21 



NOTES ON COMET 1, FOR 1865, 



Together with additional Notes on the Nebula sue- 



EOUNDiNG V Aegus. Bij Fraucis Ahhoit, F.B.A.S. 



Read before the Royal Society, March 14th, 1865. 



Various conjectures are fi'equently offered respecting the 

 physical aspect, nature, origin and office of Comets, and these 

 subjects are at the present time, still involved in great 

 obscurity. A Comet when examined by proper optical means, 

 has the appearance of an ill-defined gaseous or nebular 

 substance surrounding a dark nucleus, which nucleus has 

 different degrees of opacity in different Comets, and which in 

 general becomes more brilliant as the Comet approaches its 

 perihelion. 



A second characteristic is their internal condensation. Most 

 of them have a minute stellar point, called the nucleus, which 

 occupies the position of maximum density. This centre of 

 condensation, or brightest point is, with rare exceptions, placed 

 on the side which is nearest to the sun, and is always very 

 close to the centre of gravity. 



Another feature of importance is the tail (so-called) although 

 it sometimes precedes the nucleus in its motion, and usually 

 in a direction opposite to that of the sun — the convex, and 

 brightest side of the tail being ordinarily presented to the 

 region towards which the Comet is moving. 



To determine the orbit of a Comet requires a very difficult 

 and troublesome computation, chiefly because none of them 

 are visible through the whole of their revolution. Such a 

 computation would occupy a person well acquainted with the 

 subject about twenty-four hours ; but a Comet's proper course 

 may be found by observing its distance, fi'om time to time, 

 from two fixed stars, whose positions are accurately known. 

 Or by finding its altitude when in the same azimuth with two 

 known stars. By either of these means the place of a Comet 

 may be computed for each night, and thence its course, and if 

 a great circle be drawn through three distant places thus laid 

 down, it -will intersect the Ecliptic, and show approximately 

 the place of the node. The inclination of the Ecliptic being 

 thus found from several triplets, independent of each other, 

 a mean of the results may be considered tolerably correct. 



In a similar way a Comet's distance may be approximately 

 known by parallax. A Comet shortly before it disappears 

 moves so slowly, that for several days it appears to have but 

 little motion among the Stars, let it be first observed when it 

 is high above the horizon, take any two stars between which 

 the Comet lies iu a right line parallel to the horizon, extend a 



