452 NOTICE OF THE ORBIT OF THE 
Captain W. 8S. Jacob’s Orbit of a and? Centauri. 
Position of perihelion, er = 26° 24 
Inclination to the plane of projection, y = AT “56 
Position of ascending node, : oO = Shay 
Angular distance of perihelion from ay on the “saab of the orbit, A = 291 22 
Kecentricity, d . : : F s ; = + 0:960 
Kpoch of perihelion passage, rs = 1851:50 year 
Periodic time, : : : t ; P = 77:0 years 
Mean motion, ; : : : ; » =) 4e6ro 
Semiaxis major, . : : ; : e =) loom 
Mass = 3 of the Solar 
Apparent Orbit. 
21:85" at 207°5° 
0:50 ... ay 
2° 40’ 
Maximum distance, 
Minimum, 
Greatest daily motion, 
II 
(See Plate XI.) 
We thus have here altogether one of the most, if not the most, interesting and 
important sidereal system in the heavens; the only one which can compare with 
it is y Virginis, and that has been looked upon as being amongst the double stars, 
what HALLEy’s comet is amongst comets; but though so well and frequently 
observed of late years, it was not instrumentally measured so early as « Centauri, 
and it isa much smaller star, with an orbit of only one-fourth the apparent dimen- 
sions, and a period of time double the length of its southern rival; so that, while 
the actual observation for the purpose of carrving theory with fact would be eight 
times more difficult in the case of y Virginis, and loaded with eight times the pro- 
bable error of observation, there is the further objection, that on account of the 
greater length of the period, but a small portion of the orbit could be determined 
by one observer, or even by one instrument. 
But the crowning importance of the binary system of a Centauri, is the accu- 
rate determination of its parallax or distance from us, by the late Professor 
HENDERSON, as we are thereby enabled to speak, not only of the proportions of 
the different parts of the orbit, but of their actual size, and the weight of the two 
bodies. Thus the least distance of these two suns is only half that of the earth 
from the sun, or a little less than that of Venus, while the greatest distance is a 
little more than that of Uranus; and the mass of the two stars comes out three- 
quarters of that of our sun, their distance from us being 226,100 times our dis- 
tance from the sun. 
Well, therefore, may Sir J. Herscuex have said, “that no subject more 
worthy of diligent and continued inquiry can possibly be urged on the attention 
of southern astronomers.” 
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