Founded in 1955.5 AD, this is a measure of a specific atom of Caesium 133 isotope to a pulse of radiation. It measures the passage of time more accurately and atomic clocks are situated around the world for this purpose/
Occurs when the moon's disc diameter is less than Sun's and a 'ring of fire' takes place that can be seen during the eclipse sequence. Magnitude is always < 1.0000
An epoch used prior to 1984 as a co ordinate system
Solar atmosphere extending from the Sun's circumference outwards
ECLIPTIC : A frame of reference that maps outwards from the observer to the night sky from the path of the sun (ecliptic). Co ordinates are in Latitude and Longitude.
EQUATORIAL : with reference to the plane of the earth extended into space. Co ordinates measured in Right Ascension and Declination.
HORIZON : with refernce to the obsever's location on the earth. Co ordinates are Altitude (vertical) and azimuth (horizontal)
Periphereal atmosphere of Sun ,can be seen during total eclipses
This is the difference between atomic time as measured by precise clocks and the earth's rotation. Delta time can only be measured directly or indirectly by eclipses, transits and movements of artificial satellites. It cannot be predicted back in time i.e. it can only be measured empirically.
Delta T = UT - TDT (DT)
Ephemeris Time is a time scale used before 1984 AD to calculate tables of planets etc. It was superceded by DT (TDT).
With reference to earth's centre
The varying speed of the moon in orbit around the earth; AKA Secular Lunar Acceleration;Lunar Tidal Acceleration;Lunar Acceleration Parameter. The Lunar Orbital Acceleration is madeup of the following parts :-
Solar Acceleration of the Sun (c')
Lunar Acceleration (c)
Mean motion of sun & moon (q)
The following formula is used :-
n-dot = 2( c - qc') arcsec/cy squared
e.g. c = + 5.22
q = 13.37
c' = 1.23
n-dot = 2(5.22 - (13.37x1.23))
= - 22.44 arcsec/ cy squared
Spencer Jones measurement (1939)
Usually defined as the passage of a stellar object behind the moon. Occultations can be precisely timed and values of Delta T gained indirectly as a result.
Angle measured in degrees usually between moon's centre and earth and sun
Gases given off by sun,seen during total eclipses or conditions that mimic them
A Babylonian lunar cycle consisting of 6585.32 days (18 years 11 days) during which sun and moon return to almost same positions
The angular separation of an object from the Sun or Moon measured in degrees
(Temps Atomic Internationale) A close definition of Atomic time (see elsewhere) that differs by 32.184 seconds. TAI = TDT(Dynamic Time) + 32.184 secs
Terrestrial Dynamic Time is a uniform time scale based on Atomic Clock Time. TDT = TAI + 32.184 seconds that is used to calculate the change in time from the rotation of the earth to atomic time.
Passage usually of Mercury or Venus across the Sun's disc as seenfrom earth.
When Moon's disc diameter is >= Sun's diameter . Magnitude is always >= 1.000
Universal Time; the rotation of the earth in 24 hours. It differs from Terrestrial Dynamic Time (TDT) by a measure called Delta Time.