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Astro*Dictionary by Michael Erlewine

 

 

 

 

6 articles for "Ephemeris"

Ephemerides [Astro*Index]

\ef-e-'mer-e-dez     or     i-'fem'-e-ridz\

The plural of ephemeris.

 

Ephemeris [Astro*Index]

A tabulation of position (sometimes velocity also) of a celestial object for (usually) regular intervals of time. Frequently, such tabulations are provided in printed form, but may also be recorded in machine-readable form to facilitate computer usage (such as magnetic tape or disk). When a 24-hour time-interval is used, values are given for midnight (0h) or noon (12h) in ET (Ephemeris Time) or UT (Universal Time). Often, the Moon's position is given each 12 hours. The AENA (American Ephemeris and Nautical Almanac) gives a lunar ephemeris (Right Ascension and Declination) for every hour of ET. For the outer planets (Jupiter through Pluto), tabulations at 40-day, and even 400-day intervals are not uncommon. And, to facilitate computer operations, the ephemeris may be represented in a compressed form, e.g., by some numerical approximation (such as Chebychev Polynomials). The plural of ephemeris is ephemerides (pronounced eph-a-mehr'-a-deez), and is a collection of such tabulations for more than one body, or covering a period longer than one year. A Geocentric ephemeris is computed to represent the position of a body as it would be "seen" from the center of the Earth; a Heliocentric ephemeris represents the position of a body as viewed from the center of the Sun. The ephemeris is the astrologer's primary tool.

 

See also:
♦ Apparent Ephemeris ♦ Geometric Ephemeris ♦ Astrometric Ephemeris ♦ Ephemeris Time ♦ Universal Time ♦ Right Ascension ♦ Declination ♦ Geocentric ♦ Heliocentric
Ephemeris [Munkasey M.]

A book of tables showing the positions of the planetsat periojic intervals.

 

See also:
♦ Apparent Ephemeris ♦ Geometric Ephemeris ♦ Astrometric Ephemeris ♦ Ephemeris Time ♦ Universal Time ♦ Right Ascension ♦ Declination ♦ Geocentric ♦ Heliocentric
Ephemeris [DeVore]

pl. Ephemerides.

An almanac listing the ephemeral or rapidly changing position which each of the solar system bodies will occupy on each day of the year: their Longitude, Latitude, Declination, and similar astronomical phenomena. The astronomer's Ephemeris lists these positions in heliocentric terms; that of the astrologer, in geocentric terms. A set of Ephemerides which includes the year of the native's birth, is essential in the erection of a horoscope. Ephemerides were first devised by astrologers to facilitate the erection of a horoscope. Finally, when they became of common use to navigators and astronomers, they were given official recognition by the Government, and issued as the Nautical Almanac. The oldest almanac in the British Museum bears the date 143I. It is said that Columbus navigated by the aid of an Astrologer's Ephemeris.

Some of the notable ephemerides have been: Vincent Wing, 1658-81; John Gadbury, 1682-1702; Edmund Weaver, 1740-46; Thomas White, 1762-1850 (also reappeared in 1883); George Parker, in Celestial Atlas, 1780-90; John Partridge, in Merlinus Liberatus, 1851-59; E. W. Williams, in the Celestial Messenger, 1858; W. J. Simmonite, 1801-61; Raphael, 1820 to date.

The old astronomical day which began at noon was abolished on Jan. 1, 1925, and since then the astronomical day has begun at midnight. Gradually this is reflected in the making of Ephemerides. Therefore it is important to verify whether the ephemeris one is using for any given year since around 1930 shows the planets' places at noon or midnight. This can be determined at a glance by noting the sidereal time on Jan. 1: if it is around 18h the ephemeris is for noon; if around 6h, it is for midnight; if neither of these, it is probably calculated for some longitude other than that of Greenwich.

 

See also:
♦ Apparent Ephemeris ♦ Geometric Ephemeris ♦ Astrometric Ephemeris ♦ Ephemeris Time ♦ Universal Time ♦ Right Ascension ♦ Declination ♦ Geocentric ♦ Heliocentric
Ephemeris Time [Astro*Index]

A uniform measure of time, the determination of which depends upon the laws of Dynamics. ET (Ephemeris Time) is the independent variable for the gravitational theories of the Sun, Moon, and planets, and is the argument used for the fundamental ephemerides. The IAU (International Astronomical Union) has adopted the following definition for Ephemeris Time: "Ephemeris time is reckoned from the instant, near the beginning of the calendar year AD1900, when the geometric mean longitude of the Sun was 279o41'48.04", at which instant the measure of ephemeris time was 1900 JAN 00d12h precisely." For astrologers, ET is normally obtained from UT (Universal Time) by adding the value of delta-T, which is tabulated for previous years, and extrapolated for future years.

 

See also:
♦ Universal Time ♦ Delta-t
Ephemeris Time [Munkasey M.]

The time which is obtained directly from the obser-vation and comparing of the positions of the Sun, Moon, and planets. This requires an extended observational period over time. ET is defined by the apparent annual motion of the Sun in longitude.

 

See also:
♦ Universal Time ♦ Delta-t

 

Astro*Index Copyright © 1997 Michael Erlewine