sidereal time and time scales

 

 

Sidereal Time is based on the Earth's sidereal rotation period (rotation with respect to the stars). The length of the sidereal day is 24 sidereal hours, or 23h56m4s Solar time. The difference is due to the angular motion of the earth around the Sun. In a year there is one more sidereal day than Solar day. The sidereal time is a measured locally. The local sideral time (LST) is the right ascension of the zenith. The hour angle HA of an object with right ascension RA is given by HA=LST-RA

GMST (Greenwich Mean Sidereal Time) at 0h UT = 24110.54841 + 8640184.812866 TU + 0.093104TU^2 - 6.2x10-6TUT^3 seconds

GAST (Greenwich Apparent Sidereal Time) = GMST + the equation of the equinoxes.
The Equation of the Equinoxes is the total nutation in longitude time the cosine of the true obliquity of the ecliptic. It ranges from +0.8 to +1.2 seconds.

LAST (Local Apparent Sidereal Time) is the hour angle of the ascending node of the ecliptic.

LMST (Local Mean Sidereal Time) = GMST - geodetic longitude.

Note: both the sideral and solar days are 86,400 seconds long. The sidereal second is shorter than the solar second by 0.997269566414 - 0.586x10-10TE

Right ascension is generally measured in units of time, not degrees of arc. 24 hours of time corresponds to 360 degrees of arc at the equator. Therefore,

    1h=15o/cos(δ)
    1m=15'/cos(δ), and
    1s=15"/cos(δ)

where δ is the declination of the target. You can use analogous formulae to convert degrees of arc to distances on the earth's surface.  

Astropy's approach

get earth rotation table from IERS

to calculate apparent or mean sidereal time

use time functions, eg, .sidereal time, eg from UTC

so, it's time to worry about where to find "equation of equinox", "equation of time"

Dynamical Time Scale

TDT : Terrestrial Dynamical  Time 地球力學時

TDB: Barycentric Dynamical  Time質心力學時

sunysb

harvard 

astropy example on reionization ( radio astronomy)

esa navipedia

sofa canonical model 

Alan Eliasen in frink language

wavemetrics app on sun, moon

almanac archive online:

explanatory supplement almanac 1992 archive

almanac 2019 archive

Chandler wobble and negative leap second