Time Zones
The concept of time is built into the biological rhythm of living things. In this scheme, time and sunlight are inextricably linked; Morning glories open and roosters crow at dawn, night-blooming jasmine unfurls and fire-flies begin their amatory signaling at dusk. Time operates on a larger, seasonal scale as well. Geese and other birds migrate at the same time each year; bears hibernate at the onset of winter.
Early humans responded to the same kinds of biological rhythms, but over time they developed ways to standardize measures of time in order to predict and gain control over events. The positions of the sun and the other stars in the sky during Earth's rotation provide the foundamental unit; the day. The Earth does not rotate at a uniform speed, so the length of all solar days in a year have been averaged, giving us a 24-hour day.
Back when there was little communication among different areas, each town set its clocks by observing the sun's position. When it was noon in Washington, D.C., it was 12;12 p.m. in New York City. As transportation systems advanced, especially the railroads, the need for a standardized time system increased . In the United States, this happened in 1883, when the country's 60 degrees of longitude was divided into four time zones. All the localities in a zone would observe the time at the center of the zone. These four zones still broadly define today's eastern, central, mountain, and pacific time zones.
Soon a system of global time zones was created, using the longitudinal line that runs through Greenwich, England, as the prime meridian. Since then, continuing small adjustments have been made to national and international time-zone systems.
- Where does One day End and the Next Begin?
In theory, the international date line represents the 180 degrees meridian of longitude. When this meridian is crossed from west to east, the date advances by one day. In crossing the line in the opposite direction, from east to west, the date reverts by one day. In reality, however, a number of situations cause adjustments to the international date line. political frontiers, such as Russia's Kamchatka peninsula, push out the line to keep a nation's territory intact, datewise. Similarly, the line takes detours to keep together all islands in some pacific Ocean groups. These local modifications move the international date line off the 180 degrees meridian for much of its length.
- Can Clocks save Daylight?
During World War I, a number of countries pushed their clocks forward an hour to extend daylight as an energy-saving measure. This concept was expanded during World War II, with many countries observing a onehour advancement year-round.
In the United states, the adjustment of daylight hours lasted continuously, from february 9, 1942, to september 30, 1945. To gain an even greater benefit during the war, Britain instituted a two-hour extension known as ''double summer time'' in the summer that reverted to a singlehour extension in the winter.
Many contries retained a partialyear daylight adjustment after the war, and others adopted it Congressional measures in the United states, beginning in 1966 and continuing to 2005, have instituted the plan nationally. Currently daylight saving time in the U.S. starts the second Sunday in march and ends the first Sunday in November.
Today, some 70 countries worldwide observe summer daylight saving time. Some have adopted and then rejected the plan, including japan and china. Numerous social, economic, and political influences affect the decision to keep, extend, or discontinue daylight saving time.
- Why do we need Leap Years?
Since it takes Earth approximately 365 1/4 days to orbit the sun, an extra day's worth of time accumulates every four years. In a leap year, february receives an extra day to compensate for the difference between the astronomical year and the calendar year.
If no adjustment were made, the calendar and the seasons would drift apart by 24 days every 100 years. The plan of adding a day every four years was devised by the Egyptians. The Romans created a standard leap day, february 29, in 46 B.C.
Adding one day per four years didn't perfectly correct the problem, however, because Earth orbits in 11 minutes and 14 seconds less than 365 1/4 days. A correction, established in 1582, adds a leap day only to century years divisible by 400. Thus 2000 was a leap year, but 2100, 2200, and 2300 will not be.
0 Comments