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Topic: Universal time

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Coordinated Universal Time

Standard time

Civil time

Sidereal time

Ephemeris time

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International Atomic Time

History of standard time in the United States

Solar time

Greenwich Mean Time

Barycentric Dynamical Time

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	Universal Time - Wikipedia, the free encyclopedia
		Universal Time (UT) is a timescale based on the rotation of the Earth.
		It is a modern continuation of the Greenwich Mean Time (GMT), i.e., the mean solar time on the meridian of Greenwich, England, which is the conventional 0-meridian for geographic longitude.
		However, because Universal Time is synchronous with night and day, and more precise atomic-frequency standards drift away from this, UT is still used to produce a correction called leap seconds to atomic time to obtain a broadcast form of civil time that carries atomic frequency.
	en.wikipedia.org /wiki/Universal_Time (1309 words)

	universal time. The Columbia Encyclopedia, Sixth Edition. 2001-05 (Site not responding. Last check: 2007-10-07)
		(UT), the international time standard common to every place in the world, it nominally reflects the mean solar time along the earth’s prime meridian (renumbered to equate to civil time).
		Universal time is also called world time, Z time, and Zulu time.
		Because the rate of the earth’s rotation is gradually slowing, it is occasionally necessary to add an extra second, called the leap second, to the length of the UTC year; synchronization is obtained by making the last minute of June or December contain 61 seconds.
	www.bartleby.com /65/un/univtime.html (377 words)

	Coordinated Universal Time - Wikipedia, the free encyclopedia
		Originally, the local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard at the 1884 International Meridian Conference, leading to the widespread use of Greenwich Mean Time in order to set local clocks.
		International standard UTC time can only be determined to the highest precision after the fact, as atomic time is determined by the reconciliation of the observed differences between an ensemble of atomic clocks maintained by a number of national time bureaus.
		UTC is a hybrid time scale: the rate of UTC is based on atomic frequency standards but the epoch of UTC is synchronized to remain close to astronomical UT.
	en.wikipedia.org /wiki/Coordinated_Universal_Time (1044 words)

	Encyclopedia: Coordinated Universal Time (Site not responding. Last check: 2007-10-07)
		Time zones around the world are expressed as positive and negative offsets from UT. UTC differs by an integral number of seconds from International Atomic Time (TAI), as measured by atomic clocks and a fractional number of seconds from UT.
		Time zones are areas of the Earth that have adopted the same standard time, usually referred to as the local time.
		Times which may be labeled "Universal Time" or "UT" in data provided by the Astronomical Applications Department of the U.S. Naval Observatory (for example, in the annual almanacs) conform to this definition.
	www.nationmaster.com /encyclopedia/Coordinated-Universal-Time (2686 words)

	Time Standards
		Universal Time is the mean solar time of the prime meridian plus 12 hours, determined by measuring the angular position of the Earth about its axis.
		Mean Solar Time is simply apparent solar time corrected for the effects of orbital eccentricity and the tilt of the Earth's axis relative to the ecliptic plane; that is, corrected by the equation of time which is defined as the hour angle of the true Sun minus the hour angle of the mean Sun.
		Coordinated Universal Time (UTC) is a coordinated time scale maintained by the Bureau International des Poids et Mesures (BIPM), which forms the basis of a coordinated dissemination of standard frequencies and time signals.
	www.beaglesoft.com /mainfaqtime.htm (2368 words)

	Space Today Online - time explained - universal time - what time is it?
		Time is a human perception defined as the length of an interval separating two points on a nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.
		A clock is a freestanding instrument that measures and records time, usually displaying the passage of time through a moving pointer on a dial or else by a digital readout.
		Known as Coordinated Universal Time (UTC) and Greenwich Mean Time (GMT), the universal standard time is the mean solar time at the zero-degree meridian at Greenwich, England.
	www.spacetoday.org /Time/TimeExplained.html (905 words)

	CLHS: Section 25.1.4.2 (Site not responding. Last check: 2007-10-07)
		Universal time is an absolute time represented as a single non-negative integer---the number of seconds since midnight, January 1, 1900 GMT (ignoring leap seconds).
		Thus the time 1 is 00:00:01 (that is, 12:00:01 a.m.) on January 1, 1900 GMT.
		Because universal time must be a non-negative integer, times before the base time of midnight, January 1, 1900 GMT cannot be processed by Common Lisp.
	www.lisp.org /HyperSpec/Body/sec_25-1-4-2.html (137 words)

	universal time on Encyclopedia.com (Site not responding. Last check: 2007-10-07)
		UNIVERSAL TIME [universal time] (UT), the international time standard common to every place in the world, it nominally reflects the mean solar time along the earth's prime meridian (renumbered to equate to civil time).
		Pont Alexandre III was built between 1896 and 1900 to commemorate the 1892 French-Russian alliance, and in time for the Universal Exhibition in 1900.
		Pont Alexandre III (background) was built between 1896 and 1900 to commemorate the 1892 French-Russian alliance, and in time for the Universal Exhibition in 1900.
	www.encyclopedia.com /html/u1/univtime.asp (1433 words)

