200 



KNOWLEDGE. 



[September 1, 1900. 



same interval afterwards, that is to say when the 

 change is about half completed. 



The observation is a simple one, with no accessories 

 of brilliant lights or pleasing colours. Yet the young 

 observer cannot, we think, but exjserieuce a real pleasure 

 when for the first time his observations, carefully and 

 systematically made and dul)' recorded show him beyond 

 a doubt that he is witnessing the dimming of the Demon 

 Star; that he is watching across untold millions of 

 millions of miles of space the signalling of that far 

 distant sun. There will be a sense of achievement, 

 greater and not less because it has been accomplished 

 by his unaided sight, than if he had had the help of 

 some great instrument, and if there be in him anything 

 of the stuflt of which astronomers are made, he will 

 turn eagerly to look for other objects of study, and will 

 wait with much interest for other opportunities of watch- 

 ing Algol. 



He will not soon exhaust this field of work which 

 Algol has to offer him. Minimum after minimum 

 should be carefully watched so as to determine the 

 period. This of course is now known with the utmost 

 exactness, even to the thousandth part of a second, and 

 the purpose of the student's making an independent 

 determination is for his own training in the work, not 

 for a closer approximation to the true elements of the 

 star. Nevertheless it has been by the continual re- 

 jjetition of such observations, long after the period was 

 precisely known, that minute variations in it have been 

 discovered, and the student should certainly not drop 

 Algol from his observing list until he has been able 

 not only to work out a period for himself, and so to 

 predict in advance future minima, but also to detect 

 an apparent irregularity in the period which is known 

 as " the equation of light," and which is due to the fact 

 that light takes some 16 minutes to cross the orbit of 

 the earth. Minima which are observed in November, 

 therefore, when the earth is at its nearest position to 

 Algol, come earlier than the average; those in Maixh 

 and June come later. 



It is of course well known now that the variability of 

 Algol is due to its having a dark companion which 

 revolves round it in about 69 hours. The variation in 

 Beta Lyrse is of a more complicated kind. Here there 

 are two minima, one less pronounced than the other, 

 and we infer therefore that in this case both stars 

 are bright and that they alternately eclipse each other. 

 The variation is less than with Algol, being but little 

 more than a single magnitude. 



Delta Cephei has a variation of much the same 

 amount as Beta Lyr<e, but it differs from that star in 

 that it has a slow decline and a quick recovery — the 

 decline being 91 hours, the recovei-y 38. 



It is, however, rather with the variables of longer 

 period that the student will most occupy himself, and, 

 therefore, it is especially desirable that the beginner 

 should turn his attention to the last of the four stars 

 which I have named, Mira Ceti, before it again fades 

 into invisibility. 



JUPITER AND HIS MARKINGS. 



By W. F. Denning, f.r.a.s. 



Jupiter is the most interesting planet of the solar system, 

 considered as a subject for telescopic investigation; and 

 he is certainly one of the easiest objects we have, on 

 account of his large size and the conspicuous character 

 of many of the markings he displays. It is in the 



study of changes in the figure and motion of these 

 markings that Jupiter offers attractions of a more 

 distinct and special kind than those of any other planet. 

 Venus is beautiful, as a crescent, but her disc shows 

 no more than mere suspicions of dusky areas, of which 

 it is exceedingly difficult to trace the outlines or discern 

 variation. Mars displays an interesting configuration, 

 in reference to which we have still much to learn, but 

 he is of small dimensions, and only visible to the best 

 advantage at comparatively long intervals. Saturn 

 exhibits a novel and picturesque effect, but his details 

 are somewhat faint, and this, combined with his great 

 distance and relatively small apparent diameter, has 

 occasioned a good many dubious observations of late 

 years. No doubt there are occasional irregularities 

 in the belts, and definite spots now and then appear, 

 for there is every reason to believe that the surface 

 phenomena of the planet is somewhat similar to that 

 operating on Jupiter. 



In 1878 and 1879, when the great red spot developed 

 into striking prominence and became an attractive object 

 for study, the planet Jupiter was surveyed in nearly 

 every telescope, and our knowledge of his phenomena 

 was much enhanced. It was soon found that the dark 

 belts and bright zones represented a series of different 

 longitudinal currents. The red spot indicated a rotation 

 in 9h. 55m. 34s., the white equatorial spots in about 

 9h. 50m., while in 1880, some dai-k spots in the north 

 hemisphere returned in 9h. 48m. Other objects showed 

 proper motions, and the rates seemed to vary with the 

 time. It is true these features were not entirely new, 

 for Cassini, about two centuries before, had seen a 

 white equatorial marking rotating in 9h. 50m., while a 

 great southern spot (possibly identical with the red spot 

 of our own times) moved in 9h. 55m. 58s. 



The spots having considerable proper motion and 

 being subject to extensive changes cannot be regarded as 

 material parts of the planet's surface. They are pro- 

 bably situated in the outer envelopes of Jupiter, and 

 do not accurately indicate the true rotation period of 

 the planet's globe. It is probable that the time differs 

 little from 91i. 56m., but it is doubtful to several seconds 

 In the case of Mars we know the rotation period to 

 the tenth of a second, his principal lineaments being 

 durable surface markings, which have been followed 

 during the two and a half centuries which have elapsed 

 since the times of Huygens and Hooke. 



During the last few years Jupiter has received much 

 further investigation. The red spot is still present, though 

 only as a dusky stain in the bay or hollow in the south 

 side of the gi-eat southern equatorial belt. The latter 

 feature has certainly been intermittently visible since 

 1831, Sejstcmber, when Schwabe drew it, and there is 

 every prospect that it will remain visible for many 

 years. In fact, it appears to be an object which, like 

 the red spot, is subject to fluctuations, not only of 

 velocity but of apjjearance, and is also liable to tem- 

 porary obliteration. Its mean rate of rotation between 

 1831, September, and 1899, September, was 

 9h. 55m. 36.4s. (from 60,074 rotations), while its present 

 rate is about 9h. 55m. 41.7s., but in recent years it has 

 varied as under : • — ■ 



The values for the last two years are slightly less than 

 those given in my paper in Monthly Notices. Vol. LIX., 

 p. 580, and they are more correct, depending as they 



