Ephemeris

For the 18th-century Greek newspaper, seeEfimeris.

Inastronomyandcelestial navigation,anephemeris(/ɪˈfɛmərəs/;pl.ephemerides/ˌɛfəˈmɛrədiːz/;fromLatinephemeris'diary', andGreekἐφημερίς(ephemeris)'diary, journal')is a book with tables that gives thetrajectoryof naturally occurringastronomical objectsas well asartificial satellitesin thesky,i.e., theposition(and possiblyvelocity) overtime.Historically, positions were given as printed tables of values, given at regular intervals of date and time. The calculation of these tables was one of thefirst applicationsofmechanical computers.Modern ephemerides are often provided in electronic form. However, printed ephemerides are still produced, as they are useful when computational devices are not available.

The astronomical position calculated from an ephemeris is often given in thespherical polar coordinate systemofright ascensionanddeclination,together with the distance from the origin if applicable. Some of the astronomical phenomena of interest to astronomers areeclipses,apparent retrograde motion/planetary stations, planetaryingresses,sidereal time,positions for the mean and truenodes of the moon,thephases of the Moon,and the positions of minorcelestial bodiessuch asChiron.

Ephemerides are used in celestial navigation and astronomy. They are also used byastrologers.GPS signalsinclude ephemeris data used to calculate the position of satellites in orbit.

History

A Latin translation ofal-Khwārizmī'szīj,page fromCorpus Christi CollegeMS 283

Alfonsine tables

Page fromAlmanach Perpetuum

• 1st millennium BC – Ephemerides inBabylonian astronomy.
• 2nd century AD – theAlmagestand theHandy TablesofPtolemy
• 8th century AD – thezījofIbrāhīm al-Fazārī
• 9th century AD – thezījofMuḥammad ibn Mūsā al-Khwārizmī
• 11th century AD – thezījofIbn Yunus
• 12th century AD – theTables of Toledo– based largely on Arabiczījsources ofIslamic astronomy– were edited byGerard of Cremonato form the standard European ephemeris until theAlfonsine Tables.
• 13th century AD – theZīj-i Īlkhānī(Ilkhanic Tables) were compiled at theMaragheh observatoryin Persia.
• 13th century AD – theAlfonsine Tableswere compiled in Spain to correct anomalies in theTables of Toledo,remaining the standard European ephemeris until thePrutenic Tablesalmost 300 years later.
• 13th century AD - theDresden Codex,an extant Mayan ephemeris
• 1408 –Chineseephemeris table (copy inPepysian Library,Cambridge, UK (refer book '1434'); Chinese tables believed known toRegiomontanus).
• 1474 –Regiomontanuspublishes his day-to-day Ephemerides in Nürnberg, Germany.
• 1496 – theAlmanach PerpetuumofAbraão ben Samuel Zacuto(one of the first books published with amovable typeandprinting pressinPortugal)
• 1504 – While shipwrecked on the island of Jamaica,Christopher Columbussuccessfully predicted a lunar eclipse for the natives, using the ephemeris of the German astronomerRegiomontanus.
• 1531 – Work ofJohannes Stöffleris published posthumously at Tübingen, extending the ephemeris of Regiomontanus through 1551.
• 1551 – thePrutenic TablesofErasmus Reinholdwere published, based onCopernicus's theories.
• 1554 –Johannes StadiuspublishedEphemerides novae et auctae,the first major ephemeris computed according to Copernicus'heliocentric model,using parameters derived from thePrutenic Tables.Although the Copernican model provided an elegant solution to the problem of computing apparent planetary positions (it avoided the need for theequantand better explained theapparent retrograde motionof planets), it still relied on the use ofepicycles,leading to some inaccuracies – for example, periodic errors in the position of Mercury of up to ten degrees. One of the users of Stadius's tables isTycho Brahe.
• 1627 – theRudolphine TablesofJohannes Keplerbased on elliptical planetary motion became the new standard.
• 1679 –LaConnaissance des Tempsou calendrier et éphémérides du lever & coucher du Soleil, de la Lune & des autres planètes,first published yearly byJean Picardand still extant.
• 1975 –Owen Gingerich,using modern planetary theory and digital computers, calculates the actual positions of the planets in the 16th century and graphs the errors in the planetary positions predicted by the ephemerides of Stöffler, Stadius and others. According to Gingerich, the error patterns "are as distinctive as fingerprints and reflect the characteristics of the underlying tables. That is, the error patterns for Stöffler are different from those of Stadius, but the error patterns of Stadius closely resemble those ofMaestlin,Magini,Origanus,and others who followed the Copernican parameters. "

Modern ephemeris

For scientific uses, a modern planetary ephemeris comprises software that generates positions of planets and often of their satellites,asteroids,orcomets,at virtually any time desired by the user.