	Time Zones and "Z" Time
		GMT time is also referred to as "Z" or, phonetically, "Zulu" time, for the letter identifying that time zone centered on the Greenwich Prime Meridian (See the time zone labels along the top of the accompanying World Map of Time Zones).
		Civil time zones were initially instituted in the U.S. and Canada in the 1880's by the railroads to standardize time keeping.
		Time in the Z or UTC system is reckoned from the local standard time observed along the Greenwich Meridian.
	www.meteor.wisc.edu /~hopkins/aos100/z-time.htm (746 words)

	Read about Coordinated Universal Time at WorldVillage Encyclopedia. Research Coordinated Universal Time and learn about ... (Site not responding. Last check: 2007-10-07)
		Greenwich mean time, the astronomical basis for civil time.
		Time zones around the world are expressed as positive and negative offsets from UT. UTC differs by an integral number of
		International standard UTC time can only be determined to the highest precision after the fact, as atomic time is determined by the reconciliation of the observed differences between an ensemble of atomic clocks maintained by a number of national
	encyclopedia.worldvillage.com /s/b/Coordinated_Universal_Time (552 words)

	Astronomical Times
		Several important time scales still follow the rotation of the earth, most notably civil and sidereal time, but of these are now derived from atomic time through a combination of earth rotation theory and actual measurements of the earth's rotation and orientation.
		Universal Time (UT1) is a measure of the actual rotation of the earth, independent of observing location.
		It might seem strange that UT1, a solar time, is determined by measuring the earth's rotation with respect to distant celestial objects, and GMST, a sidereal time, is derived from it.
	www.cv.nrao.edu /~rfisher/Ephemerides/times.html (2060 words)

	Space Today Online -- Solar System Planet Earth -- Angkor
		Coordinated Universal Time (UTC) is the modern implementation of Greenwich Mean Time (GMT).
		Coordinated Universal Time (UTC) is the time at Earth's zero or reference meridian, which is located on the globe at Greenwich, England.
		Major users of highly precise universal time include astronomers, spacecraft tracking stations, science labs, military and civilian ships and aircraft, radio and TV stations, seismographers, geologists, power companies and ham radio operators.
	www.spacetoday.org /Time/UTC.html (1293 words)

	Why Time is Absolute, and Relative, But Never Universal
		Time is reckoned by noting the intervals that occur by the motion of material things.
		After every standard unit of time passes, clock A triggers a gunshot aimed at the target on clock B. The observer (or machine/computer) at clock B records the impact and notes the constant arrival of a bullet (assuming the cartridges have exact loads of powder and that other factors remain the same).
		Time is not something a God brought forth with the imaginary 6,000-10,000 year old creation of the universe that is the biblical, or the equally imaginary 10-20 billion year age Big Bang model of the universe.
	home.pacbell.net /skeptica/time.html (3640 words)

	EXPLORES! Basic Skills: Time Conventions (Site not responding. Last check: 2007-10-07)
		Universal time is a world-wide standard of time observation which is based on converting local time anywhere in the world to the time along the prime meridian (0 degE, 0 degW) (see
		A knowledge of a universal system for measuring time allows observations to be made on a "synoptic" (from the Greek 'synoptikos' meaning 'general view of the whole') basis.
		Universal time calculation is based on the 24-hour clock (sometimes referred to as "military time" because of the military's exclusive use of the 24-hour clock).
	www.met.fsu.edu /explores/utc.html (365 words)

	A Guide To Metric Time
		Most proposed day-based decimalized time systems are basically the same in that one tenth of a day is one tenth of a day for all of them.
		This is especially problematic with the metric hour which is almost two and a half times the length of the ABT hour -- a significant period of time for a scheduling mishap.
		The SI second is defined as the period of time that it takes a specific number of cesium isotope radiation emissions to occur such that it is as close to a mean ABT second (1/86400 day) as feasible given the variance of the earth's rotation.
	zapatopi.net /metrictime.html (2483 words)

