transit (transit)

[F. transit = Sp. trán sito Pg. trans i to = It. trans i to, a going over, a pa ing, pa age, transition, For the adaptation of his [man's] moral being to an ultimate destination, by its transit through a world full of moral evil, the economy of the world appears to contain no adequate provision. Whewell. The nece ity of subjecting the you sands of tons of provisions consumed daily by a large army to such long and complicated transits limits the transportation by wagons considerably, and renders the powerful a istance of steam indispensable, both by water and by rail. Comte de Paris , Civil War in America (trans.), I. 202. 2. A line of pa age or conveyance through a country: as, the Nicaragua transit.-3. In astron.: (a) The pa age of a heavenly body acro the meridian of any place. The right ascension of such a body is the sidereal time of its upper transit. (b) The pa age of a celestial body (specifically either of the planets Mercury and Venus) acro the sun's disk, or of a satellite, or the shadow of a satellite, acro the face of its primary. The pa age of the moon acro the sun's face, however, is called an eclipse. The planet Mercury pa es acro the sun's face usually at intervals either of 13 or of 7 years, transits at the planet's ascending node occurring in November, and those at the descending node in May. November transits have occurred or will occur in 1651, 1664, 1677, 1690, 1697, 1710, 1723, 1736, 1743, 1756, 1769, 1776, 1782, 1789, 1802, 1815, 1822, 1835, 1848, 1861, 1868, 1881, 1894, 1907, 1914, 1927, 1940, 1953, 1960, 1973, 1986, 1999, and May transits in 1674, 1707, 1740, 1753, 1786, 1799, 1832, 1845, 1878, 1891, 1924, 1937, 1970, 2003. Owing to the proximity of Mercury to the sun, its transits do not have the astronomical importance of those of Venus, as they are le suitable for determining the solar parallax. Transits of Venus occur at intervals of 8, 122, 8, 105, 8, 122, years, and always in June or December. They are of great importance to the astronomer, for they afford an excellent method of determining the sun's parallax. The actual calculation of this from a transit is very intricate, as many slight corrections and sources of error have to be considered. The principle involved, however, will be understood from the dia IMG:content-0737.png:[blocks in formation] 6432 gram, in which AB represents the earth, and V and S Ve nus and the sun. Observers at A and B see Venus projected on the sun's disk at A' and B' respectively, the observations being made simultaneously. The apparent positions A', B' are carefully determined by photography, by micrometric measures, or otherwise; and a subsequent comparison of notes gives the angle a. If Randr denote the respective distances of the earth and Venus from the sun, the angle ẞ is given by the equation a: ẞ =r: R. The ratior: R is known with great precision from the sidereal periods of Venus and the earth, and since a was found by observation, the foregoing equation determines B. The angle AB'B (being the angle subtended by the earth's diameter at the sun's distance) is equal to double the solar parallax, or to 27. From the triangle AVB' it follows that β = a + 2, or π = (β-α) = a (li/r-1). The transit of 1769 was observed by expeditions sent out expre ly for the purpose by the British, French, Ru ian, and other governments. The celebrated expedition of Captain Cook to Otaheite was one of them. The transits of December 8th, 1874, and December 6th, 1882, were also observed by various government expeditions. The next two transits of Venus will take place on June 8th, 2004, and June 6th, 2012, respectively. The satellites of Mars, Uranus, and Neptune to be seen in transit, and even Titan an unsatisfactory object to follow acro the face of Saturn. Great interest attaches, however, to transits of the satellites of Jupiter, or of the shadows of these satellites. When one of them cro es a dark belt it can usually be followed entirely acro the disk as a round shining spot. The brightne of the satellites is variable, however, and sometimes they look like dusky or even black spots when seen against the disk of the planet. The transit of a satellite's shadow is readily observed. The shadow may be on the disk when the satellite casting it is off, or the two may be seen on the disk at the same time. The shadows are not always black, but are sometimes so bright as to be invisible. They are often, and perhaps usually, different in size from the satellites casting them; and they have repeatedly been seen elliptical in outline. On a few occasions comets are thought to have been seen in transit. transitionally ment properly, and not for the determination of zenith distance or declination. The idea of having an instrument fixed in the plane of the meridian is as old at least as the time of Ptolemy. The first transit-instrument, as the word is now understood, was constructed in 1689 by the Danish astronomer Olaus Roemer. In 1704 Roemer constructed a private observatory near Copenhagen, into which he put a transit-instrument combined with a vertical circle for measuring declinations. This was the first transit-circle made. - Prime vertical transit-instrument. See prime.