

A full moon is opposite the sun, so if you know it's a full moon tonight, and if the full moon is directly south, then you know it's right around solar midnight, that is, 12 hours past solar noon. It's not exact, though, because the moon doesn't trace through the sky at exactly the same speed as the sun.

If you're familiar with the area and practiced, you could use the full moon to estimate hours other than midnight and not be far off, say, within an hour. Note, however, that a sundial (moondial?) trick wouldn't work without some fiddling because the moon is only rarely in the same plane as the sun. The gnomon would need to be adjusted to the right angle - something an astronomer could figure.

Similarly, if a First-Quarter moon is directly south, then you know it's right around 6 hours past solar noon. If a Full moon is south then it's 12 hours past noon, and if a Third-Quarter moon is south then it's 18 hours past noon, that is, 6 hours before noon. A New moon that's south is zero hours past noon, so you can't see it, and it's useless for telling time at night.

None of the above is very precise, but it can give you a rough guide to use when you wake up in the middle of the night for a bathroom break and forgot your watch back in the sleeping bag.

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+1. I especially appreciate the image of trying to stuff an astronomer into an emergency kit (hehe), and also I did not know that a first-quarter, full, or third-quarter moon could help you estimate time like that. –  Clare Steen Feb 15 '12 at 16:24
    
Thanks. Here's a helpful diagram I found that presents this pretty well. upload.wikimedia.org/wikipedia/commons/7/72/… –  Kent Fisher Feb 15 '12 at 18:14
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