
Systems of Time With atomic time the unit of a second is defined as; "the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium133 atoms in their ground state undisturbed by external fields" Pretty precise standards. In fact, it is the most accurate realization of a unit that mankind has yet achieved. Today, cesium* clocks measure frequency with an accuracy of from 2 to 3 parts in 10 to the 14th, i.e. 0.00000000000002 Hz; this corresponds to a time measurement accuracy of 2 nanoseconds per day or one second in 1,400,000 years! A cesium clock operates by exposing cesium atoms to microwaves until they vibrate at one of their resonant frequencies and then counting the corresponding cycles as a measure of time. The frequency involved is that of the energy absorbed from the incident photons when they excite the outermost electron in a cesium atom to jump ("transition") from a lower to a higher orbit. Where is it used? The National Institute of Standards and Technology (NIST) in Boulder, Colorado, as well as other International research laboratories maintain cesium clocks for scientific purposes. The U.S. Naval Observatory keeps track of Coordinated Universal Time (UTC) by using many atomic clocks and devices called "timeinterval counters" that compare each clock's time against that of one "Master Clock," whose frequency is steered to match its time to the average of the other clocks. Universal Time (UT) is counted from 0 hours at midnight, with unit of duration the mean solar day, defined to be as uniform as possible despite variations in the rotation of the Earth. Dynamical Time Terrestrial Dynamical Time (TDT) views time from the earth's position and motion. Terrestrial Time Sometimes represented as (TT), or Terrestrial Dynamical Time, (TDT), with unit of duration 86400 SI seconds on the geoid, It was defined as being equal to TAI (Atomic time) plus 32.184 (atomic) seconds at the instant beginning 1 January 1977. Barycentric Dynamical Time (TDB) This is time at the center of mass of the solar system. TDB has various forms depending on the theory of relativity adopted. Geocentric Coordinate Time (TCG) This is a coordinate time having its spatial origin at the center of mass of the Earth. TCG differs from TT as: TCG  TT = Lg x (JD 2443144.5) x 86400 seconds, with Lg = 6.969291e10. Barycentric Coordinate Time (TCB) TCB is a coordinate time having its spatial origin at the solar system barycenter (the center of the mass of our solar system). TCB differs from TDB in rate. The two are related by: TCB  TDB = iLb x (JD 2443144.5) x 86400 seconds, with Lb = 1.550505e08. Sidereal Time Closely connected with the Mean Solar Time is the Sidereal Time, which is defined as the RA (Right Ascension) of the Local Meridian: when the Vernal Point passes the meridian it is 00:00 Sidereal Time. Delta T is the difference between Earth rotational time (UT1) and dynamical time (TDT). Predicted values of UT1  UTC are provided by the Earth Orientation Department. Julian Day Julian Day Number is a count of days elapsed since Greenwich mean noon on 1 January 4713 B.C., Julian proleptic (meaning the calender is being used for a time period before it actually was invented) calendar. The Julian Date is the Julian day number followed by the fraction of the day elapsed since the preceding noon. Scientists and chronologists frequently make use of the Modified Julian Date (MJD), which is defined as MJD = JD  2400000.5. An MJD day thus begins at midnight, civil date. Julian dates can be expressed in UT , TAI, TDT, etc. Julian dates are used primarily in scientific applications to measure elapsed time over periods of months and years, because the Gregorian Calendar continually reuses/repeats the same months every year and the calendar isn't a very accurate measure of time. Julian Date Numbers will also give you the exact hour of day because the numbers are expressed as a whole number and a fraction (the numbers after the decimal) of a day. Military Time A system of time that may seem more familiar and mundane, compared to the other methods of precision timekeeping mentioned above, is based on the number of hours in a day  24. Each hour is numbered from one to 24, starting at midnight (labeled 00:00 hours) and numbered consecutively as the day progresses towards midnight the following night. For example, 5:00am in the morning is 05:00 hours. 5:00pm in the evening, however, is easily distinguished from morning as 17:00 hours  the 17th hour of the day (see the table at right).Zulu Time is another military designation for GMT. *************************************************************************** *Cesium 
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