HARDWICKE'S SCIENCE-GOSSIP. 



75 



Towards the end of last November it was possible, 

 with a powerful telescope, to see no fewer than seven 

 comets at the same time. A comet does not gene- 

 rally attract popular attention unless it is very bright 

 or large, or so situated with respect to our line of 

 sight as to appear very large. None of these seven 

 comets fulfilling any of these conditions, none 

 attracted public attention. Only one presented any 

 peculiarity. This was discovered on November 6, by 

 Mr. Holmes, of Islington, who was well acquainted 

 with the constellation Andromeda. On that night 

 he was observing, with his twelve-inch reflector, that 

 group of stars for a special purpose, when he detected 

 a nebulous object which was not charted. It was 

 soon found to be a comet, and it, also, created 

 a sensation in the astronomical world. Comets 

 move in parabolic, hyperbolic, or elliptic orbits. To 

 determine the shape of a comet's orbit, the direction 

 of its motion, its inclination, and the time of its 

 perihelion passage, at least three accurate observations 

 of its place at different times are required. When 

 Comet Holmes was first seen, its apparent motion 

 was very small ; consequently, the difficulty of 

 determining its orbit was very great. One calculator 

 produced an orbit, in accordance with which the 

 comet was rapidly approaching the earth, and some 

 little alarm was occasioned in certain quarters by this 

 announcement, while another physicist assigned to it 

 a motion in the opposite direction. Both of these 

 orbits have since proved to be erroneous. 



The light proceeding from this comet varied, 

 though not exactly as one would have expected, but 

 its changes in apparent size are, at present, in- 

 explicable. When, theoretically, it should have 

 subtended a smaller angle than it did at the time of 

 its discovery, it was really more diffused, and, more 

 wonderful still, on January 18, it presented the 

 appearance of a nebulous star of the eighth magni- 

 tude. 



I have said that photographs of the heavens are 

 being constantly taken. The time of exposure varies 

 with the object to be photographed ; if a picture 

 of stars is required, the exposure may be only 

 for a few seconds, but a spectrum or a nebula may 

 require an exposure of some hours. This is possible 

 by means of an equatorial driven by accurate clock- 

 work. Should it be desired to photograph a comet, 

 the rate of whose apparent movement differs from 

 that of the stars, the rate of the clock-work must be 

 adjusted accordingly, or the comet will appear on 

 the plate only as a trail, or sort of long patch. In 

 a photograph taken at Hammersmith on October 18, 

 there is a peculiar object which at first was taken 

 for Comet Holmes. Authorities, however, differ ; 

 some think it is the comet, others look upon it as a 

 mere imperfection. The first actual discovery of a 

 comet by photography was made on October 12, by 

 Mr. Barnard, the discoverer of Jupiter's fifth satellite. 

 The plate was exposed for 4 hours 20 minutes and, 



on being developed, a very peculiar patch was ob- 

 servable. Shortly afterwards the comet was visually 

 discovered. 



When one views the sun through a telescope, the 

 eye being of course properly protected from the 

 excessive heat and light, dark spots (as well as 

 other peculiarities) are frequently seen on its surface. 

 These are not scattered irregularly over the disc, but 

 are usually confined to a zone extending for about 

 35° on either side of the solar equator. ' They vary 

 very much in size and appearance as well as in their 

 persistence. Some are visible for only a short time, 

 while others are much more permanent, and occa- 

 sionally spots are seen for several solar rotation- 

 periods of twenty-five days each. Some are so 

 enormous that they may be seen without a telescope, 

 either by using a dark glass, or through the inter- 

 vention of a thin cloud. To be seen by the naked 

 eye, they must be at least three times the size of 

 the earth, and subtend an angle of at least 50" of arc 

 — the mean apparent diameter of the sun is about 30'. 

 They appear singly or in groups. No one knows 

 their origin ; no one can predict either their appear- 

 ance or, when they have once shown themselves, 

 their disappearance. There is, however, roughly 

 speaking, an eleven-year period of maximum and 

 minimum frequency. A sun-spot usually consists of 

 three parts — the spot proper, which is dark, its 

 penumbra, which is much lighter, and its nucleus or 

 centre, which is the darkest 'part of all, but is not 

 always distinguishable. Some spots have a com- 

 paratively small penumbra, whereas others are sur- 

 rounded by a penumbra of enormous size, presenting 

 most varied and fantastic shapes. When at the edge 

 of the sun's disc, spots have sometimes been seen as 

 actual depressions or notches on the limb. 



A magnificent group of spots was observed 

 November 15, 1 89 1, and was watched through five 

 semi-rotations. On its first appearance it extended 

 in area over 400 millionths of the sun's visible 

 hemisphere, and was soon distinguished by a beauti- 

 ful network of bright interlacing bridges, which 

 divided its nucleus into a perfect archipelago. By 

 February 13, its area had increased to 3500 

 millionths, or, measured in miles, its length may be 

 taken as 162,000, and its breadth as 75,000; this 

 extensive group had, on March 5, diminished to less 

 than one-fifteenth of that size, and was not seen after 

 March 17. When it was at its maximum in February, 

 a great magnetic disturbance took place on the earth, 

 as so frequently happens during the formation or 

 existence of large solar spots. That there is a close 

 connection between these two phenomena can hardly 

 admit of doubt, but what, exactly, that connection is, 

 science has yet to discover. 



Besides spots, light streaks called faculae are seen 

 on the sun's disc. These are not so easy to observe, 

 except when near the edge, owing the brightness of 

 the sun. Round the circumference of the disc are 



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