Delta T, delta-T, deltaT, ΔT, or DT is the time difference obtained by subtracting Universal Time from Terrestrial Time. Universal Time (UT) is a timescale based on the rotation of the Earth. ... Terrestrial Time (TT) is the modern time standard for time on the surface of the Earth. ...

Universal Time (UT) is a time scale based on the Earth's rotation, which is somewhat irregular over the short term (less than a century), thus any time based on it cannot have an accuracy better than 1 : 10⁸. But the principal effect is over the long term. Over many centuries tidal friction inexorably slows Earth's rate of rotation by 2.3 ms/day/cy. However, the melting of continental ice sheets at the end of the last ice age removed their tremendous weight, allowing the land under them to begin to isostatically rebound upward in the polar regions, which continues to this day, causing Earth's rate of rotation to speed up by 0.6 ms/day/cy. The net tidal acceleration or the change in the length of the mean solar day (LOD) is +1.7 ms/cy. The Earths rotation is the rotation of the solid earth around its own axis, which is called Earths axis or rotation axis. ... It has been suggested that Tidal friction be merged into this article or section. ... Variations in CO2, temperature and dust from the Vostok ice core over the last 400 000 years For the animated movie, see Ice Age (movie). ... It has been suggested that Tidal friction be merged into this article or section. ...

Terrestrial Time (TT) is a uniform time scale based on International Atomic Time (TAI), but corrected to equal the former Ephemeris Time (ET) by adding 32.184 s (TT = TAI + 32.184 s). ET is the independent variable of time in Simon Newcomb's Tables of the Sun, which formed the basis of all astronomical ephemerides from 1900 through 1983. ET, in turn, was actually the average mean solar time between 1750 and 1890 (centered on 1820), because that was the period during which the observations on which those tables were based were performed. TAI and hence TT is strictly uniform (every second is the same as every other second), with an accuracy of about 1 : 10¹⁴. International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earths geoid. ... Ephemeris Time (ET) is a now obsolete time scale used in ephemerides of celestial bodies, in particular the Sun (as observed from the Earth), Moon, planets, and other members of the solar system. ... Simon Newcomb. ... 1900 (MCM) was an exceptional common year starting on Monday of the Gregorian calendar, but a leap year starting on Saturday of the Julian calendar. ... 1983 (MCMLXXXIII) was a common year starting on Saturday of the Gregorian calendar. ...

Earth's rate of rotation must be integrated to obtain time, which is Earth's angular position (specifically, the orientation of the meridian of Greenwich relative to the fictitious mean sun). Integrating +1.7 ms/d/cy and centering the resulting parabola on the year 1820 yields (to a first approximation) 31×((Year − 1820)/100)² seconds for ΔT. Smoothed historical measurements of ΔT using total solar eclipses are about +16800 s at the year −500, +10600 s at 0, +5700 s at 500, +1600 s at 1000, and +180 s at 1500. During the telescopic era, measurements were made by observing occultations of stars by the Moon. ΔT continued to decrease until it reached a plateau of +11±6 s between 1680 and 1866. For about three decades immediately before 1902 it was negative, reaching −6.64 s. Then it increased to +63.83 s at 2000. It will continue to increase at an ever increasing parabolic rate in the future. This will require the addition of an ever greater number of leap seconds to UTC as long as UTC is kept within one second of UT1. Physically, the meridian of Greenwich in Universal Time is almost always to the east of the meridian in Terrestrial Time, both in the past and in the future. +16800 s or 4 ²/₃ h corresponds to 70°E. This means that at −500 Earth's faster rotation would cause a total solar eclipse to occur 70° to the east of its location calculated using the uniform TT. Solar time is based on the idea that, when the sun reaches its highest point in the sky, it is noon. ... Photo taken during the 1999 eclipse. ... Bulk silicate composition (estimated wt%) SiO2 44. ... A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ... Coordinated Universal Time (UTC) is a high-precision atomic time standard. ...

All values of ΔT before 1955 depend on observations of the Moon, either via eclipses or occultations. Conservation of angular momentum in the Earth-Moon system requires that the angular momentum lost by the Earth due to tidal friction be transferred to the Moon, increasing its angular momentum, which means that its moment arm (its distance from the Earth) is increased, which via Kepler's laws of planetary motion causes the Moon to revolve around the Earth at a slower rate. The cited values of ΔT assume that the lunar acceleration due to this effect is Γ = −26"/cy². This is close to the best estimate for Γ as of 2002 of −25.858±0.003"/cy² so ΔT need not be recalculated given the uncertainties and smoothing applied to its current values. Nowadays, UT is the observed orientation of the Earth relative to an inertial reference frame formed by extra-galactic radio sources, modified by an adopted ratio between sidereal time and solar time. Its measurement is performed by the International Earth Rotation and Reference Systems Service (IERS). This article does not cite its references or sources. ... Sidereal time is time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical with, the motion of stars. ... The International Earth Rotation and Reference Systems Service is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups. ...

References

• F.R. Stephenson, L.V. Morrison. "Long-term fluctuations in the Earth's rotation: 700 BC to AD 1990". Philosophical Transactions of the Royal Society of London, Series A 351 (1995) 165-202. JSTOR link.
• F.R. Stephenson. Historical Eclipses and Earth's Rotation. Cambridge University Press, 1997. ISBN 0-521-46194-4
• Historical Eclipses and Earth's Rotation by F. Richard Stephenson 242KB PDF file

Cover of Cover of the first volume of , published in 1665 The Philosophical Transactions of the Royal Society, or Phil. ...

External links

• Delta T by Felix Verbelen
• Delta T by Jean Meeus
• Delta T by Peter Meyer
• Eclipse Predictions and Earth's Rotation by Fred Espenak

<STATEMENT> it is not only used for universal time but also for temperature.