306 Professor Piazzi Smyth on a Centauri, 
Orbit of a! and a? Centauri. 
Position of perihelion, ; : es = 26° 24 
Inclination to the plane of projection, y = 47 56 
Position of ascending node, : Q =) 36, 07 
Angular distance of perihelion from node on 
the plane of the orbit, ‘ ave 29, 222 
Excentricity, : : £ 0:950 
Epoch of perihelion passage, + =1851'50 year 
Periodic time, . . Z P = 77-0 years 
Mean motion, f 5; . v = 4°675 
Semiaxis major, - ¢ : @ = 15-50" 
Mass= 2 of the Solar 
Apparent Orbit. 
Maximum distance, 3 : = 21°85” at 207°5° 
Minimum, ; . = 10°50) enone 
Greatest daily motion, : : = 749, 
We have thus 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 a Cen- 
tauri, and it is a much smaller star, with an orbit of only one-fourth 
the apparent dimensions, and a period of time double the length of 
its southern rival ; so that, while the actual observation for the pur- 
pose of comparing theory with fact would be eight times more difficult 
in the case of y Virginis, and loaded with eight times the probable 
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 Centuari, 
is the accurate determination of the parallax or distance from us, by 
the late Professor Henderson, as we are thereby enabled to speak, 
