ASTRONOMY READER TIME 2.1 TIME Astronomical time Time is defined by astronomical cycles. The day is the average time from noon to noon (the solar day). The month was originally based on the average time from new moon to new moon (the lunar month). The year is the amount of time from spring equinox to spring equinox (the tropical year). This time is very nearly the time it takes the Earth to orbit around the Sun. Sun time Clock time is based on the sundial, which tells the position of the Sun in the sky. True noon is the time when the Sun is highest in the sky; that is, the moment when it crosses the meridian, the line that goes north to south through the zenith. "Meridian" comes from the Latin word for "noon", meridies. Sun time is the number of hours since noon. It turns out that the Sun is an imperfect clock, however. As it moves north and south of the ecliptic, it speeds up at the equinoxes. and slows down at the solstices. For timekeeping purposes, astronomers define an imaginary sun that moves along the celestial equator at a constant rate. This is the mean sun, where "mean" means "average". Local mean time is the time defined by the mean sun; that is, the hours since the mean sun crossed the meridian. Local mean time is the true local time. Prior to 1883, clocks were set to local mean time.
2.2 TIME ASTRONOMY READER World Time Since the world is round, noon doesn't happen everywhere on the world at the same time. Indeed, it isn't daytime everywhere in the world at the same time. Since the Earth rotates to the east, noon occurs earlier is places to the east of where you are. It occurs later in places farther to the west. For example. every day noon occurs first in Florida, three hours later in California, and a couple of hours later in Hawaii. Recall that your location on the Earth is specified by your latitude and longitude. Your longitude is how far east or west you are. (Your latitude is how for north or south of the equator you are.) Every place at the same longitude experiences noon at the same time. Perhaps for this reason, the lines of longitude are called meridians. By international agreement, the Prime Meridian is the line of longitude that runs through the Royal Observatory in Greenwich, England (a suburb of London). Longitude is measured east or west of the Prime Meridian. Los Angeles, for example, is 118 west of the Prime Meridian. Greenwich Mean Time (GMT) s the local mean time on the Prime Meridian. It is used as a world-wide time standard. Astronomers use GMT for telling when astronomical events occur. Another word for GMT is Universal Time (UT). There are different technical ways of defining UT. The one that is currently used to define the official time is called Co-ordinated Universal Iime (UTC). The official government time is based on UTC. UT is usually expressed in 24-hour time, rather than am/pm time. For example, 1:00 am is 1:00, but 1:00 pm is expressed as 13:00. Local Time Prior to the industrial age, clocks were set to Local Mean Time in each town or city. Since LMT is later to the east and earlier to the west, every town's clock was set to a different time. Every degree of longitude you go to the east, LMT is 4 minutes later. Every degree of longitude you go to the west, LMT is 4 minutes earlier. A degree of longitude is roughly 50 miles. There was no problem because in those days travel was slow. Only if you undertook a long journey would you find that you had to reset your watch every now and then if you were wealthy enough to own a watch. The invention of the railroad changed all that. The railroad enabled people to travel from town to town in a few hours. Every time they came to a new town, they would have to reset their watches. This was a minor problem for travelers but a major source of confusion for the railroads, who needed to maintain a schedule. It was too inconvenient for railroad conductors to reset their watches every time they came to a new town. So the railroads began the use of railroad time, the use of one time standard for the railroad. This time standard was usually the LMT in the railroad's hub city. In the U.S., the major hub citites were New York, Boston, Washington,