	MSI – Time Activity: Universal Sundial
		The simple sundial tells time accurately for a specific location, but does not work when the device is moved to a different latitude.
		Instead of building thousands of simple sundials all over the world at every latitude, the universal sundial was developed which can tell time by the sun anywhere on the planet.
		Universal sundials were used through the beginning of the 19th Century.
	www.msichicago.org /scrapbook/scrapbook_exhibits/time/educ_pages/act_univ_sundial.html (545 words)

	Einstein \| American Museum of Natural History
		Time is what clocks measure, ticking off the steady beat of seconds, minutes and hours.
		The notion of the regular passage of time is so ingrained in human consciousness that our languages have developed special ways to distinguish events that occur in the past, present or future.
		Speed is a measure of distance divided by time (for example, kilometers per hour or miles per hour).
	www.amnh.org /exhibitions/einstein/time/revolution.php (286 words)

	Date and Time Formats
		Times are expressed in local time, together with a time zone offset in hours and minutes.
		A time zone offset of "+hh:mm" indicates that the date/time uses a local time zone which is "hh" hours and "mm" minutes ahead of UTC.
		A time zone offset of "-hh:mm" indicates that the date/time uses a local time zone which is "hh" hours and "mm" minutes behind UTC.
	www.w3.org /TR/NOTE-datetime (692 words)

	Universal Time: UTC, UT1 (Site not responding. Last check: 2007-10-07)
		Coordinated Universal Time UTC is the basis of civil timekeeping.
		Universal Time UT, or more specifically UT1, is in effect the mean solar time.
		It is needed for computing the sidereal time, an essential part of pointing a telescope at a celestial source.
	www.hartrao.ac.za /nccsdoc/slalib/sun67.htx/node219.html (451 words)

	Meteorological Time Defintion - The use of the Universal Time Constant for meteoroligical & hydrological data. (Site not responding. Last check: 2007-10-07)
		The time expressed in hydrometeorological products is important for correct interpretation for their use and also varies according to the product.
		These times [the times of preparation, transmission and the forecast valid time] are expressed in UTC.
		Hence, the time in the bulletin heading will be the standard time, usually ending in minutes of zero (00), and the first time within the forecast will be the preparation time and will end in 30, representing 30 minutes after the hour.
	www.nws.noaa.gov /tg/time.html (536 words)

	Definition: universal time (Site not responding. Last check: 2007-10-07)
		In celestial navigation applications, the time which gives the exact rotational orientation of the Earth obtained from UTC by applying increments determined by the U.S. Naval Observatory.
		A measure of time that conforms, within a close approximation, to the mean diurnal rotation of the Earth and serves as the basis of civil timekeeping.
		Note: Universal Time (UT1) is determined from observations of the stars, radio sources, and also from ranging observations of the Moon and artificial Earth satellites.
	www.its.bldrdoc.gov /projects/devglossary/_universal_time.html (182 words)

	Universal Time Computer Encyclopedia Enterprise Resource Directory Complete Guide to Internet (Site not responding. Last check: 2007-10-07)
		UT is tied to the rotation of the Earth in respect to the fictitious "mean Sun".
		{Greenwich Mean Time} (GMT) was measured from Greenwich mean midday until 1925 when the reference point was changed from noon to midnight and the name changed to "Universal Time".
		{Coordinated Universal Time} is kept within 0.9 seconds of UT1, by addition of leap seconds to {International Atomic Time}.
	jaysir.com /computer-encyclopedia/u/universal-time-computer-terms.htm (131 words)

	Venus Transit brief InfoSheet B3
		Civil time as shown on our watches is ultimately derived from the rotation of the Earth and the daily passage of the Sun across the sky.
		We can establish a universal time scale by relating events to the passage of the mean sun relative to the Prime Meridian of Greenwich at zero longitude.
		Equipped with a clock adjusted to Universal Time, or differing from it by an integral number of hours, we are ready to time each event when Venus crosses the Sun.
	www.vt-2004.org /Background/Infol1/BIS-B3.html (604 words)

	Z-Time - by Harold F. Maybeck
		This is because the United Nations, still considering French as the international language, have designated the official designator for Coordinated Universal Time as UTC, as the initials would appear in French.
		The first zone centered on 15° West longitude became time zone "N", the next time zone west centered on 30° West longitude became time zone "O", etc, etc. Approaching the International Date Line from the east, Bowditch designated the remaining "half zone" from 172.5° West longitude to 180° longitude as time zone "Y".
		This notation, "Z", is appended to times on most navigational, meteorological, and astronomical charts today to indicate that the time of the chart is in "Greenwich Mean Time".
	www.maybeck.com /ztime (941 words)

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