376 



SCIENCE- GOSSIP. 



increased to 10-53 magnitude. It had previously- 

 been catalogued as having a peculiar spectrum. 



The New Star in Perseus continues to de- 

 crease in brightness, but not with regularity, 

 having shown remarkable fluctuations. By March 

 18th it was 3-71 magnitude, and on 19th 5 - 25 ; by 

 20th it had again reached 3 - 7, then recommenced 

 falling. On 21st it was 4-0 magnitude, but by 

 22nd again fell to 5"25 ; on 24th it was 4 0, on 25th 

 5-38, and on 26th 3-9 magnitude. It decreased to 

 a little less than 4-0 magnitude on 28th, and seems 

 to have become a trifle brighter on 30th. On 

 March 31st it was a little below 4 - magnitude, 

 but by April 1st had fallen below 5th magnitude. 

 On April 5th it had nearly reached 4th magnitude, 

 and was still about the same brightness on the 8th, 

 but by the 10th had fallen to an equality with 30 

 Persei, which is about 5th magnitude. A photo- 

 graph of the region showing stars to below 11th 

 magnitude, taken only 28 hours before Dr. Ander- 

 son's discovery, by Mr. A. Stanley Williams, of 

 Hove, shows no trace of the Nova. 



Another new variable star in Cygnus has 

 been discovered by Dr. T. D. Anderson, known as 

 2.1901, in E.A. 19h. 12-2m., Dec. N. 29° 55'. 

 Variation was observed from 9'5 magnitude on 

 December 26th, to 10'4 on 16th of February. 



ANSWERS TO CORRESPONDENTS. 



John Clark (Birmingham). — We cannot spare 

 space to enter into the discussion. The moon's 

 path described on p. 317 is not theory, but ascer- 

 tained fact. The mass of the moon is to that of 

 the earth as 1 : 88, nearly ; so that the centre of 

 gravity of the system is well within the globe 

 of the earth. 



CHAPTERS FOR YOUNG ASTRONOMERS. 



By Frank C. Dennett. 



(Continued from p. 318.) 



JUPITER'S SATELLITES. 



Since Galileo first discovered three of these little 

 bodies on January 7th, 1610, and the other a few 

 dayslater, they have been constant objects of interest 

 to the observer. In size, the four which are within 

 the reach of the ordinary telescopes do not differ 

 much from that of our own moon. They are 

 usually known as I., II., III., and IV. in the order 

 of their distance from the planet. Their real and 

 apparent diameters, and distances from Jupiter, 

 are : — 



1 2,390 miles 



II 2,120 „ 



III 3,980 „ 



IV 2,970 „ 



1".048 .... 267,000 miles 



0".911 .... 425,000 „ 



1".513 .... 628,000 „ 



1".278 .... 1,192,000 „ 



From time to time they have been seen with the 

 naked eye, especially when two or more of them have 

 been near together, and when the brilliance of the 

 planet has been modified by twilight. The mis- 

 sionary Stoddart at Oroomiah, in Persia, could often 

 see them under the latter conditions, as also could 

 Mrs. McCance at Putney Hill, London, when II., 

 III. and IV. were near together on October 12th, 

 1878. About 20 minutes before the auroral dis- 

 play on April 21st, 1859, Levander and others at 

 Devizes were able to see two of the satellites in 

 twilight. Dr. R. F. Hutchinson, of Mussoorie, 

 India, has twice seen two of the moons with the 



naked eye. Jacob, Banks, Webb, Mason, Boyd, 

 Buffham and others also add their testimony of 

 seeing these tiny bodies without artificial aid. The 

 slightest optical aid will bring them in view. 

 Their apparent stellar magnitudes being about 7, 

 7, 6 and 7. The tiny fifth satellite discovered by 

 the great Lick telescope is too small to be con- 

 sidered in this matter, being hopelessly invisible, 

 except with the very largest telescope in existence. 



If it were only for their motions around Jupiter, 

 the satellites would be very interesting ; but their 

 eclipses, occupations, transits and shadow transits 

 add much for observation and study, as also do 

 their variations in brightness. M. Camille Flam- 

 marion from observation in 1874-5 found that IV. 

 was dark and sombre compared with the others, 

 but its brightness is very variable, sometimes 

 being equal to the 6th, and at other periods falling 

 to the 10th magnitude. As regards dimensions he 

 places the satellites in the decreasing order III., 

 IV., I., II. For the intrinsic light reflected from 

 equal areas of surface he places them in the order, 

 I., II., III., IV., and found the decreasing order of 

 their variability to be IV., I., II., III. The writer, 

 as a result of observations from 1877 to 1881, 

 found, in decreasing order, the intrinsic surface 

 brightness, II., I., III., IV. Relative variability, 

 IV., I., III., II. I. was usually brightest in the 

 half of its orbit nearest to the earth ; II. in the 

 eastern half ; and III. also in the eastern half ; 

 whilst IV. was dullest in the quadrant nearest the 

 earth on the western side, and a little the brightest 

 in the other quadrant on the same side. These 

 observations lead to the conclusion that the Jovian 

 satellites, like our own moon, turn once on their 

 own axis during one revolution of their orbit ; in 

 other words, always present the same face to the 

 planet. This result has been confirmed more or 

 less completely by several observers at different 

 times during the past century. The study is one 

 which does not need great instrumental power ; 

 but it does necessitate persistent observation, 

 month in and month out, on every available oppor- 

 tunity. It is necessary to note the brilliance of 

 each satellite and also the position of each in its 

 orbit east or west of the planet, and also whether 

 in the superior — or farther, or inferior — or nearer, 

 half of its path, which may be found by reference 

 to the " Nautical Almanac." The time of each 

 record should be noted. There must also be 

 meteorological changes on these moons, for no 

 revolution will alone explain many of the changes 

 to be observed. 



Notwithstanding that the moons, with the ex- 

 ception of V., are all more distant from Jupiter 

 than our own moon is from the earth, their motions, 

 owing to the much greater attractive power of 

 their primary, are much quicker than that of the 

 moon. I. has a month which lasts only 42 h. 

 29 m. ; II., 3 d, 13 h. 13 m. ; III., 7 d. 4 h. ; 

 and even IV. only 16 d., 18 h. 5 m. Barnard's 

 tiny V. only takes 11 h. 49 m. to travel the circuit 

 of its orbit. There is a peculiar relationship 

 between the motions of the first three satellites 

 which makes it impossible for them all three to be 

 in conjunction together. If II. and III. are in 

 conjunction, I. is always at the part of its orbit on 

 the opposite side of the planet. 



The colours of the moons are not identical. I. is 

 usually found to be white or bluish, II. and III. 

 yellow, and IV. dusky red. 



(To be continued.) 



