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# Some Definitions of Time Related Terms [8/17/99]

*On September 26, 2002*

# Some Definitions of Time Related Terms

**Posted by Mike Disher on August 17, 1999 at 09:37:51:**

** Bulletin Board Post Number: 790**

Posted from Host: 1cust32.tnt12.sfo3.da.uu.net (153.37.33.32)

**Coordinated Universal Time**: See Universal Time, Coordinated (UTC), below.

**Ecliptic plane**: the plane of the Earth’s revolution around the Sun.

**Egyptian Solar Calender**: One of three calender systems used by the ancient Egyptians (the other two being stellar and lunar). The Egyptian solar calender year consisted of 365 days, and it provided the foundation for the calender used in the western world today.

**Ephemeris Time (ET)**: Measurements with highly accurate atomic clocks show that the rotation period of the Earth is slightly irregular. Ephemeris Time is introduced to remove the dependence on the Earth rotation, and is calculated from the observed motion of the Moon. In practice, differences between the rates of ET and Universal Time (UT, see below) may be neglected. The absolute difference has increased over the last 100 years to about 60 seconds.

**Equation of Time**: Difference between the Mean Solar Time (see below) and the real solar time (see below). This varies between a minimum of -15 and a maximum of +15 minutes during the year.

**Equinox**: Moment at which the Sun as viewed from the Earth appears to cross the celestial equator. This occurs at about 21 March – the vernal equinox – and at about 22 September – the autumnal equinox.

**Greenwich Mean Time (GMT)**: Mean solar time (see below) at the “prime meridian” of Greenwich, England. GMT was formerly used as a basis for global standard time. GMT has been replaced by Universal Time Coordinated (UTC, see below). GMT arose out of our understanding that the concept of time is linked to the rotation of the earth. We defined the length of the day as the 24 hours it takes the earth to spin once on its axis. As timepieces became more accurate and communication became global, a need arose for an agreed single point from which all other world times would be derived. Given Great Britain’s maritime power when the concept of latitude and longitude came to be, the starting point for designating longitude was the “prime meridian” which is zero degrees longitude and runs through the Royal Greenwich Observatory, in Greenwich, England, just outside London. When the concept of time zones was introduced, the “starting” point for calculating the different time zones was/is at the Royal Greenwich Observatory. Note: Prior to 1925, in astronomical and nautical almanacs, a day of Greenwich Mean Time began at noon. This reckoning of Greenwich Mean Time is now called Greenwich Mean Astronomical Time, and is no longer used. Persons using old editions of the almanacs for historical research should be aware of the previous convention.

**Gregorian Calender**: The Gregorian calendar is the one commonly used today in the western world.. It was proposed by Aloysius Lilius, a physician from Naples, and adopted by Pope Gregory XIII in accordance with instructions from the Council of Trent (1545-1563) to correct for errors in the older Julian Calendar (see below). In February, 1582, Pope Gregory XIII issued a Papal Bull requiring that 10 days should be dropped from October 1582 so that 15 October should follow immediately after 4 October, and from then on the reformed calendar should be used. This was observed in Italy, Poland, Portugal, and Spain. Other Catholic countries followed shortly after, but Protestant countries were reluctant to change, and the Greek orthodox countries didn’t change until the start of this century. In the Gregorian calendar, the tropical year is approximated as 365 97/400 days = 365.2425 days. Thus it takes approximately 3300 years for the tropical year to shift one day with respect to the Gregorian calendar. The approximation 365 97/400 is achieved by having 97 leap years every 400 years. These are calculated as follows: Every year divisible by 4 is a leap year. However, every year divisible by 100 is not a leap year. However, every year divisible by 400 is a leap year after all. So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600, 2000, and 2400 are leap years.

