Time Zones and Daylight Saving Time
For purposes of this article, the term "midnight sun" refers to the phenomena of 24 consecutive hours of sunlight north of the Arctic Circle or south of the Antarctic Circle. There are, however, some instances which are sometimes referred to as "midnight sun", even though they are in reality due to meandering time zones and the observance of daylight saving time. For instance, in Fairbanks, Alaska, which is located south of the Arctic Circle, the sun sets at 12:47 a.m. on the summer solstice. This is because Fairbanks is one hour ahead of its idealized time zone (due to meandering for the purpose of keeping most of the state on one time zone) and because the state of Alaska observes daylight saving time. This means that solar culmination occurs at roughly 2 p.m. instead of at 12 noon, as in most places.
If a precise moment for the genuine "midnight sun" is required, the observer's longitude, the local civil time and the equation of time must be taken into account. The moment of the sun's closest approach to the horizon coincides with its passing due north at the observer's position, which occurs only approximately at midnight in general. Each degree of longitude east of the Greenwich meridian makes the vital moment exactly 4 minutes earlier than midnight as shown on the clock, while each hour that the local civil time is ahead of coordinated universal time (also known as GMT) makes the moment an hour later. These two effects must be added. In addition the equation of time (which depends on the date) must be added: a positive value on a given date means that the sun is running slightly ahead of its average position, so the value must be subtracted.
As an example, at the North Cape at midnight on June 21/22, the east longitude of 25.9 degrees makes the moment 103.2 minutes earlier by clock time; but the local time, 2 hours ahead of GMT, takes it 120 minutes later by clock time. The equation of time at that date is -2.0 minutes. So the sun's lowest elevation occurs 120 - 103.2 + 2.0 minutes after midnight, i.e. at 00.19. On other nearby dates the only thing different is the equation of time, so this remains a reasonable estimate for a considerable period. The sun's altitude remains within half a degree of the minimum of about 5 degrees for about 45 minutes either side of this time.
Read more about this topic: Midnight Sun
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