2011-06-21

Survival Guide: Tell time using the Sun or Moon

The June solstice occurred today (2011-06-21) at 17:16 UTC. A solstice is when "the Sun's apparent position in the sky reaches its northernmost or southernmost extremes" ["Solstice" - Wikipedia]. For the Northern Hemisphere of our planet, that means it is the middle of traditional Summer. For the Southern Hemisphere, the middle of traditional Winter. Knowledge of today's solstice will be useful in the first of the survival guide series when "telling time using only the Sun or Moon".

CAUTION: Daylight saving time can cause confusion in some of the calculations, so I recommend ignoring DST entirely until you have the final times you are seeking. Only then should you try to adjust it according to the strange daylight "saving" time system or make any adjustments based on your preferences or location.

ESTIMATE DAY LENGTH
The length of the day depends almost entirely on Latitude. (Your location North or South of the Equator.)
- At the Equator, the day is always twelve (12) hours and the night is also twelve.
- During the equinox (mid-Spring and mid-Autumn), the day is roughly twelve (12) hours, regardless of Latitude.
- During the solstice (mid-Summer and mid-Winter), the day is roughly fourteen (14) hours (Summer) or ten (10) hours (Winter) at the "midpoint" between the equator and a pole.
- During the solstice, the day is either all hours (Summer) or non-existent (Winter) at the poles.

I try to remember those as the "10, 12, 14 facts" - 10 in the Winter, 12 in the Spring, 14 in the Summer.

You can roughly guess how long the day is at your location using the "10, 12, 14 facts" I listed above and a rough guess at your Latitude. (Keep in mind the days are longer closer to the pole in Summer and shorter closer to the pole in Winter. Also, remember the "10, 12, 14 facts" apply best at the approximate "midpoint" between a pole and the equator.)
CALCULATE TIME OF SUNRISE AND SUNSET
STEP 1: Find your estimated day length.
STEP 2: Divide the STEP 1 result by 2.
STEP 3: Subtract the STEP 2 result from 12 to get the time of sunrise (in hours). Add the STEP 2 result to 12 to get the time of sunset.
STEP 4 (optional): Convert to AM/PM, if you must. Personally, I think the 24hr clock is cleaner.

(Do not adjust for daylight saving time, if you intend to use these times in other calculations.)
NOTE: Knowing how to roughly estimate the sunrise and sunset times can be helpful even if you have a clock. Sometimes it's just nice to know how much time you might have remaining in your day or night.

ESTIMATE THE TIME OF MOONRISE AND MOONSET USING EARTH'S SHADOW
(We use the Earth's shadow on the Moon to estimate the Moon's rise and set times based on sunset and sunrise. Since a full Moon has no shadow on it, it needs no adjustments; a full Moon rises at sunset and sets at sunrise.)
STEP 1: "Read" the Moon from its right edge to its left. The right edge is sunset, the left is sunrise.
STEP 2: Imagine that the moon is divided (from top to bottom) into a number of segments equal to the number of hours in the night.
STEP 3: Estimate how many of those segments are covered in the Earth's shadow.
STEP 4: Look at where the Earth's shadow falls on the Moon. Is the shadow to the left or the right of the light?
- If the shadow's on the right, add the STEP 3 result to the time of sunset to get the time of moonrise.
- If the shadow's on the left, subtract the STEP 3 result from the time of sunrise to get the time of moonset.
STEP 5: The moon transits the sky in approximately 12 hours.
- If you know the Moon's rise time, add 12 hours to estimate the set time.
- If you know the Moon's set time, subtract 12 hours to estimate the rise time.
ESTIMATE THE TIME USING THE SUN OR MOON
STEP 1: Pick an "orb" that you can see. Sun or Moon?
STEP 2: Imagine the path of the orb across the sky. (Remember, depending on your location and the season, the orb may not ever pass directly overhead; it may only arch in some portion of the sky.)
STEP 3: Divide the path of the orb into segments of the same number as the hours of its transit. (For the Sun, this is day length in hours. For the Moon, this is 12 hours.)
STEP 4: Note the segment which the orb is currently in, counting from left (rise) to right (set).
STEP 5: Add the STEP 4 result to the rise time of the orb.
STEP 6: This is the estimated current time.

Knowing how soon dawn or dusk will occur is something which I consider a necessary life-or-death survival skill. (We'll discuss this further in future guides.) Beyond that essential skill, knowing how to tell time using only the natural environment is, at the least, a trick which can entertain a few interested friends.
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