An
ephemeris (plural:
ephemerides; from the
Greek word "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.
[Paul Schlyter. 2008. retrieved 7 July 2008] 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
- 13th century — the Zij-i Ilkhani, or Ilkhanic Tables, were compiled in Persia
- 13th century — the Alfonsine tables were compiled in Spain to correct anomalies in the Tables of Toledo, remaining the standard European ephemeris until the Prutenic Tables almost 300 years later
- 1408 Chinese Ephemeris Table [copy in Pepysian Library, Cambridge, UK (refer book '1434')Chinese tables believed known to Regiomontanus.]
- 1504 — While shipwrecked on the island of Jamaica, Christopher Columbus successfully predicted 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 astrologers who study
sidereal astrology (
Indian 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 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.
See also
