The Sun's celestial coordinates are determined by its position in its apparent orbit of the Earth. Although the Earth actually orbits the Sun, it seems to us that it is the Sun which is in orbit about the Earth, and for the purposes of calculating the Sun's position it is convenient to regard this as the case. We can then compute the ecliptic longitude of the Sun for a given instant and, since the Sun-Earth orbit defines the plane of the ecliptic, we can take the ecliptic latitude to be 0. The right ascension and declination can be found by using routine EQECL (2000) followed by EQHOR (1500) if we wish to know the Sun's azimuth and altitude as well.

Subroutine SUN calculates the true geocentric ecliptic longitude of the Sun, SR, and its distance from the Earth, RR, at the instant given by UT (Universal Time, hours), DY (integer days), MN (months), and YR (years). The time may also be expressed as a fraction of a day and added to the value of DY, in which case UT must be set to 0. The true ecliptic latitude may be assumed to be 0 (always correct to within 1.2 arcseconds). SR is the longitude referred to the mean equinox of date and is the quantity required in the calculation of geocentric planetary positions.