For your informaiton, Daylight Saving Time begins for most of the United States at 2 a.m. on the first Sunday of April. Time reverts to standard time at 2 a.m. on the last Sunday of October. To remember which way to turn the clocks, "Spring ahead, and fall back".

You probably have heard the expression, "Time is of the essence", well, that is particularly true for astronomical obervations. Since astronomers are scattered all over the world, a standard time needed to be adopted. So the IAU (International Astronomical Union) has decided to use UTC as its standard. When an astronomical event occurs, the time of the event is of critical importance. Often you will see UTC used for the timing of an event in the future. For example, "The occultation of a star by Jupiter will occur at 10 March at 02:00 UTC".(this is hypothetical. I don't know if it is true). If you want to watch the occultation, you will need to convert the UTC to local (civil) time.

When you make an observation, you will need to convert from your local civil time to UTC. Especially if you are going to report it to a governing body, such as AAVSO (American Association of Variable Star Observers). In your notes, you should report both local and UTC times.

Standard time in the U.S. and its territories is observed within eight time zones. Standard time within each time zone is an integral number of hours offset from UTC.

To obtain	Atlantic Daylight Time	subtract 3 hours from UTC
	Atlantic Standard Time	subtract 4 hours from UTC
	Eastern Daylight Time	subtract 4 hours from UTC
	Eastern Standard Time	subtract 5 hours from UTC
	Central Daylight Time	subtract 5 hours from UTC
	Central Standard Time	subtract 6 hours from UTC
	Mountain Daylight Time	subtract 6 hours from UTC
	Mountain Standard Time	subtract 7 hours from UTC
	Pacific Daylight Time	ubtract 7 hours from UTC
	Pacific Standard Time	subtract 8 hours from UTC
	Alaska Daylight Time	subtract 8 hours from UTC
	Alaska Standard Time	subtract 9 hours from UTC
	Hawaii-Aleutian Daylight Time	subtract 9 hours from UTC
	Hawaii-Aleutian Standard Time	subtract 10 hours from UTC
	Samoa Standard Time	subtract 11 hours from UTC

Thus, 11:00 UTC is the same as 5:00 CST. When converting zone time to or from UTC, dates must be properly taken into account. In our example above, 10 March at 02:00 UTC is the same as 9 March at 18:00 PST. (6:00 P.M.) Therefore you know that you will not be able to observe the occultation. It will not be dark enough. The table can also be used to determine the difference between the time observed in any two zones. For example, the table shows that Eastern Standard Time is three hours "ahead" of Pacific Standard Time.

History of Standard Time in the U.S.

Standard time in time zones was instituted in the U.S. and Canada by the railroads on 18 November 1883. Before then, time of day was a local matter, and most cities and towns used some form of local solar time, maintained by some well-known clock (for example, on a church steeple or in a jeweler's window). The new standard time system was not immediately embraced by all, however.

Use of standard time gradually increased because of its obvious practical advantages for communication and travel. Standard time in time zones was not established in U.S. law until the Act of March 19, 1918, sometimes called the Standard Time Act. The act also established daylight saving time, itself a contentious idea. Daylight saving time was repealed in 1919, but standard time in time zones remained in law, with the Interstate Commerce Commission (ICC) having the authority over time zone boundaries. Daylight time became a local matter. It was re-established nationally early in World War II, and was continuously observed until the end of the war. After the war its use varied among states and localities. The Uniform Time Act of 1966 provided standardization in the dates of beginning and end of daylight time in the U.S. but allowed for local exemptions from its observance. The act also continued the authority of the ICC over time zone boundaries. In subsequent years, Congress transferred the authority over time zones to the U.S. Department of Transportation (DOT), modifed (several times) the beginning date of daylight time, and renamed the three westernmost time zones.

Time zone boundaries have changed greatly since their original introduction and changes still occasionally occur. DOT issues press releases when these changes are made. Generally, time zone boundaries have tended to shift westward. Places on the eastern edge of a time zone can effectively move sunset an hour later (by the clock) by shifting to the time zone immediately to their east. If they do so, the boundary of that zone is locally shifted to the west; the accumulation of such changes results in the long-term westward trend. The process is not inexorable, however, since the late sunrises experienced by such places during the winter may be regarded as too undesirable. Furthermore, under the law, the principal standard for deciding on a time zone change is the "convenience of commerce." Proposed time zone changes have been both approved and rejected based on this criterion, although most such proposals have been accepted.


	Riverside Astronomical Society * Bringing the Joys of Astronomy to Southern California * Founded 1957 *

Universal Time vs. Local Time

History of Standard Time in the U.S.