## Explanatory Supplement to the Astronomical AlmanacThe Explanatory Supplement provides a detailed description of how to perform practical astronomy or spherical astronomy, how the computations for the Astronomical Almanac are performed, and the underlying information required by someone using the Almanac or wanting to perform related computations. This completely revised and rewritten edition of the Explanatory Supplement is an authoritative source on astronomical phenomena and calendars and includes details for FK5 J2000.0 reference system and transformations; details on celestial and terrestrial coordinate systems; current values of astrometric quantities; explanation of eclipse of Sun and Moon computations; details of time scales and their relationships; and detailed descriptions of ephemerides and characteristics of the planets. |

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### Contents

INTRODUCTION TO POSITIONAL ASTRONOMY | 1 |

TIME | 39 |

CELESTIAL REFERENCE SYSTEMS | 95 |

Copyright | |

15 other sections not shown

### Common terms and phrases

aberration accuracy altitude American Ephemeris angular apparent place ascension and declination Astrometric Astron Astronomical Almanac Astrophys axis barycentric Besselian body calculated calendar catalog celestial clock coefficients computed constant coordinate system corrections day numbers defined determined difference distance diurnal dynamical Earth Earth's rotation ellipsoid ephemeris epoch of observation equator and equinox equatorial equinox Figure formulas frequency fundamental geocentric geodetic geoid given gravitational Greenwich Greenwich mean heliocentric hour angle International Julian century Julian date Jupiter latitude light-time lunar mean longitude mean sidereal measured meridian method Moon Nautical Almanac Newcomb nutation object parallax parameters planet planetary polar motion pole position precession precision prime meridian proper motions reference frame reference system refraction right ascension satellites Saturn Section solar eclipse solar system spherical standard stars stellar surface Table tabulated terrestrial theory timescale topocentric transit U.S. Naval U.S. Naval Observatory unit Uranus values variations vector velocity VLBI