1861.] 
The great Comet of 1861. 
279 
The great Comet o/‘1861 . — Bg the Rev. Dr. Mackay. 
The following paper contains the computations by which the ele- 
ments of the great Comet of 1861 were determined. They deserve 
record, partly as being the earliest determination of the elements 
known in India, but chiefly as being founded on observations taken 
with a common sextant, reading to 10", and as showing how much 
can be done, and with very considerable accuracy, with means appar- 
ently so insufficient. 
The method adopted was to choose two stars, one to the south, 
the other to the east, of the comet’s apparent position, and, about 
the same time every available evening, to measure the comet’s 
distance from both very carefully, and, by the mean of several obser- 
vations, to reduce them to a fixed time. From these observations 
the comet’s A. It. and Declination were computed ; and, to ensure 
greater accuracy, the distance of the comet from a third star was 
first measured by the sextant, and then computed with its E. A. and 
Dec., as already found. If the measured and computed distances 
agreed, the observations were looked upon as trustworthy. The 
differences, one time with another, averaged a little more than 1' of 
arc : and as the united effects of nutation, precession, and aberration, 
partly balancing each other, will not amount to more than a few 
seconds of arc, while the uncertainty of observation is more than a 
minute, not wishing to affect accuracy where it was not attainable, 
I have omitted them altogether. In like manner, not knowing before- 
hand the comet’s distance from the earth, I was unable to correct its 
position for parallax, leaving the parallax to be applied in the com- 
putation of another set. 
The stars chosen were Vega and Arcturus (the 3rd being Eta of 
the G-reat Bear) : and the observations fixed on were those of July 
5th, 10th, and 15th. 
My normal triangle therefore was P A V, where 
P A and P Y are the North Polar distances of Arctu- 
rus and Vega, and A P V the difference of their right 
ascensions, all taken from the Nautical Almanac. A V 
was then computed, and the other two angles found 
by the common rules ; thus 