After introduction of electronic computers in the 1950s it became feasible to usenumerical integrationto compute ephemerides. TheJet Propulsion Laboratory Development Ephemerisis a prime example. Conventional so-called analytical ephemerides that utilize series expansions for the coordinates have also been developed, but of much increased size and accuracy as compared to the past, by making use of computers to manage the tens of thousands of terms.Ephemeride Lunaire ParisienneandVSOPare examples.

Typically, such ephemerides cover several centuries, past and future; the future ones can be covered because the field ofcelestial mechanicshas developed several accurate theories. Nevertheless, there aresecular phenomenawhich cannot adequately be considered by ephemerides. The greatest uncertainties in the positions of planets are caused by the perturbations of numerousasteroids,most of whose masses and orbits are poorly known, rendering their effect uncertain. Reflecting the continuing influx of new data and observations,NASA's Jet Propulsion Laboratory (JPL) has revised itspublished ephemeridesnearly every year since 1981.

Solar Systemephemerides are essential for the navigation ofspacecraftand for all kinds of space observations of theplanets,theirnatural satellites,stars,andgalaxies.

Scientific ephemerides for sky observers mostly contain the positions of celestial bodies inright ascensionanddeclination,because these coordinates are the most frequently used on star maps and telescopes. Theequinoxof 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 one of the "standard" equinoxes, typicallyJ2000.0,B1950.0,or J1900. Star maps almost always use 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 planetSaturnalso sometimes contain the apparent inclination of its ring.

Celestial navigationserves as a backup toSatellite navigation.Software is widely available to assist with this form of navigation; some of this software has a self-contained ephemeris.When software is used that does not contain an ephemeris, or if no software is used, position data for celestial objects may be obtained from the modernNautical AlmanacorAir Almanac.

An ephemeris is usually only correct for a particular location on the Earth. In many cases, the differences are too small to matter. However, for nearbyasteroidsor theMoon,they can be quite important.

Other modern ephemerides recently created are the EPM (Ephemerides of Planets and the Moon), from the Russian Institute for Applied Astronomy of theRussian Academy of Sciences,and the INPOP (Intégrateur numérique planétaire de l'Observatoire de Paris) by the FrenchIMCCE.

See also

• Almanac
• American Ephemeris and Nautical Almanac
  • TheAstronomical Almanac(new name)
• Ephemera
• Ephemeris time
• Epoch (astronomy)
• Epoch (reference date)
• Fundamental ephemeris
• January 0orMarch 0
• Keplerian elements
• Nautical almanac
• Osculating orbit
• Ptolemy's table of chords
• Two-line elements
• William of Saint-Cloud

References

• Duffett-Smith, Peter (1990).Astronomy With Your Personal Computer.Cambridge University Press.ISBN0-521-38995-X.
• "ephemeris".American Heritage Dictionary of the English Language(3rd ed.). Boston:Houghton Mifflin.1992.
• MacCraig, Hugh (1949).The 200 Year Ephemeris.Macoy Publishing Company.
• Meeus, Jean (1991).Astronomical Algorithms.Willmann-Bell.ISBN0-943396-35-2.
• Michelsen, Neil F. (1990).Tables of Planetary Phenomena.ACS Publications, Inc.ISBN0-935127-08-9.
• Michelsen, Neil F. (1982).The American Ephemeris for the 21st Century - 2001 to 2100 at Midnight.Astro Computing Services.ISBN0-917086-50-3.
• Montenbruck, Oliver (1989).Practical Ephemeris Calculations.Springer-Verlag.ISBN0-387-50704-3.
• Seidelmann, Kenneth (2006).Explanatory supplement to the astronomical almanac.University Science Books.ISBN1-891389-45-9.

External links

• The JPL HORIZONS online ephemeris
• Introduction to the JPL ephemerides(archived 26 February 2005)
• "Ephemerides-IMCEE".