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When talking about the duration of sunset, we are speaking of the period of time from when the lower limb of the Sun first hits the horizon to when the top limb of the sun hits the horizon. Yes, the duration of sunset varies over the course of the year.
In a nutshell, the year's fastest sunsets (and sunrises) happen around the September and March equinoxes. The slowest sunsets (and sunrises) happen around the June and December solstices. Sunset photo, courtesy of 2-Dog-Farm's Photostream.
What Happens on the Equinox?
On the day of an equinox, the Sun is at zenith (straight overhead) at the Earth's equator, and everyone sees the Sun rising due east and setting due west all over the world. The Earth's North And South Poles stand as an exception to this rule, because east and west are non-existent at the poles. From the North Pole, every direction is south; from the south pole, every direction is north. At the poles, there is only one sunset and one sunrise during the course of one year.
What Happens on the Solstice?
On the day of the solstice, the zenith Sun is at a maximum distance north or south of the Earth's equator. On the June (or Northern) solstice, the zenith Sun is farthest north for the year, shining directly over the tropic of Cancer. On the December (Southern) solstice, the zenith Sun is farthest south, shining straight above the tropic of Capricorn. (See globe.) On the day of the solstice, the Sun sets farthest north or farthest south of due west.
Places north of the Arctic Circle and south of the Antarctic Circle (see globe) stand as an exception to this rule. On the day of the solstice, the Sun neither rises nor sets, because the Sun is either above the horizon or below the horizon all day long.
Explanation for Fastest Sunset
When the sun sets due west - as it does on the day of an equinox - the setting Sun hits the horizon at the steepest angle possible. Therefore, the fastest sunsets come around the equinox time of the year.
Formula for Fastest Sunset
Although all latitudes have the fastest sunset around the equinox, the duration of sunset varies by latitude. The closer you are to the equator, the faster the sunset. The farther north or south you are of the equator, the slower the sunset. We can calculate the duration for the equinox sunset at any latitude, thanks to the formula given to us by Astronomy Answers (click on 12. Length of Sunrise and Sunset):
128/cosine latitude = duration of equinox sunset/sunrise in seconds
Answer for Fastest Sunset
If you live at 45 degrees latitude (like I do) here is the answer:
|128/cos 45 = duration of equinox sunset|
|128/0.707 = duration of equinox sunset|
|128/0.707 = 181 seconds (3 minutes 1 second)|
Explantion for Slowest Sunset
On the day of a solstice, the Sun sets a maximum distance north or south of due west. When this happens, the sunset hits the horizon at the shallowest possible angle for the year. Consequently, the shallowest angle of the setting Sun gives the year's slowest sunset. Thanks to Astronomy Answers, we also have a formula for the slowest sunset:
Formula for Slowest Sunset
142/cosine (1.14 x latitude) = duration of solstice sunset in seconds
Answer for Slowest Sunset
For my latitude of 45 degrees, the answer is as follows:
|142/cos (1.14 x 45) = duration of solstice sunset|
|142/cos 51.3 = duration of solstice sunset|
|142/0.625 = 227 seconds (3 minutes 47 seconds)|
copyright 2011 by Bruce McClure
|Dr. Bob's Duration of Sunset Equation|
August 2011 Feature * October 2011 Feature