User's Guide for Almanac, version 1.03. Explanation of Astronomical EventsAlmanac calculates the equatorial coordinates of the Sun, Moon, and the nine major planets using formulae from: "Low-Precision Formulae for Planetary Positions", by T.C.Van Flandern and K.F.Pulkkinen, U.S.Naval Observatory, The Astrophysical Journal, Supplement Series, 41:391-411, 1979 NovemberThese formulae are used in Almanac to calculate the positions of these celestial bodies to within 1 minute of arc for any epoch within 300 years of the date of the standard epoch of 2000 January 1.5 ephemeris time. Equatorial Coordinates Right Ascension (RA) and Declination (Dec) are a system of angular coordinates used to define the position of distant objects in the sky. These coordinates measure an objects position with respect to the celestial equator and a fixed point in the sky called the first point of Aries or vernal equinox. Right Ascension measures the position of an object in the sky to the east of the vernal equinox. It is usually given in units of time where one revolution is 24 hours (which means 1 hour equals 15 degrees). This is to simplify calculation of position as seen by an observer on the earth's surface. The celestial equator and the earth's equator are in the same plane. Declination is given in degrees above (positive) or below (negative) the celestial equator. Distance Units Distances to celestial objects, except the Moon, are given in Astronomical Units (AU) which is defined as the mean distance between the Earth and the Sun. It is defined as:
1 AU = 149,597,870.691 kilometers
Distance to the Moon is given in units of earth radii (ER). It is defined as:
1 ER = 6,378.140 kilometers (IAU,1976)
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