INTRODUCTION TO ASTRONOMY. xxxvii 



them at stated times proves that they revolve round the sun ; but in orbits 

 so extremely eccentric, and running- to such a distance from the sun, that 

 they disappear for a great number of years. If they are inhabited, it must 

 be by a species of beings very different, not only from the inhabitants of 

 this, but from those of any of the other planets, as they must experience 

 the greatest vicissitudes of heat and cold : their heat in that part of their 

 orbit nearest the sun is computed to be greater than that of red-hot iron. 

 In this part of its orbit the comet emits a luminous vapour, called the tail, 

 which it gradually loses as it recedes from the sun ; arid the comet itself 

 totally disappears from our sight in the more distant parts of its orbit, 

 which, in most cases, extends considerably beyond that of the furthest 

 planet. 



The number of comets belonging to our system cannot be ascertained, 

 as some of them are several centuries before they make their re-appear- 

 ance. The number that are known by their regular re-appearance is very 

 small. 



The ancients, in order to recognise the fixed stars, formed them into 

 groups, to which they gave the names of the figures delineated on the 

 celestial globe. In order to show their proper situations in the heavens, 

 they should be painted on the internal surface of a hollow sphere, from 

 the^centre of which you should view them : you would then behold them 

 as they appear to be situated in the heavens. The twelve constellations, 

 called the Signs of the Zodiac, are those which are so situated, that the 

 earth in its annual revolution passes directly between them and the sun. 

 Their names are Aries, the Ram ; Taurus, the Bull ; Gemini, the 

 Twins ; Cancer, the Crab ; Leo, the Lion ; Virgo, the Virgin ; Libra, 

 the Balance; Scorpio, the Scorpion; Sagittarius, the Archer; Capri- 

 cornus, the Wild Goat ; Aquarius, the Water-carrier ; Pisces, the Fishes : 

 the whole occupying a complete circle, or broad belt, in the heavens, 

 called the Zodiac (Jig. 7). Hence, a right line drawn from the earth, 

 and passing through the sun, would reach one of these constellations, and 

 the sun is said to be in that constellation at which the line terminates. 

 Thus when the earth is at A, the sun would appear to be in the constella- 

 tion or sign Aries ; when the earth is at B, the sun would appear in 

 Cancer ; when the earth was at C, the sun would be in Libra ; and 

 when the earth was at D, the sun would be in Capricorn. This circle, 

 in which the sun thus appears to move, and which passes through the 

 middle of the Zodiac, is called the Ecliptic. 



We have no means of ascertaining the distance of the fixed stars. 

 When therefore they are said to be in the Zodiac, it is merely implied that 

 they are situated in that direction, and that they shine upon us through 

 that portion of the heavens which we call the Zodiac. 



Whether the apparent difference, of size and brilliancy of the stars 

 proceed from their various degrees of remoteness, or of dimension, is a 

 point which astronomers are not enabled to ascertain. Considering them 

 as suns, we know no reason why they should not vary in size, as well as 

 the planets belonging to them. 



It may perhaps be objected to this system of the universe, that it is 

 directly in opposition to the evidence of our senses, to which it is plain, 

 and obvious that the earth is motionless, and that the sun and stars revolve 

 round it. But our senses, or at least the inferences we draw from them, 

 too often mislead us, for us to place implicit reliance on them. When 

 sailing on the water with a very steady breeze, the houses, trees, and every 

 object appear to move, whilst we are insensible of the motion of the 



