Ephemeris

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Template:Wikisourcehas An ephemeris (plural: ephemerides) (from the Greek word ephemeros= daily) was, traditionally, a table providing the positions (given in a Cartesian coordinate system, or in right ascension and declination or, for astrologers, in longitude along the zodiacal ecliptic), of the Sun, the Moon, the planets, asteroids or comets in the sky at a given moment in time; the astrological positions are usually given for either noon or midnight depending on the particular ephemeris that is used. In 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.

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. The biggest uncertainties, nowadays, are due to the perturbations on the planets of numerous asteroids, most of whose masses are poorly known, rendering their effect uncertain.

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.

An ephemeris may also provide data on astronomical phenomena of interest to astrologers and astronomers such as solar and lunar 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, Lilith, and other minor celestial bodies. 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.

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 ephemerides 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.

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Scientific ephemeris

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. Of course the date for which the equinox is valid must be given. It is in nearly all cases either the actual equinox, that means the equinox valid for the day, or that of the year, which is closest to the year for which the ephemeris is valid, which can be divided by 50 without rest (many scientific ephemerides for 2006 use the equinox 2000.0). Latter option makes it easier to draw the exact position of the celestial body in star maps. Scientific ephemerides often contain further useful data of the moon, planet, asteroid or comet than the pure coordinates in the sky. Two such items, which are useful in nearly every case are the elongation to the sun and the brightness. For planets and the moon they contain sometimes the appearent diameter in the sky or the phase angle. Ephemeris of the planet Saturn also contain sometimes the inclination of its ring. Sometimes also the values of rise, culmination and set are part of ephemeris. Although these data are very useful for observers, their value is only exactly correct for one location on earth.

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References

{{cite book

| first = Oliver | last = Montenbruck
| title = Practical Ephemeris Calculations
| publisher = Springer-Verlag
| year = 1989
| id = ISBN 0387507043
}}

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