How to tell the time at night

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.

posted almost 13 years ago

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G. Jay Kerns‭

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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. :-)

posted almost 13 years ago

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Tomas‭

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What is the simplest way to approximate time after the sun has set?

If all you want is a rough approximation, this this is extremely easy. For example, if you wake up in the middle of the night, you may just want to be able to look up and get some idea of how much time has passed and how long it is until dawn. Once it gets dark, pick anything in the sky that's easily visible on that particular night, preferably something that's somewhere over in the eastern side of the sky. This could be any object such as the moon, a planet, or a constellation with a distinctive shape. You don't even have to know what it is, as long as you're able to recognize it again later in the night. In 6 hours, that object is going to travel 90 degrees across the celestial sphere. So for example if you wake up and notice that it's traveled about 45 degrees, then about 3 hours have passed.

posted about 10 years ago

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Ben Crowell‭

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There are very simple ways to use both the moon and the stars to tell time at night. The moon can be used without complicated charts, and the stars can be used with a simple star dial. Here are links: Telling time with the moon without complicated charts, like the one here

To tell time using the stars (if you are a newbie and haven't observed the big dipper in relation to Polaris very often) you can print and use this very simple stardial to tell time at night. Here is the link to 2D-Nocturnal-Celestial-Stardial

posted about 10 years ago

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WayfinderAli‭

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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.

posted almost 13 years ago

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Big General‭

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