Hi 



ASTRONOMY. 



[PRECESSION OP TUB BQl'INOXBS. 



the eclipse* always happened earlier than th 

 time ; and when the planet wai near conjunction, tho 

 eclipaes happened later, the whole difference amounting 

 to upwards of sixteen minute*. The tables of Castini 

 were founded on an extensive *erie* of observations at 

 all part* of the orbit of the planet, and would therefore 

 give a result free from aberration at the mean distance 

 of the planet from the earth. Thus, in Fig. 34, 

 T T'T' represent the earth's orbit, 8 the sun, and J J' J" 

 the orbit of Jupiter. Between T and T* the planet is at 

 opposition, and U at its leaat distance from the earth 

 the eclipse* would, at this position, happen earlier at T 

 the planet is at conjunction, or at its greatest distance, 

 when the eclipses would, of course, happen later about 

 this period. Huygens and others continued this asser- 

 tion of Roemer, out the subject does not appear to hare 

 been further attended to till Dr. Bradley, in the year 

 1728, communicated to the Royal Society the theoretical 

 cause of the displacement of a star, termed the " Aber- 

 ration of Light. To the same astronomer we are like- 

 wise indebted for the discovery of the Nutation of the 

 earth's axis. Without these two corrections of Aberra- 

 tion and Nutation, there would be a discordance in 

 the prediction of the apparent place of a star to the 

 amount of 1' nearly of right ascension, and 30" of 

 North Polar distance. To the first of these discoveries, 

 in chronological order, we now beg the reader's atten- 

 tion. 



1'icard, and other astronomers, in observations made 

 for the purpose of determining the annual parallax of 

 certain stars, found an unaccountable difference of 40" 

 (annually) after the application of the Prussian and all 

 other known corrections. Dr. Bradley, who confirmed, 

 by observations of several stars, this difference, explained 

 the theoretical cause in the following manner : Taking 



Kg. M. 



the velocity of light combined with that of tho earth in 

 its orbit on a stars position. 



In Fig. 36 S represents the sun ; T, T, T", <tc., the 

 earth in its orbit ; the place of a star. When tho 



Tif-H. 



earth is at T, an observer at T will see it in the direction 

 of T E ; the corresponding direction that an observer at 

 the sun would see it, would be found by making e E 

 parallel and equal to T S, the radius of the earth's orbit ; 

 and drawing Se equal and parallel to TE, the star's 

 place would then be, to an observer at S, in the position e : 

 similarly when the earth has moved to T', making e' E 

 equal and parallel to 1" S', and completing the parallelo- 

 gram, the star's place would be found me. In the same 

 manner, tracing the position of the star throughout 

 the year, it will be found to describe a curve equal as 

 e e' e" e"' is to the orbit of the earth, and parallel to its 

 plane. 



Now, to transfer these appearances to an example, let 

 us take, in the next figure (Fig. 37), a sphere of which 

 O is the centre, A B C D the ecliptic, K its pole, K S a 



Fig. 37. 



for granted that light took 8' 3* to pass from the sun to 

 the earth at ita mean distance, we have the motion of the 

 earth in it* orbit during this interval equal to 20" -5. In 

 the preceding figure, when the earth is at T, the star 

 being in the direction E T, it is evident that the telescope 

 will not be in this position to see the star ; it must have 

 a certain inclination, T A or T A', such that the cross of 

 wires placed in T describe the distance T T by virtue of 

 the motion of the earth, whilst that light describes the 

 distatice, A'T. We see, in fact, that the light wind. 

 pastes the optical centre of the object-glass A', when tho 

 telescope occupies the position "rA', arrives in T wh.-n 

 the telescope has taken the position T A, and can, conse- 

 quently, meet the cross wires, which are then found at 

 the point T. 



In order that the reader may see clearly tho effect of 

 the aberration of light on a star's position, we shall com- 

 menco with the most simple appearances. And, in the 

 first place, we will consider the effect of the varying posi- 

 tion of the earth in its orbit on the place of a fixed star. 

 For this purpose we shall take as a point of reference the 

 I'Uoe that a Ur would successively take, when seen from 

 th centre of the sun, in the position* of the earth in it* 

 orbit In the next place, we shall treat of the effect of 



circle of latitude passing through a star as -y Draconix ; 

 then, according to the preceding example, when the star 

 is at S, the star will appear, with reference to the centre 

 of the sphere, in a line E e, equal ami parallel to O S, 

 which cuts the cone in the direction p of the small ellipse 

 mp nq. It may be remarked that each star will appear 

 to describe an ellipse in the heavens, which will, for a 

 star situated at the pole of the ecliptic, have its major 

 and minor axes equal, or will become a circle, and will 

 gradually become more and more elongated as it ap- 

 proaches the ecliptic, when it becomes merely a right 

 The major axis, m n, of this ellipse is parallel to 



In, 



the ecliptic ; and the minor axis p q, is perpendicular to 

 it. We have thus found that when the sun is at S, or 

 at the foot of the circle of latitude, the star should 

 appear at p; and as the sun progresses in its orbit, it should 

 successively be found in n, q, and m. 



Such are the positions that Dr. Bradley calculated that 



