ECLIPSE

in astronomy, complete or partial obscuring of a celestial body by another. An eclipse occurs when three celestial objects become aligned. The Sun is eclipsed when the Moon comes between it and the Earth; the Moon is eclipsed when it moves into the shadow of the Earth cast by the Sun. Eclipses of natural or artificial satellites of a planet occur as the satellites move into the planet's shadow. The two component stars of an eclipsing binary star move around each other in such a way that their orbital plane passes through or very near the Earth, and each star periodically eclipses the other as seen from the Earth. When the apparent size of the eclipsed body is much smaller than that of the eclipsing body, the phenomenon is known as an occultation. Examples are the disappearance of a star, nebula, or planet behind the Moon, or the vanishing of a natural satellite or space probe behind some body of the solar system. A transit occurs when, as viewed from the Earth, a relatively small body passes across the disk of a larger body, usually the Sun or a planet, eclipsing only a very small area: Mercury and Venus periodically transit the Sun, and a satellite may transit its planet. in astronomy, complete or partial obscuring of a celestial body by another. An eclipse occurs when three celestial objects become aligned. The many eclipse phenomena known to astronomers are of two distinctly different types. In the first, the eclipsing body comes between an observer and the eclipsed object; the latter appears to the observer totally or partly covered by the eclipsing object. Eclipses of the Sun, occultations of stars by the Moon, transits of Venus or Mercury across the Sun's disk, and eclipses of binary stars are of this kind. Eclipses of the second type affect only planets or natural satellites that are not self-luminous. In this case, the eclipsing body intervenes between the Sun and the eclipsed object. The latter remains in view of the observer, but its illumination by the Sun is interrupted, and it becomes darkened by entering into the shadow of the eclipsing object. Examples of this kind of eclipse phenomenon are eclipses of the Moon. Solar and lunar eclipses have long been of interest, because they are readily observable to the unaided eye and offer an impressive spectacle. Primitive peoples were struck with fear by the falling darkness during a total solar eclipse or by the strange sight of the eclipsed Moon. Accounts of such eclipses are found among the oldest records of history, and the successful prediction of eclipses constitutes one of the earliest achievements of the scientific investigation of nature. Solar eclipses. A solar eclipse occurs when the Moon, revolving in its orbit around the Earth, moves across the disk of the Sun so that the shadow of the Moon sweeps over the face of the Earth. No sunlight penetrates the umbra, the inner part of the shadow. To observers on the Earth within the umbra, the disk of the Sun will appear completely covered by that of the Moon. Such a solar eclipse is said to be total. Because the umbra is narrow at its intersection with the Earth, a total eclipse can be observed only within a very narrow areathe zone of totality. Furthermore, because of the relative motion of the bodies, the conical shadow moves rapidly over the terrestrial surface; the totality of the solar eclipse thus lasts only a short time (less than eight minutes at any one place on the Earth). To observers located within the penumbra, the outer portion of the Moon's shadow, the disk of the Moon will appear to be projected against the Sun so as to overlap it in part. This gives rise to a partial solar eclipse. Because the Earth revolves around the Sun in an elliptical orbit, the distance of the Sun changes slightly during the course of a year. Similarly, the apparent size of the lunar disk changes to some degree during a month because of the elliptical shape of the Moon's orbit. If an eclipse occurs when the Sun is closest to the Earth and the Moon is farthest away, the Moon will not completely cover the Sun, thereby leaving the rim of the latter all around it. This form of eclipse is known as an annular eclipse. Eclipses of the Sun occur two to four times a year. In rare instances, more may occur, as in 1935, when there were five solar eclipses. Partial solar eclipses yield little information of astronomical interest. Total eclipses, on the other hand, have contributed much knowledge about the nature of the chromosphere and corona, the thin external layers of the Sun that are usually lost in the brilliant glare from the shining solar surface (the photosphere). At total eclipse the Moon acts as a screen outside the Earth's atmosphere, cutting off the direct rays from the photosphere. The brillance of the sky is decreased greatly, and the fainter appendages of the Sun become visible. The astronomical value of eclipse observation has decreased in recent years, largely as a result of the invention of the coronagraph. This instrument obscures the photosphere artificially and thereby makes it possible for investigators to conduct studies of the solar chromosphere and corona without waiting for eclipses to occur. Additional reading Bryan Brewer, Eclipse (1978); and David Allen and Carol Allen, Eclipse (1987), explain and recount the history of eclipses for the general reader. Donald H. Menzel and Jay M. Pasachoff, Solar Eclipse: Nature's Super Spectacular, National Geographic, 138(2):222233 (August 1970), chronicles the events of a solar eclipse expedition. Frank Dyson and R.v.d.r. Woolley, Eclipses of the Sun and Moon (1937); and J.B. Zirker, Total Eclipses of the Sun (1984), discuss in considerable detail the history, methods, and results of eclipse observations. W.M. Smart, Textbook on Spherical Astronomy, 6th ed., rev. by R.M. Green (1977), presents the basic mathematical tools for calculating occultations and eclipses. Great Britain Nautical Almanac Office, Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac (1961, reissued 1977), presents in comprehensive fashion data for the calculation of astronomical phenomena. F. Link, Eclipse Phenomena in Astronomy (1969), treats modern developments in eclipse problems, excluding solar eclipses and eclipsing variables. Works on historical eclipses include F. Richard Stephenson, Historical Eclipses, Scientific American, 247(4):170183 (October 1982); Said S. Said, F. Richard Stephenson, and Wafiq Rada, Records of Solar Eclipses in Arabic Chronicles, Bulletin of the School of Oriental and African Studies, 52:3864 (1989); and F. Richard Stephenson and S.S. Said, Non-tidal Changes in the Earth's Rate of Rotation as Deduced from Medieval Eclipse Observations, Astronomy and Astrophysics, 215(1):181189 (1989). Catalogs of eclipse data include Th. Ritter Von Oppolzer, Canon of Eclipses (1962; originally published in German, 1887), astronomical data of all solar eclipses between 1207 BC and AD 2161 and eclipses of the Moon from 1206 BC to AD 2163, with maps of the central lines of the solar eclipses over the Earth; George Van Den Bergh, Eclipses in the Second Millennium BC (-1600 to -1207) (1954), a demonstration of a method for computing each of these eclipses with simple arithmetic, and Periodicity and Variation of Solar (and Lunar) Eclipses (1955; originally published in Dutch, 1951), an arrangement of all the eclipses in Oppolzer's Canon into the saros and the inex periods in a newly developed panorama; Robert R. Newton, Ancient Astronomical Observations and the Accelerations of the Earth and Moon (1970), and Medieval Chronicles and the Rotation of the Earth (1972); and Hermann Mucke and Jean Meeus, Canon of Solar Eclipses -2003 to +2526 (1983), and Canon of Lunar Eclipses -2002 to +2526, 2nd ed. (1983), more accurate data and maps of central lines over the Earth of all solar and lunar eclipses over 4,500 years. Joseph Needham, Science and Civilisation in China, vol. 3, Mathematics and the Sciences of the Heavens and the Earth (1959), includes a discussion of eclipses placed in the context of Chinese astronomy with extensive references to original literature. Bernard Lovell (ed.), Astronomy, 2 vol. (1970), contains discourses in the physical sciences from 1851 to 1939, a large number of which are devoted to solar eclipses. Jakob Houtgast F. Richard Stephenson