ASTRONOMY. 133 



THE PLANETS. 



Mercury : The mass of Mercury. — Dr. Backlund has published in the 

 Bnlletin astronomique for October a new mass of Mercury, obtained in- 

 cidentally in his discussion of the motion of Encke's comet. The new 

 result in question is ae^iyom ^^^ sun's mass being unity, and this is the 

 largest value of the mass of the planet yet obtained. Dr. Backlund 

 states that, even supposing the acceleration of the comet's mean motion 

 to have been constant during the entire period, 1871-'85, it is not pos- 

 sible to represent satisfactorily the five apparitions of the comet during 

 that i)eriod on the assumption of a mass of Mercury less than 5-00^^0^0. 



Venus : Semi- diameter of Venus. — Mr. Thackeray, discussing the ob- 

 servations of Venus made at Greenwich from 1866 to 1884, finds that 

 the amount of personality in the measures is much greater than the cor- 

 rection due to the instrument, and that, though a greater number of 

 observers by compensating one another might give increased accu- 

 racy to the value of the semi-diameter, it is just as likely that they 

 should not. 



The Earth : Geodetic Congress. — We learn from Nature that the In- 

 ternational Geodetic Conference met at Berlin in October, 1886, to settle 

 the organization of the central geodetic bureau (which is to have its per- 

 manent seat at Berlin), and to determine upon the best method of exe- 

 cuting the resolutions passed at Eome and Washington in 1883 and 

 1884, respecting the actual measurement of a degree on the earth's sur- 

 face. The adoption of Greenwich as a first meridian is to be strictly en- 

 forced, but the introduction of international normal time is postponed 

 on account of insuperable practical difficulties. 



The proposed change in the beginning of the astronomical day. — It is to 

 be regretted that no agreement has yet been reached by astronomers 

 upon the proposition to change the beginning of the astronomical day 

 from noon to midnight. 



The general sentiment is opposed to making any change until it is 

 clear that it will be adopted by a majority of astronomers, and until the 

 proper modifications have been introduced into our principal ephemeri- 

 des. The new day has been provisionally adopted by Mr. Christie, at 

 Greenwich, and the board of visitors have recommended that it be in- 

 troduced into the Nautical Almanac for 1891. On the other hand, the 

 superintendents of the German and American ephemerides oppose any 

 change ; and there seems to be great danger that the agitation of the 

 question by the Washington Meridian Confereuce in 1884 may introduce 

 new confusion rather than remove the old. At present there is little 

 prospect of the plan meeting with anything like a general acceptance 

 before the beginning of the next century. 



Theory of the moon^s motion. — Several valuable papers upon the lunar 

 theory have been published by Hill and others. Reference should be 

 made to the papers themselves, cited in our Bibliography. 



