- Apparent right ascension (RA) and apparent declination (Dec) of the Sun are the astronomical coordinates of the Sun on the celestial sphere in the equatorial coordinate system. "RA" corresponds to the longitude on the celestial sphere, measured eastwards along the celestial equator with respect to the zero hour of RA which passes through the two celestial poles and the vernal equinox point. It is expressed in hours ( h ), minutes ( m ) and seconds ( s ), and if expressed in terms of angles, 1h=15°, 1m=15', 1s=15". "Dec" corresponds to the latitude on the celestial sphere, measured north or south of the celestial equator in degrees ( ° ), minutes ( ' ) and seconds ( " ). Declinations north of the celestial equator are positive, while those to the south are negative.
- Terrestrial Time (TT) is the time standard used for precise calculation of astronomical events observed from the geoid. On the geoid, gravitational potential is the same everywhere and the gravitational force is perpendicular to the geoid. TT is independent of the irregular rotation of the Earth and its relation to the International Atomic Time (TAI*) is TT = TAI + 32.184 seconds.
- For more information about TAI, please refer to the following hyperlink: http://www.hko.gov.hk/en/gts/time/basicterms-TAI.htm