Definitions

ephemeris

ephemeris

[ih-fem-er-is]
ephemeris (pl., ephemerides), table listing the position of one or more celestial bodies for each day of the year. The French publication Connaissance de Temps is the oldest of the national astronomical ephemerides, founded in 1679. The Nautical Almanac and Astronomical Ephemeris (usually abbreviated to the Nautical Almanac), an annual publication by the British Royal Observatory at Greenwich since 1767, has been a leading compilation of ephemerides since its inception. Its original purpose was to provide the astronomical information necessary to derive longitude at sea. In 1852 the U.S. Naval Observatory began publishing a book called the American Ephemeris and Nautical Almanac, which contained similar information to that published at Greenwich but adjusted for the meridian at Washington, D.C. Beginning with the edition for 1958, Great Britain and the United States, in a joint effort, issued ephemerides that are identical in content, although they remain separate publications with different names (the British volume was renamed The Astronomical Ephemeris); in 1981 the British and American publications were combined as The Astronomical Almanac. This ephemeris (adapted to the Greenwich meridian) is issued well in advance of the dates covered and contains such information as the daily right ascension and declination of the sun, moon, planets, and other celestial bodies, and daily data on the sunrise, sunset, moonrise, and moonset. Among other publications issued are The Ephemeris (U.S.) and The Star Almanac for Land Surveyors (Brit.), which are star ephemerides used by surveyors, and the Air Almanac (Brit./U.S.), used in air navigation. By international agreement the basic calculations of astronomical ephemerides are shared among a number of countries including France, Germany, Spain, and Russia. The Ephemerides of Minor Planets is compiled and published annually by the Institute of Theoretical Astronomy, St. Petersburg (formerly Leningrad). In addition, the International Astronomical Union issues ephemerides for every newly discovered comet and for many newly found asteroids. Through the Minor Planet Center in Cambridge, Mass., astronomers can obtain ephemerides of any asteroid or comet.

Table of the positions of celestial bodies at regular intervals, often with supplementary information. Constructed as early as the 4th century BC, ephemerides are still essential to astronomers and navigators. Modern ephemerides are calculated, with heavy computing and careful checking, after a mathematical description of a heavenly body's observed motion has been evolved. Various national ephemerides are published regularly; the U.S. ephemeris, first published in 1852, became the best and is now published jointly with the U.K. as The Astronomical Almanac.

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An ephemeris (plural: ephemerides; from the Greek word ἐφήμερος ephemeros "daily") is a table of values that gives the positions of astronomical objects in the sky at a given time or times. Different kinds are used for astronomy and astrology. Even though this was also one of the first applications of mechanical computers, an ephemeris will still often be a simple printed table.

The position is given to astronomers in a spherical polar coordinate system of right ascension and declination or to astrologer in longitude along the zodiacal ecliptic, and sometimes declination. Astrological positions may be given for either noon or midnight.

An astronomical ephemeris may also provide data on astronomical phenomena of interest to astrologers and astronomers such as eclipses, apparent retrogradation/planetary stations, planetary ingresses, sidereal time, positions for the Mean and True nodes of the moon, the phases of the Moon, and sometimes even the position(s) of Chiron, and other minor celestial bodies. Astrologers also use other ephemerides that include tables of imaginary celestial bodies, such as Lilith, a term they use variously for the apogee of the Moon or the second focus of the Moon's orbit. Some ephemerides also contain a monthly aspectarian, while others often include the declination of the planets as well as their longitudes, right ascensions or Cartesian coordinates.

History

1504 - While shipwrecked on the island of Jamaica, Christopher Columbus successfully predicts a lunar eclipse for the natives, using the Ephemeris of the German astronomer Regiomontanus

1554 - Johannes Stadius published a well-known work known as Ephemerides novae at auctae that attempted to give accurate planetary positions. The effort was not entirely successful, and there were, for example, periodic errors in Stadius’ Mercury positions of up to ten degrees.

Scientific ephemeris

For scientific uses, a modern planetary ephemeris comprises software that generates positions of the planets and often of their satellites, or of asteroids or comets at virtually any time desired by the user. Often there is an option to find the velocities of the bodies of interest, as well.

Typically, such ephemerides cover several centuries, past and future; the future ones can be covered because celestial mechanics is an accurate theory. Nevertheless, there are secular phenomena, factors that cannot adequately be considered by ephemerides. The biggest uncertainties on planetary positions are due to the perturbations of numerous asteroids, most of whose masses are poorly known, rendering their effect uncertain. Therefore, despite efforts to overcome these uncertainties, the JPL has to revise its published ephemerides at intervals of 20 years.

Solar system ephemerides are essential for the navigation of spacecraft and for all kinds of space observations of the planets, their natural satellites, stars and galaxies.

Scientific ephemerides for sky observers mostly contain the position of the mentioned celestial body in right ascension and declination, because these coordinates are the most often used on star maps and telescopes. The equinox of the coordinate system must be given. It is in nearly all cases either the actual equinox (the equinox valid for that moment, often referred to as "of date" or "current"), or that of the one of the "standard" equinoxes, typically J2000.0, B1950.0, or J1900. Star maps are almost always in one of the standard equinoxes.

Scientific ephemerides often contain further useful data about the moon, planet, asteroid, or comet beyond the pure coordinates in the sky, such as elongation to the sun, brightness, distance, velocity, apparent diameter in the sky, phase angle, times of rise, transit, and set, etc. Ephemerides of the planet Saturn also sometimes contain the apparent inclination of its ring.

An ephemeris is usually only correct for a particular location on the Earth. In many cases the differences are too small to matter, but for nearby asteroids or the Moon they can be quite important.

GPS navigation satellites transmit electronic ephemeris data consisting of health and exact location data that GPS receivers then use (together with the signal's elapsed travel time to the receiver) to calculate their own location on Earth using trilateration.

Astrological ephemeris

The majority of astrologers study tropical astrology, involving planetary positions referenced to the vernal (spring) equinox position along the ecliptic (the equinox being the nexus of Earth's rotational plane and Earth's orbital plane around the Sun). They use exactly the same referential frame of the astronomers, except for a small minority of astrologers who study sidereal astrology and use a different ephemerids, based on the constellations.

Though astrology is and always has been geocentric, heliocentric astrology is an emerging field; for this purpose a standard ephemeris cannot be utilized, and because of this specialized heliocentric ephemerids must be calculated and used instead of the default geocentric ephemerides that are used in standard Western astrology to construct the astrological chart/natal chart.

See also

References

  • Duffett-Smith, Peter (1990). Astronomy With Your Personal Computer. Cambridge University Press. ISBN 0-521-38995-X.
  • MacCraig, Hugh (1949). The 200 Year Ephemeris. Macoy Publishing Company. (no ISBN indicated).
  • Meeus, Jean (1991). Astronomical Algorithms. Willmann-Bell. ISBN 0-943396-35-2.
  • Michelsen, Neil F. (1990). Tables of Planetary Phenomena. ACS Publications, Inc.. ISBN 0-935127-08-9.
  • Michelsen, Neil F. (1982). The American Ephemeris for the 21st Century - 2001 to 2100 at Midnight. Astro Computing Services. ISBN 0-917086-50-3.
  • Montenbruck, Oliver (1989). Practical Ephemeris Calculations. Springer-Verlag. ISBN 0-387-50704-3.
  • Seidelmann, Kenneth (2006). Explanatory supplement to the astronomical almanac. University Science Books. ISBN 1-891389-45-9.

External links

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