**Julian Calender**: The Julian calendar was introduced by Julius Caesar in 45 B.C. as a modification of the Egyptian solar calender. The Julian calender had a year consisting of 365.25 days, plus a leap year every 4 years to catch up the quarter days added together to make one day. In fact, a year has 365.2422 days, or 365 days, 5 hours, 48 minutes and 46 seconds. The actual year is in fact 11 minutes and 14 seconds shorter than Caesar supposed. The Julian calendar introduced an error of 1 day every 128 years. This resulted in a deviation of some 13 days by the 16th century. This derivation was corrected by Pope Gregory VIII in 1582 (see Gregorian Calender, above).

**Local solar time**: The time at which the sun is at its zenith at noon in any given location on earth. Local time is set according to the longitude of a place.

**Mean Solar Time**: Mean Solar Time is 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. Stated a different way, it is time measured with respect to the motion of a fictitious body called the Mean Sun which moves at a constant rate (another way to state this assumption is that the earth moves in a circular orbit around the sun, and that the axis of rotation is perpendicular to the orbital plane (ecliptic). The time interval between two meridian crossings of the Mean Sun is exactly one solar day. Due to the combined effects of the eccentricity of the Earth’s orbit and the tilt of the Earth rotation axis, the real sun arrives at our local meridian a little early at certain times of the year, and a little late at other times. The difference between real solar time and mean solar time is called the Equation of Time (see above).

**Roman Calendar**: Believed to have been devised originally by Romulus (circa 750 BC), the founder of Rome. It was essentially a lunar calendar and had ten months, six of 30 days and four of 31 days making a total of 304 days. The year started with the month of March and ended with the month of December. This was then followed by a gap before the next year started. The next year would start on a new moon to bring the calendar back into step with the lunar cycle.

**Sidereal Time**: Time required for the Earth to rotate once on its axis relative to the stars. This occurs in 23h 56m 4s of ordinary mean solar time. A sidereal day consists of 24 sidereal hours. The sidereal day starts when the Vernal Equinox crosses the Greenwich meridian. Sidereal time is therefore equal to the Hour Angle of the Vernal Equinox.

**Universal Time (UT)**: Local mean solar time (see above) on the Greenwich meridian, also called Greenwich Mean Time (GMT) or Zulu Time (Z).

**Universal Time, Coordinated (UTC)**: 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. UTC is the basis for civil standard time in the U.S. and its territories. Standard time within U.S. time zones is an integral number of hours offset from UTC. The UTC timescale is kept by a large number of highly precise atomic clocks at facilities around the world, including the U.S. Naval Observatory, and there is international coordination in maintaining UTC to better than a nanosecond (billionth of a second) per day. The length of a UTC second is defined in terms of a count of radiation cycles of a certain atomic transition of the element cesium, and is not directly related to any astronomical phenomena. UTC is the time broadcast by radio stations WWV and WWVH and it can also be obtained from GPS satellites. NOTE: A UTC clock has the same rate as a Temps Atomique International (TAI) clock or international atomic time clock but differs by an integral number of seconds called leap seconds. The UTC scale is adjusted by the insertion or deletion of seconds (positive or negative leap seconds) to ensure approximate agreement with UT1 (also known as the Julian Date)

**Universal Time Family**: The various forms of Universal Time (UT). In applications requiring precise timekeeping, it is necessary to specify the form of UT such as UT1 which is directly related to polar motion and is proportional to the rotation of the Earth in space. UT1 is a natural time scale kept by the rotation of the Earth. UT1 has a precise technical definition that relates it to the positions of stars passing over the meridian of Greenwich. UT1 is affected by small variations in the rotation of the Earth, and UT1 can differ from civil time on the Greenwich meridian by up to 0.9 second. By international agreement, UTC cannot differ from UT1 by more than 0.9 second. When the two kinds of time begin to diverge toward this limit, a one-second change called a “leap second” is introduced into UTC. This occurs on average about once a year. The UT1 is further corrected empirically for annual and semiannual variations in the rotation rate of the earth to obtain UT2.