PLANETS 



217 



being about 1J times as large as all the others 

 together ; and, as we proceed farther outwards, we 

 find Saturn less than Jupiter, and Uranus than 

 Saturn, though Neptune, the remotest, is some- 

 what larger than Uranus. 



With reference to their distance from the sun, as 

 compared with that of the Earth, the planets are 

 divided into superior and inferior ; Mercury and 

 Venus are consequently the only ' inferior ' planets, 

 all the others being ' superior.' The inferior planets 

 must always be on the same side of the Earth as 

 the sun is, and can never be above the horizon of 

 any place (not in a very high latitude) at mid- 

 night ; they are always invisible at their superior 

 and inferior con junctions, except when, at the latter, 

 a transit (see SUN) takes place. The superior 

 planets are likewise invisible at conjunction, but 

 when in opposition they are seen with the greatest 

 distinctness, being then due south at midnight. 

 The time which elapses from one conjunction to 

 its corresponding conjunction is called the synodic 

 period of a planet, and in the case of the inferior 

 planets mast always be greater than the true 

 period of revolution. 



Fig. 1. Diagram showing the comparative sizes of Sun and Planets. 



Mercury, the planet which is nearest the sun, 

 is also, with the exception of the Planetoids, the 

 smallest (being only three times the bulk of the 

 moon ), and performs its revolution round the sun 

 in the shortest time. Its greatest elongation is 

 never more than 27 45', and consequently it is 

 never above the horizon more than two hours after 

 sunset, or the same time before sunrise ; on this 

 Hoi-omit, and from its small apparent size (5" to 13"), 

 it is seldom distinctly olwervable by the naked eye. 

 It shines with a peculiarly vivid white or rose- 

 coloured light, and exhibits no spote. Its year (or 

 sidereal period of revolution round the sun) is 

 87 "969 days. 



Veniu, the next in order of distance and period, 

 in to us the most brilliant of all the planets. Its 

 orbit is more nearly a circle than any of the others, 

 and when at its inferior conjunction it approaches 

 nearer the Earth than any other planet. Its appar- 

 ent angular dimensions thence vary from 10* at 



the superior to 66" at the inferior conjunction. Its 

 greatest elongation varies from 45 to 47 12', and 

 therefore it can never be above the horizon for much 

 more than three hours after sunset, or the same 

 time before sunrise. While moving from the in- 

 ferior to the superior conjunction Venus is a morn- 

 ing star, and during the other half of its synodic 

 period an evening star. When this planet is at an 

 elongation of 40 its brilliancy is greatest, far sur- 

 passing that of the other planets, and rendering a 

 minute examination through the telescope im- 

 possible. At this period it sometimes becomes 

 visible in the daytime, and after sunset is so bright 

 as to throw a distinct shadow. Astronomers have 

 repeatedly attempted to ascertain the nature and 

 characteristics of its surface, but its brightness so 

 dazzles the eyes as to render the correctness of 

 their observations at best doubtful. From the 

 changes in the position of dusky patches on its 

 surface, which have been frequently noticed, it is 

 concluded that it revolves on its axis, and that its 

 equator is inclined to the plane of its orbit at an 

 angle of 75 ; but many astronomers doubt these 

 conclusions. Its year is 2247 days. Professor 

 Schiaparelli has shown reason for 

 believing that both Mercury and 

 Venus always present the same face 

 to the sun. Both Venus and Mercury 

 necessarily exhibit phases like the 

 moon. For transits of Venus, see 

 SUN. 



The Earth, the next planet in order, 

 will be found under its own name ; it 

 has a single satellite, the Moon (q.v.). 

 Its year is 365-256 days. 



Mars, the first of the superior 

 planets, is much inferior in size to 

 the two previous, its volume being 

 aliont $th of the Earth's, and, after 

 Mercury, its orbit is much more 

 eccentnc than those of the other 

 planets. When it is nearest to the 

 Earth (i.e. in opposition ) its apparent 

 angular diameter is 30" ; when farthest 

 from it(i.e. in conjunction), not more 

 than 4". Mars revolves on its axis 

 ( which is inclined at an angle of 28 

 27') in 24 hours 37 minutes, and its 

 year is 687 days long. In 1877 Hall 

 of Washington discovered that it had 

 two satellites, now named Phobos and 

 Deimos. It shines with a fiery red 

 light, and is a brilliant object in the 

 heavens at midnight when near 

 opposition ; when seen through the 

 telescope its surface appears to be 

 covered with irregular blotches, some 

 of them of a reddish, others of a 

 greenish colour, while at each pole is a spot of 

 dazzling white. The red spots are surmised to be 

 land ; the green, water ; but the markings on 

 Mars appear to have changed considerably since 

 they were first observed. The white spots at 

 the poles are with some reason supposed to be 

 snow, since they decrease when most exposed to 

 the sun, and increase under the contrary circum- 

 stances. The Phases (q.v.) of Mars range be- 

 tween full, half, full (in conjunction, if visible), 

 and half. 



The Planetoids. After Mars in order come the 

 Planetoids (see below), formerly but improperly 

 called Asteroids. 



Jupiter, the next in order, is the largest of all the 

 planets, its bulk being more than 1400 times that of 

 the Earth, though, from its small density, its mass 

 is only 300 times more. After Venus it is the 

 brightest of the planets and the largest in apparent 

 size, its angular diameter varying from 30" to 50". 



