However, because the precession of the orbital plane is exactly matched by the precession of the rotational axis of the moon, the angle remains constant " with respect to the Earth ", but not " with respect to the stars " ( see sidereal time ).
42.
"' Section B : TIME-SCALES AND COORDINATE SYSTEMS "'contains calendar information, relationships between time scales, universal and sidereal times, Earth rotation angle, definitions of the various celestial coordinate systems, frame bias, precession, nutation, obliquity, intermediate system, the position and velocity of the Earth, and coordinates of Polaris.
43.
The observer position vector ( in Equatorial Coordinate System ) of the observation points can be determined from the latitude and local sidereal time ( from Topocentric Coordinate System ) at the surface of the focal body of the orbiting body ( e . g ., Earth ) via either:
44.
This could include the location of the sun and moon in the sky, the age and Lunar phases, the position of the sun on the ecliptic and the current zodiac sign, the sidereal time, and other astronomical data such as the moon's nodes ( for indicating eclipses ) or a rotating star map.
45.
Because Earth orbits the Sun once a year, the sidereal time at any given place and time will gain about four minutes against local civil time, every 24 hours, until, after a year has passed, one additional sidereal " day " has elapsed compared to the number of solar days that have gone by.
46.
Greenwich mean sidereal time ( GMST ) and UT1 differ from each other in rate, with the second of sidereal time a little shorter than that of UT1, so that ( as at 12 : 00 noon on 2000 January 1 ) s of mean sidereal time was equal to 1 s of UT1.
47.
Greenwich mean sidereal time ( GMST ) and UT1 differ from each other in rate, with the second of sidereal time a little shorter than that of UT1, so that ( as at 12 : 00 noon on 2000 January 1 ) s of mean sidereal time was equal to 1 s of UT1.
48.
Greenwich mean sidereal time ( GMST ) and UT1 differ from each other in rate, with the second of sidereal time a little shorter than that of UT1, so that ( as at 12 : 00 noon on 2000 January 1 ) s of mean sidereal time was equal to 1 s of UT1.
49.
Thus the aim developed, to provide a new time scale for astronomical and scientific purposes, to avoid the unpredictable irregularities of the mean solar time scale, and to replace for these purposes Universal Time ( UT ) and any other time scale based on the rotation of the Earth around its axis, such as sidereal time.
50.
Sidereal time, at any moment ( and at a given locality defined by its geographical longitude ), more precisely local apparent sidereal time ( LAST ), is defined as the hour angle of the vernal equinox at that locality : it has the same value as the right ascension of any celestial body that is crossing the local meridian at that same moment.
