Asked in ScienceAstronomyTimeStars
How can the sun be used to measure or estimate time of day?
September 11, 2008 11:05AM
# Make note of the position of the sun. If you're in the northern hemisphere, face the south; if you're in the southern hemisphere, face north. (If you don't have a compass, use one of these techniques.) In other words, look at the equator--that is the line the sun generally follows in the sky. It always rises in the east (which is to your left if you're facing south, to your right if you're facing north) and sets in the west.
#* If the sun is in the exact center of the sky, it's exactly noon. A nickname for this is "high noon" because the sun is at its highest at noon and the time will be 12:00 PM, but this assumes no daylight saving time, and that you're in the middle of your time zone. For instance, in Salt Lake City, Utah (US), "high noon" is actually at 1:30 PM in the summer because there's an hour added by daylight saving and another thirty minutes added because the city is that time distance (west) away from the center of the time zone. #* If the sun is not in the exact center, you will have to do more figuring. If it is morning, the sun will be in the eastern half of the sky. If it is afternoon, the sun will be in the western half. You can use fractions to divide the sky into hours, and find the approximate time. # Estimate the number of hours between sunrise and sunset. This varies depending on the season and location. Winter days are shorter than summer ones: about ten and fourteen hours, respectively. Spring and fall days tend to be about twelve hours long, especially close to the equinox (late March or late September). # Divide the sun's path into segments. If you're looking towards the equator, you can imagine the sun following an imaginary arc from east to west, beginning and ending at the horizon, even if it's obscured. Visualize dividing that arc into equal segments; the number of segments should equal the number of hours in the day. If you know there are twelve hours in a day, you would divide the arc into twelve equal parts, six on the eastern half and six on the western half.
#* If you're having trouble visualizing the sky in segments, you can use your hand or fist to "measure" segments. Going hand over hand, count the amount of fists from one end of the arc to the zenith (the highest point of the sky). Take that number as half of a day. For instance, if you counted 9 fists, and you know the day is 12 hours long, nine fists would be equal to six hours. To figure out how much time each fist represents, divide the number of hours by the number of fists. One fist, then would equal 6 divided by 9 - or about 2/3 hours (40 minutes). This is your hours-per-fist. # Determine which segment the sun is in. Starting in the east, count how many segments there are before you get to the segment that the sun is in. That will tell you how many daytime hours have passed. The segments that the sun hasn't touched yet indicate how many daytime hours are remaining. If you know the time of high noon, sunrise, or sunset in your area, then you can approximate the current time.
#* Using the Salt Lake City example given earlier, let's say there are fourteen segments (because it's summer) and the sun is on the ninth segment (from the east). The eighth segment (right after the highest point) begins at 1:30 PM. The ninth segment begins one hour after that, so if the sun is in the ninth segment, it's probably between 2:30 and 3:30 PM. If the sun was in the sixth segment, the time would be between 11:30 AM and 12:30 PM. With practice, you'll be able to estimate time without consciously dividing the sky. #* If you used the fists method, count the number of fists from the eastern end of the arc to the sun. Multiply that number times the hours-per-fist measurement. Let's say you counted three fists from east to west. Three hours times forty minutes equals 120 minutes, or two hours. So it's been two hours since sunrise. If you know the time of sunrise in your area and season, you can approximate what time it is.