SIR 



PLANE 



PLANETS 



and ornament in the smith of Europe. Many fine 

 trees exist in England, but they were at one time 

 mnch more numerous, great part having died in 

 the end of the 18th century. The sprint- frosts and 

 the insufficient duration of the summer for the 

 proper ripening of the wood render Scotland less 



Fig. 1. Oriental Plnne (Platanm orientali) : 

 a, branch ; 6, flower, and c, fruit, on larger scale than o. 



suitable for its cultivation, yet there is a tree at 

 Gordon Castle 66 feet high. No tree better endures 

 the atmosphere of a large city, and there are no 

 finer trees within the precincts of London than 

 its plane-trees. Noble specimens are to be seen 

 in Hyde Park and Russell Square, in London, 

 and the avenue of the Thames Embankment is 

 formed with this tree. In the East the plane 

 attains an immense size. One tree in the meadow 

 of Buyukdere, on the banks of the Bosporus, is 

 141 feet in circumference at the base, extends its 

 branches !."> feet from the tnink, and is believed 

 to be more than 2000 years old. The wood of 

 the plane, when young,, is yellowish white ; when 

 old it is brownish, fine grained, takes a high polish, 

 and is esteemed for cabinet-making. A rich 

 alluvial soil and the vicinity of water are most 



Fig. 2. Branch of Platanut orridentalit. 



ui table to this tree. The North American Plane, 

 or Buttonwood (/*. oectdtHtalu), is a very similar 

 tree. It i the largi-st deciduous tree of the lulled 

 Stated, and abounds on the hanks of the great 

 river* of the middle staU-s. Its timber is not very 

 valuable, and is very liable to decay. A tree of 



this species on the bank of the Thames, in Chelsea. 

 Hospital gardens, is 115 feet high, with a trunk & 

 feet in diameter. The name plane-tree is commonly 

 given in Sr.it hind to what in England is known 

 as the Sycamore (Acer pseitdo-platantu), which 

 resembles the true planes in its foliage, bat U 

 neither a plane nor a true sycamore, being really 

 the Greater Maple. See MAPLE. 



Planetarium. See ORRERY. 



Planetoids " r ASTEROIDS, are now usually 

 known at, .Minor Planets. See PLANETS. 



Planets (Or. jilimiien, 'a wanderer") are those 

 heavenly bodies (including the earth) which be- 

 long to our solar system, and revolve in elliptic 

 orbits round the sun. They are often denominated 

 primary planets, to distinguish them from their 

 moons or satellites, which are called secondary 

 planets. The name planet is of considerable an- 

 tiquity, and was applied to these dependent* of the 

 sun to distinguish them from the myriads of lumin- 

 ous bodies which stud the sky, and which present 

 to the naked eye no indication of change of place 

 (see STARS). The planets at present known me, in 

 the order of their distance from the sun, Mercury, 

 Venus, the Earth, Mars, the Planetoids (q.v.), 

 Jupiter, Saturn, Uranus, and Neptune. Six of 

 these, Mercury, Venus, the Earth (which was not, 

 however, then reckoned a planet), Mars, Jupiter, 

 and Saturn, were known to the nncients ; Uranus 

 was discovered by Sir William Herschel (q.v.) in 

 1781 ; and Neptune, after having its position and 

 elements determined theoretically by Leverrier and 

 Adams, was discovered by Professor Challis and 

 Dr Galle in 1846. The Planetoids, of which HOHH- 

 300 are now known, have all been discovered since 

 January 1, 1801. Six of the planets, the Earth, 

 Mars, Jupiter, Saturn, Uranus, and Neptune, are 

 attended by one or more satellites ; Uranus (gener- 

 ally), Neptune, almost all the Planetoids, and all 

 the satellites except the Moon are invisible to the 

 naked eye. The visible planets can lie at once 

 distinguished from the fixed stars by their clear 

 steady light, while the latter have a sparkling or 

 twinkling appearance. The planets, as oliserved 

 from the Earth, move sometimes from west to 

 cast, sometimes from east to west, and for some 

 time remain stationary at the jmint where pro- 

 gression ends and retrogression commences. This 

 irregularity in their movements was very puzzling 

 to the. ancient astronomers, who invented various 

 hypotheses to account for it. See PTOLEMAIC 

 SYSTEM. The system of Copernicus, by assuming 

 the sun, and not the earth, as the centre of the 

 system, explained with admirable simplicity what 

 seemed liefore a maze of confusion. 



The planetary orbits differ considerably in their 

 degrees of eccentricity, the Planetoids, Mars, and 

 Mercury l>eing most, and the larger planets least 

 eccentric. No two planet* move exactly in the 

 same plane, though, as a general rule, the planes 

 of the larger planets most nearly coincide with that 

 of the ecliptic. The latter are consequently always 

 to l>e found within a small strip of the heavens 

 extending on l>oth sides of the ecliptic ; while the 

 otheis have a far wider range, Pallas, one of them, 

 having the angular elevation of its orbit no less than 

 34 42" aliove the ecliptic. According to Kepler's 

 Laws (<j.v.), the nearer a planet is t<> the sun the 

 shorter is the time of its revolution. The arrange- 

 ment of the planets in the solar system l>ears no 

 known relation to their relative size or weight, for 

 though Mercury, Venus, and the Earth follow the 

 same order in size and distance from the sun, yet. 

 Mars, which is farther from the sun, is much less 

 than cither the Earth or Venus, and the Planetoids, 

 which are still farther off, are the least of all. 

 Jupiter, which is next in order, is by far the largest, 



