118 A SOUTHERN OBSERVATORY. 
summer of 1829, Dr. Young gave in charge to Prof. Rigaud a memo- 
randum urging Henderson’s superior qualifications for the post of 
superintendent of the Nautical Almanac, vacated by his own death a 
fortnight later; and the recommendation was doubtless influential in 
procuring for him after three years the offer of the Cape observatory. 
Assuming the chief command there in April, 1852, he acciunulated 
in thirteen months, a surprising number of valuable observations, still 
in part unpublished. One of the results derived from them was how- 
ever of so striking a character as to attract instant and universal at- 
tention. It was nothing less than the first authentic determination of 
the distance of a fixed star. 
After Sirius and Canopus, the brightest star in the heavens is Alpha 
Centauri. This beautiful object is easily resolved into two,—one fully 
three times brighter than the other. And these two circulate round 
each other, or rather round their common center of gravity, in a period 
of about eighty-eight years. The system thus formed was discovered 
by Henderson to have an‘ annual parallax ” of just one second of are. 
That is to say, the apparent places of the component stars as viewed 
from opposite sides of the earth’s orbit, differed, through a familiar 
effect of perspective, by z¢z's00 Of the distance from the horizon to the 
yenith. The more refined determinations of Drs. Gill and Elkin, while 
establishing its reality, have since shown that Henderson’s parallax 
was somewhat too large. The actual distance of Alpha Centauri from 
the earth is, in round numbers, 25,500,000,000 of miles. Even the 
setherial vibrations of light occupy four years and four months in span- 
ning this huge interval; yet Alpha Centauri (so far as is at present 
known) is the nearest neighbor of our sun in space. 
The attractive power of each of these coupled stars appears to be 
about equal; but while one is nearly twice, the other is only half as 
luminous, in proportion to the amount of matter it contains, as our 
own sun. Hence, according to our present notions, the darker, more 
condensed body must be considerably more advanced on the road 
towards extinction than its brilliant companion, and an attentive study 
of its spectrum ought to give interesting results. 
Henderson returned to Europe in 1833, unable, in the uncertain state 
of his health, to support the discomforts—long siice banished with the 
wolves and jackals—of a residence at Observatory Hill. He became 
astronomer royal for Scotland in 1834, and died suddenly of heart dis- 
ease ten years later. 
The third astronomer at the Cape, and the first whose term of activity 
there was prolonged to a fitting conclusion, was an Irishman. Sir 
Thomas Maclear was bornat Newtown Stewart, in County Tyrone, March 
17,1794. His career, like those of his predecessors, swerved insensibly 
towards the stars. He was a physician, practicing at Biggleswade, 
in Bedfordshire, whose astronomical proclivities had been fostered by 
the genial influence of Admiral Smyth, when summoned, as one may 
say, to the celestial charge of the southern hemisphere. 
