Ephemeris
The ephemeris (from ancient Greek ἐφήμερος Ephemeros "for a day", from ἐπί epi "on" and ἡμέρα HEMERA " day are"), the position values of moving astronomical objects based on a respective convenient astronomical coordinate system . Its name expresses that such position information is usually given for one day at a time. They are calculated from the orbital elements and in the form of tables or tables with daily position values of the sun , moon ,Planets and Comets published.
In addition to astronomy, ephemeris are also used in celestial mechanics , astrogeodesy , space travel and astrology . Until the second half of the twentieth century, they were also used as navigation aids in sea and aviation .
Applications
Ephemeris are available in printed form as astronomical yearbooks or for seafaring as nautical yearbooks . Among the earliest astronomical prints are the Ephemerides astronomicae des Regiomontanus , published in 1474 , which Christopher Columbus used for navigation on his voyages of discovery.
Ephemeris for observational astronomy use either the full circle of 360 degrees along the ecliptic or give the position in equatorial coordinates . The calculation of the ephemeris is one of the services of astronomical phenomenology and is now part of many astronomical computer programs or is available interactively on the Internet .
In the case of a satellite , the ephemeris describe the apparent orbit in mathematical form . With the Global Positioning System (GPS) and other GNSS systems, these orbital data are part of the signal transmitted by each satellite, which is then the basis for calculating the receiver position.
In order to estimate the probability of a possible collision between two bodies in the solar system, it is necessary to know exactly the positions of the bodies and the temporal development of their orbital elements . For this reason, ephemeris catalogs are created for asteroids in order to be able to more precisely determine their orbital data, which are usually inadequately known. Particular attention is paid to the earth orbit cruisers , whose impacts on earth could pose a threat.
Astrological ephemeris indicate the ecliptical length of the celestial positions as degrees of the signs of the zodiac . The specification of the ecliptical latitude is not common in all astrological ephemeris.
The calculation of the aspects of actual rise and set and the day arc of the sun is of particular importance for calculating the effectiveness of solar systems and in architecture for estimating the incidence of sunlight ; this is usually implemented as a light module in CAD software. The time and position of sunrise and sunset at a selected location is also of interest for landscape photography.
Ephemeris calculation
The ephemeris calculation as a field of activity of astronomical phenomenology is based on the actually observed positions of the celestial bodies and the theory of gravity . The expected path disruptions are taken into account for future values . For this purpose, computational models such as the planetary theory are set up in order to refine the perturbation theory .
Important historical tables for the calculation of the ephemeris were the Alfonsine tables around 1252, the Prutenic tables in 1551 and the Rudolfine tables in 1627.
Time reference and table intervals
The table intervals of the tabulated coordinates depend on their rate of change or the path speed . For fundamental stars and distant planets , 10 days are common, for the sun, moon and inner planets 1–2 days.
The arguments (times) of the tabulated coordinates can refer to different time systems :
- on terrestrial time for bodies of the solar system (sun, moons, planets, asteroids )
- or to world time (in some cases the dynamic time correction DUT1 or Delta T is necessary)
- or on sidereal time (for fundamental stars ).
In [2, 3], times are also given in fractions of a day instead of hours .
See also
literature
- Andreas Guthmann: Introduction to celestial mechanics and ephemeris calculation , Spektrum Akademischer Verlag, Heidelberg 2000, ISBN 3-8274-0574-2
- Oliver Montenbruck: Basics of the ephemeris calculation , spectrum Akademischer Verlag, Heidelberg 2005, ISBN 3-8274-1602-7
- Oliver Montenbruck and Thomas Pfleger: Astronomy with the Personal Computer , Springer Verlag, Berlin 2004, ISBN 3-540-21204-3
- Hermann Mucke (ed.): Modern astronomical phenomenology. 20th Sternfreunde Seminar, 1992/93. Zeiss Planetarium of the City of Vienna and Austrian Astronomical Association , Vienna 1992 (weblink)
- The German Ephemeris. Vol. I 1850-1889. Barth, Munich 1929
Web links
Calculations:
- Onlineplanetarium - Calculation of current ephemeris ( Memento from March 7, 2018 in the Internet Archive )
- Ephemeris calculation - planets, moon with moon phases and constellation information and as a map
- HORIZONS - Jet Propulsion Laboratory ephemeris system
Basics:
- US Naval Observatory
- ARI Astronomical Computing Institute, Heidelberg University
- Mathematical basics and example calculations on the planet Venus