ASTRONOMY READER TIME 2.3 Chicago, St. Louis, San Francisco, and a few others. Railroads in California all used San Francisco time. Tables were devised to enable travelers to convert from one time to another. For example, travelers read that when it is 12:00 noon in Washington, it is 12:12 pm in New York and 9:02 am in Sacramento. They were then breezily told that "By an easy calculation, the difference in time between the several places above named my be ascertained." In practice, travelers would change their watches when they switched from one railroad line to another. Standard Time As the railroads continued to expand, the proliferation of different times used became more and more confusing. During the 1870s, several individuals in the U.S. and Canada proposed dividing North America into four or five standard time zones. Within each zone, all cities would observe the same time. The U.S. Naval Observatory, which was responible for keeping time for the federal government, proposed a single time zone for the entire nation, which would make every clock in the nation read Washington local mean time. If that had come to pass, noon would occur when the clock read about 3pm in California. Fortunately, the government was in a state of confusion following the assassination of President Garfield in 1881 and took no action. It was the railroads that finally implemented standard time. The railroad companies had formed an co-ordinating organization, the
2.4 TIME ASTRONOMY READER General Time Convention, to co-ordinate schedules and times. The president of this board, William Allen, came up with the scheme that was finally adopted. In his scheme, North America was divided into five time zones: Atlantic, Eastern, Central, Mountain, and Pacific. The boundaries between the zone, which were the most contentious part of the business, were set to minimize disruption for the railroads. That meant that for the most part the boundary was put at the points where two different railroad lines met. The time in each zone was offset by an even number of hours from GMT: Atlantic time is GMT minus 4; Eastern is GMT minus 5, and so forth up to Pacific Standard Time, which is GMT minus 8 hours. Most railroads implemented the plan on Sunday, November 18, 1883, when most of the clocks in the nation were shifted to one of the standard times. The time in each zone is the LMT on a particular meridian of longitude called the standard meridian. Since LMT changes 1 hour in 15 of longitude, the standard meridians are 15 apart. For example, the standard meridian for the Pacific Standard Time one is 120 W. This meridian forms the border between northern California and northern Nevada. In southern California it runs through Santa Barbara: PST is Santa Barbara time. Los Angeles is east of the standard meridian. Therefore, LMT is later than PST here by about 7 minutes. That is, the Sun rises and sets here about 7 minutes earlier than it does in Santa Barbara. Standard time zones are assigned an identifying letter for global use. GMT is Z (Zulu in military jargon). The zones east of the Greenwich Meridian are denoted A. B, C, etc. The zones to the west increase to the west: N, O, P, etc. In each country there are local names for the zones. Pacific Standard Time is U time. Pitcairn Island, in the South Pacific, also observes U time, but they don't call it Pacific Standard Time. Time zones in the U.S. and Canada: Code Name Time Standard meridian Q Atlantic GMT - 4 hr 60 W R Eastern GMT - 5 hr 75 W S Central GMT - 6 hr 90 W T Mountain GMT - 7 hr 105 W U Pacific GMT - 8 hr 120 W V Alaska GMT - 9 hr 135 W W Hawaii GMT - 10 hr 150 W
ASTRONOMY READER TIME 2.5 Daylight Saving Time Most of the U.S., Canada, and Mexica observes Daylight Saving Time (DST). This means that clocks are moved forward one hour during summer actually, for more than half the year. Currently, DST is observed from the second Sunday in March to the first Sunday in November. The purpose of DST is to provide more hours of daylight in the evening. By moving the clock forward one hour, the Sun sets one hour later by clock time. It doesn't really set any later, the LMT of sunset remains the same. DST isn't observed during the winter because it makes sunrise occur one hour later according to the clock. In winter, this would result in people gaing to school and word before sunrise. The effect of DST is to switch to the next time zone to the east. That is to say, is summer, California observs T time (Mountain Standard Time). It's called Pacific Daylight Time (PDT), but it's really MST. Contrary to popular opinion, California does not observe Pacific Standard Time in the summer; in fact, it is on PST for only about 4 months of the year. Most of the year we are on Mountain Standard Time! Not all parts of the nation observe DST. Arizona does not (except for the Navajo Indian Reservation). This means that in the summer clocks in Arizona read the same as clocks in California; in winter they agree with clocks in New Mexico. The parts of the U.S. that lie in the tropics Hawaii, Puerto Rico, the U.S. Virgin Islands, and Samoa likewise do not change their clocks. Canada as a rule follows U.S. practice. Europe, where Daylight Saving Time is simply called Summer Time, changes their clocks the last Sunday in March and the last Sunday in October. [9 sept. 2013]