We've turned the corner into summer. For the northern hemisphere, daylight will still linger long into the evening, but the days will slowly begin to shorten. When exactly will it be dark?
Because of Earth's tilt on its axis, the hemisphere receiving the Sun's most direct rays experiences longer days and shorter nights. In the northern hemisphere, the longest day of the year is the June solstice (this year it occurred on June 20, 2016). The shortest day in the northern hemisphere (and conversely, the longest in the southern hemisphere) is the December solstice (which will be December 21, 2016).
But the differences in day length aren't equally distributed. At the equator, days and nights are of essentially equal duration year round. For example, in Quito, Ecuador, which sits on the equator, the sun rose at 5:58 a.m. and set at 6:05 p.m. on June 20, 2016). On the December solstice, the sun will rise at 5:55 a.m. and set at 6:02 p.m.
As one moves farther from the equator, differences in the length of the day become pronounced. Above the Arctic Circle or below the Antarctic Circle the sun never sets in the summer and never rises in the winter. For example, in Barrow, Alaska (Longitude W156° 47', Latitude N71° 17') the sun rose at 2:47 a.m. on May 10, 2016. It won't set again until 2:19 a.m. on August 1, 2016. As winter approaches and the amount of daylight diminishes, the sun will set at 1:31 p.m. on November 18, and it won't rise again until 1:17 p.m. on January 22, 2017.
Earth also revolves about its axis, and for this reason, sunrises and sunsets move progressively from east to west. The world's time zones were established to standardize the changes in hourly increments so that scheduling could more easily be coordinated. Because people have become accustomed to this time-keeping strategy, they sometimes forget that the exact moments of sunrise and sunset vary quite a bit from place to place even within the same time zone.
In honor of the American Independence Day, a day when many people in the United States eagerly await nightfall so they can enjoy celebratory fireworks, here's a look at when it will actually get dark in three different cities on July 4, 2016. These examples are based on Eastern Daylight Time, but the same principles play out irrespective of time zone.
Boston, Massachusetts (Longitude W71° 5', Latitude N42° 19'), sits near the eastern edge of the Eastern Time Zone, almost half way between the equator and the pole (Latitude N45° would be exactly half way). Sunset will occur at 8:24 p.m., and the end of civil twilight (when the sun sinks six degrees below the horizon) will occur at 8:58 p.m.
Grand Rapids, Michigan (Longitude W85° 40', Latitude N42° 58'), located near the western edge of the Eastern Time zone, is still pretty much half way between the equator and the pole. Because it is so much farther west (slightly more than fourteen degrees of longitude), sunset in Grand Rapids won't occur until 9:25 p.m. The end of civil twilight isn't until 9:59 p.m.
Miami, Florida (Longitude W80° 13', Latitude N25° 47') lies between Boston and Grand Rapids in terms of longitude (east and west), but the more southerly latitude (less than a third of the distance between the equator and the pole) leads to smaller differences in the length of summer and winter days. Therefore, instead of sunset falling between the times for the more northern cities, it actually precedes them. Sunset in Miami on July 4, 2016, will occur at 8:16 p.m. and the end of civil twilight will be 8:42 p.m.
If you want to find the times of sunrise and sunset for your location, the U.S. Naval Observatory offers a tool that will calculate it for you, "Complete Sun and Moon Data for One Day." Use Form A for U.S. Cities or Towns and Form B for other locations around the world.
This article originally appeared in the July 2016 issue of Observations: A Pier Press® Newsletter. To view the complete newsletter, visit our online archives. Also, please consider becoming a subscriber so that future editions can be delivered directly to your inbox. Subscriptions are free!