ASTRONOMY. 



[COMETS. 



The diagonal a and 7 of the same square, pro- 

 duoed on the side of 



Pollux. 



Kif . lot. 



7 above the pole, will 

 point out n, or Spica 

 UK, which forms 

 .in equilateral tri- 

 angle with Arcturus 

 and Leon is. 



The line connecting 

 ojtnd ft Unaa Majoris, 

 which points so accu- 

 rately to the I'nle 

 Star, when produced, 

 ill pass through tin- 

 constellation Leo. This 

 constellation consists 

 of four principal stars, 

 in the form of a tra- 

 pezium. The most 

 brilliant is of the first 

 magnitude ; all the 

 others are of the 

 second magnitude. ( 

 and i) Unwu Majoris, 

 being connected in a 

 liiu', will meet with a remarkable star of the first mag- 

 nitude, Arcturus, or a Bootis. 

 At the side of Arcturus, and in the direction of the 



stars 0, i, i, Jof Ursie Majoris, we find the constellation 

 of (' ilis, computed of many stars, 



in a semicircle, and of which the most brilliant is of the 

 second magnitude. 



V^ga, or a Lyno, is a conspicuous object, passing the 

 meridian of Greenwich 13 south of the zenith. It 

 forms a great triangle with Arcturus an. I 1'olai 

 wliich it occupies the summit of the right an 1. At the 

 side of a Lyra> are two stars of the third BUgnitadft, 

 ft and 7, and three of the fourth magnitude, f, i, '. 

 Tlie four stars, ft, 7, $, {, form ;i an, easily 



distinguished. 



Between Lyno and Pegasus the constellation Cygnus 

 is found, composed of five pi ineipal stars, in tin- f. 

 across. Tho line which joins CVMMUS to (lemini 

 in two equal parts by Polaris. Ihe same ,ueed 



beyond ('vunna, passes through Altair. or a Aquihi-, a 

 star of the first magnitude, n Aqnihr, is situated between 

 7 and ft Aquilir, of about the third and fourtli magni- 

 tudes respectively. 



The folio plates afford a general view of the northern 

 and southern constellations, with their most remarkaMo 

 stars. 



[The heavenly bodies, to which we havo directed atten- 

 tion, all revolve in orbits of an elliptic character. \\ e 

 shall now pass to others moving in essentially different 

 curves, which have been described in the Mathematical 

 section, p. 608, et seq. ] 



CHAPTER VIII. 

 ON COMETS. 



THESE bodies, which from the earliest ages have been the 

 subjects of attentive observation, and which, even within 

 the last few centuries, have excited the fears and appre- 

 hensions of the multitude, will next claim our attention. 

 They differ from the other planetary bodies, both in 

 their physical appearances and in the irregularities of 

 their motions. From the investigations of Sir Isaac 

 Newton, to whom this branch of astronomy is much in- 

 debted, and which have been confirmed by the labours of 

 his successors, it has been made evident that comets 



Fig. 107. 



describe renr eccentric ellipses around the sun. so much 

 to that, in the majority of owes, a parabola will perfectly 



reconcile their observed path in the heavens. The incli- 

 nations of their orbits to the ecliptic are also frequently 

 very yreat, and the directionsof their orbital motions are 

 sometimes direct, and in other instances retrograde. In 

 consequence of the great eccentricities of their orbits, 

 they are seldom visible at the utmost more than a few- 

 months about the time of the perihelion passage. The 

 following diagram (Fig- 107) will show, in aclear manner, 

 a parabolic orbit. If with the two foci, F F', an ellipse 

 be described in the usual manner, we shall find that the 

 major axis of this ellipse will be A A', and that 

 we shall obtain a curve dili'ering little from an 

 arch, and presented in a sen perpendicular to 

 the axis A A', or with a very slight degree of 

 ellipticity. A second ellipse, with the foci F F", 

 will exhibit a greater degree of ellipticity, and 

 will enclose the preceding one. A third ellipse, 

 with the foci F F", ' will (still leaving A 

 for the summit of the ellipse) show a still 

 greater degree of ellipticity, and so on. By 

 describing ellipses successively, we shall find the 

 circumference of the curve more and more re- 

 moved from the direction of the major axis A A", 

 till finally, we shall arrive at a point beyond 

 which there is no variation. The curve B A C, 

 thus described, is called a parabola. The point 

 A is called the summit of this parabola, and tli.. 

 point F its focus. A parabola is then formed 

 of two parts, A B and A C, exactly alike to one 

 another, and extending infinitely on each side 

 of the major axis, A F F F*. We can easily 

 perceive that, as the comet recedes from the 

 sun into space, it will be necessarily lost to our 

 view. For the determination of the elements 

 of the orbit of a comet, three complete observa- 

 tions are necessary as a first approximation, 

 which can be afterwards brought to greater ac- 

 curacy by means of the other additional observa- 

 tions during the time of its visibility. In a para- 

 bola the following elements are required : 

 1st, the longitude of the perihelion ; 2nd, the 

 longitude of the node ; 3rd, the inclination of the orbit 

 to the plane of the ecliptic ; 4th, the time of perihelion 



