Daylight Saving Time is observed, for the most part, in latitudes distant from the equator where sunlight varies considerably during the year. Most tropical countries and locations stay on standard time all year, because sunrise and sunset vary little.
The start and stop of Daylight Saving Time, where it is observed, is set by the laws of the countries involved. In the United States, DST starts at 2:00 AM on the first Sunday in April, and ends at 2:00 AM on the last Sunday in October.
Daylight Saving Time does not actually save any daylight. No political decision actually adjusts the rotation of the earth. Daylight Saving Time (or Summer Time) just shifts the hours of daylight to better match the waking hours of local residents.
During parts of the year in some countries, usually in the summer, laws have been enacted to advance the clocks one hour ahead of the standard time normally used. This is called Daylight Saving Time, Daylight Time, Summer Time, or Advanced Time.
For events that need to be timed and coordinated world wide, the standard time of the prime meridian is used as a reference. There are a number of names for this time - Coordinated Universal Time (UTC or UT), Greenwich Mean Time (GMT), Zulu Time (Z).
Local solar time, as shown by a sundial, is too location-dependent to be practical for a world where events need to be timed to the second over a wide area. At temperate latitudes, local solar time varies about four seconds for each mile east or west.
Suppose you went to the North Pole or to the South Pole, but did not know which. Both are icy and cold. How could you tell? The North Pole on any rotating body is the pole where objects in the sky - such as sun, moon, and stars - move clockwise.
In the tropics (between the Tropic of Capricorn and the Tropic of Cancer) the sun moves directly overhead. This happens about March 21 and September 23 at the Equator, and on June 21 at the Tropic of Cancer, and on December 21 at the Tropic of Capricorn.
At the North Pole during the months when the sun is above the horizon a shadow moves in a circle (clockwise). At the South Pole, the shadow moves in a counterclockwise circle. If you are stranded at a pole, now you know how to figure which one!
A moondial wouldn't be a very practical instrument. You can't see the moon's shadow in the daylight, and on nights when the moon is up, you would need special tables to convert the shadow's position into local time.
The design of a sundial is influenced by location-dependent factors. These factors include the latitude of the location and the number of degrees east or west of the center of the location's time zone, and the time of year it is most likely to be used.
The accuracy typical backyard sundial may be improved by calibrating it - marking the position of the shadow at various times of the day. The most accurate method is to make a separate calibration plate for every day of the year.
A sundial gives local solar time, which may differ significantly from the current legal time at a location. Local solar time varies approximately four minutes per degree of longitude, as the Earth rotates one degree every four minutes (15 deg. per hour).
After the earth completes one rotation with respect to the stars in 23h 56m 4s, a "sidereal day", because of its motion around the sun, it must continue turning another 3m 56s, on average, to face the sun again. A "solar" day is 24h 00m 00s.
The "fixed" stars are called that because they move almost impeceptibly with respect to each other during a person's lifetime. The earth rotates with respect to the "fixed" stars once every 23h 56m 4s, which is the length of a "sidereal" day.
If a star cast enough light to form a shadow, it would be possible to make a "stardial" that would keep better time than a sundial. Because of the earth's elliptical orbit around the sun, solar time is less regular than star time, called "sidereal" time.
A sundial has a shadow with fuzzy edges, making it hard to read precisely. The fuzziness is caused by the sun's size. The edge of the sundial's post blocks light from one side of the sun, but light from the other side of the sun gets by.
