 |
Business | Industries | Finance | Tax |
| Index: > A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|||||
In the late 19th century it was found that the rotation of the Earth (i.e. the length of the day) was both irregular on short time scales, and was slowing down on longer time scales. In fact, observing the position of the Moon, Sun and planets and comparing this with their ephemerides was a better way to determine the time.
Using the ephemerides based on the theory of the apparent motion of the Sun by Simon Newcomb (1898), the SI second was defined in 1960 as:
At the time, very accurate and stable atomic (caesium) clocks had become available. Comparing the clock rates with astronomical observations allowed to match atomic clocks with ephemeris time, and a new definition of the second based on caesium clocks was accepted in 1972.
The difference between ET and UT is called Delta-T; it increases irregularly with about half a second per year. International Atomic TimeTemps Atomique International TAI or International Atomic Time is a very accurate and stable time scale. It is an average of the time kept by many caesium clocks ( atomic clocks) all over the world, and has been available since 1955. True high-precision TA (TAI) was set equal to UT2 at 1 January 1958 0:00:00 . At that time, Delta-T was already about 32.18 seconds. The difference between TT (the successor to ephemeris time) and atomic time was later defined as follows:
This difference may be assumed constant, the rates of TT and TAI are designed to be identical.
P.K.Seidelmann (ed.), Explanatory Supplement to the Astronomical Almanac. University Science Books, CA, 1992 ; BooksEnthsiast.com
TimeFor alternate uses of "time", see Time (disambiguation). Time quantifies or measures the interval between events, or the duration of events. Time has long been perceived as a dimension in which each event has a definite (but not necessarily unique) positi| Politics | Business | Finance |
 |
 |
 |
|