POPULAR ASTRONOMY. 



ii 



times delay it, and sometimes hurry 

 it onward in its orbit. This explains 

 the difficulty that astronomers find in 

 fixing" on the exact date of its arrival, 

 for the amount of the disturbance that 

 it suffers from the attraction of the 

 planets depends on the mass of those 

 planets that affect its motions and also 

 on its varying" distances from them. 

 Now, the masses of Jupiter and Saturn 

 are not even yet absolutely known, so 

 that a certain limit of error must al- 

 ways be allowed in calculating' the 

 movements of the comet under their 

 influence. This has been strikingly il- 

 lustrated since our last article on the 

 comet. At that time Messrs. Cowell 

 and Crommelin, whose calculations of 

 the movements of Halley's Comet are 

 generally regarded as the most trust- 

 worthy fixed April 8th as the most 

 likely time of the perihelion passage, 

 but later they have revised their 

 figures, and now they designate April 

 13th as the proper date, with a leeway 

 of a day or two. But this is not all, 

 and the fact again demonstrates the 

 difficulty of the calculations, for in a 

 recent number of the great German 

 astronomical periodical, the Astrono- 

 mische Nachrichten, another computer, 

 working for a prize, but whose name 

 is not given, fixes the date on June 

 1 8th, at least two months later than 

 the time selected by the English com- 

 puters. Mr. Crommelin says of this 

 discrepancy that it is a little disquiet- 

 ing, both because it shows how widely 

 apart the results found by two inde- 

 pendent calculations based on similar 

 data may be, and because it introduces 

 much uncertainty for those who are 

 trying to be the first to detect the 

 comet in the heavens, coming toward 

 the sun. If it does not arrive at per- 

 ihelion until June it will be seen in 

 quite a different part of the sky from 

 that which it would occupy in April. 

 Only the event can prove which cal- 

 culation is the correct one. But the 

 great divergence is somewhat aston- 

 ishing, considering that in 1835 the 

 computers, wdio did not have as correct 

 data as those now available, hit the day 

 of perihelion passage within two days 

 of the actual time, and none of the 



calculations were as much as one 

 month in error. However, let the per- 

 ihelion be in April or in June, the 

 comet is sure to come somewhere near 

 the expected time, and is equally sure 

 to make a reputation for the first man 

 who succeeds in deserving: it. To- 

 gcther with the opposition of Mars in 

 September, this return of Halley's 

 Comet will go down in history as 

 one of the most interesting astrono- 

 mical events of the Twentieth Century. 

 What the comet will look like nobody 

 can say. As our picture shows, it 

 was a formidable looking object in 

 1835, although not comparable in that 

 respect with its appearance at some 

 previous returns, when it absolutely 

 frightened all Europe. Every time it 

 returns it shows some change of form 

 in tail and head. On this occasion it 

 may blaze with terrifying splendor or it 

 may be comparatively inconspicious. 

 But one thing bearing on this question 

 may be said : all comets that return 

 frequently to the neighborhood of the 

 sun gradually lose some of their sub- 

 stance, because they are all undergo- 

 ing a slow process of disintegration 

 and this may be happening with Hal- 

 ley's Comet, although to a less degree 

 than to comets like Encke's which 

 come to perihelion every three or four 

 years. 



Jupiter continues to be the great 

 planetary ornament of the evening sky, 

 rising in the middle of the mouth be- 

 tween 2 and 3 P. M., and setting about 

 3 A. M. For the telescopic observer 

 nothing can exceed in interest his vast 

 colored belts and swiftly moving - moons. 

 The great planet, as far as we can see, 

 is only an immense ball of clouds. 

 If it has a solid nucleus it lies deep 

 beneath the surface of the planet as 

 it is presented to our eves. 



Saturn can no longer be observed 

 in the evening. Tn the middle of the 

 month it rises about 5 A. M. After 

 April 3rd Saturn is a morning star. 



Venus becomes an evening star on 

 April 28th, but. of course, is too near 

 the sun to be seen. Her glories are 

 reserved for late in the year. 



Mercury is an evening star after 

 April 21st, reaching its greatest elong- 



