Earth Science - Other

# How to Calculate the Time a Sunset or Sunrise will Occur

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Calculating the times of sunrise and sunset may seem like a daunting challenge to someone unaccustomed to sky watching, but with some basic knowledge and a little practice, it is a relatively easy skill to learn. This article will help you get started.

Let's start with the basic knowledge. First, there are two factors which determine day length – your distance from the equator and the season of the year. At the equator itself, days and nights are 12 hours long every day of the year. The farther you get from the equator, however, the longer the daylight hours will be during summer and the shorter they will be during winter.

This phenomenon occurs because the Earth is tilted 23.5 degrees with respect to its rotational axis. Areas north of the Arctic circle (66.5 degrees N latitude) or south of the Antarctic circle (66.5 degrees S latitude) experience the midnight sun during the summer and a corresponding period of perpetual darkness during the winter.

Next, imagine the sky as a large semicircle extending 180 degrees from horizon to horizon. During an ideal 12 hour day, the sun will move westward 15 degrees each hour. To approximate 15 degrees of sky, spread your fingers apart and extend your arm straight out. The distance between the tip of your thumb and tip of your pinkie will be roughly 15 degrees.

If you have a compass, determining the cardinal directions is straightforward. Once you have determined west, place your pinkie over the sun’s disk, and measure the distance to the western horizon in 15 degree increments. During spring and fall, 15 degrees corresponds to roughly one hour. During winter, the sun’s path through the sky is shorter, and the sun will cover 15 degrees in less time (30 – 45 minutes) depending on your latitude. In the summer, the sun takes a correspondingly longer amount of time to move 15 degrees, from just over an hour in the tropics to nearly two hours at high latitudes.

Determining east and west without a compass may require some practice; however, the following is a tried and true method. Insert a long stick into the ground and mark the tip of its shadow with a stone or other small object. Wait for 10 or 15 minutes then repeat the above procedure. Take notice of where the stick’s shadow has moved relative to its position 15 minutes earlier. If the shadow has moved to the right of the first stone, you are facing due north. Conversely, if the shadow has moved to the left of the first stone, you are facing due south. With a little practice, you can estimate the time of sunset to within 15 minutes or so.

Sunrise

This is even easier to calculate than sunset. As soon as dawn breaks, you know the sun is no more than 18 degrees below the eastern horizon. At temperate latitudes, this corresponds to a time interval of about one hour. In the tropics, dawn and dusk are shorter, while at polar latitudes, twilight lasts for several hours. Sunrise tends to occur a few minutes earlier each day during the winter and spring and a few minutes later each day during the summer and fall.

The Analemma

Because Earth’s orbit around the sun is not a perfect circle, the position of the sun at noon will shift slightly over the course of the year. For observers at mid-northern or mid-southern latitudes, the sun’s path traces out a figure 8 shape called an analemma, which reveals the discrepancy between sun time (actual noon) and clock time in a given time zone. On two days every year, the vernal and autumnal equinoxes, sun time and clock time are synchronized everywhere in the world. The equinoxes correspond to the node, or intersection point, in the figure eight.

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