| Abstract from DBPedia | In astronomy and celestial navigation, an ephemeris (pl. ephemerides; from Latin ephemeris 'diary', and Greek ἐφημερίς (ephemeris) 'diary, journal') is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly velocity) over time. 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 the first applications of mechanical 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 the spherical polar coordinate system of right ascension and declination, together with the distance from the origin if applicable. Some of the astronomical phenomena of interest to astronomers are eclipses, apparent retrograde motion/planetary stations, planetary ingresses, sidereal time, positions for the mean and true nodes of the moon, the phases of the Moon, and the positions of minor celestial bodies such as Chiron. Ephemerides are used in celestial navigation and astronomy. They are also used by astrologers.天体暦（てんたいれき、ephemeris）とは、主として太陽系内の天体（太陽・月・惑星・衛星）や恒星および人工天体の時間に関する位置、および日食、月食、天体の出没などの天象の時刻・位置などを推算した予報を書き下したものである。天文暦、暦（れき）、軌道暦、天文年鑑 (astronomical almanac) とも言う。 現在、天体暦は基本暦 (fundamental ephemeris) と視天体暦 (apparent ephemeris) に分けることができる。基本暦は天体の運動方程式を積分して得られる天体の幾何学的な位置を扱い、視天体暦は基本暦を基に座標変換や惑星光行差の補正などをして得られる天体の視位置を扱っている。 天体の位置予測は、天文学・宇宙探査だけでなく、かつて外洋の航海においても船舶の位置を決定するために重要なものであった。天体暦から天測航法に必要な情報を抜粋したものを航海年鑑という。 (Source: http://dbpedia.org/resource/Ephemeris) |
