ASTRONOMICAL PHENOMENA AND PROGRESS. 



43 



marks, is equal to six periods of Saturn, fifteen 

 of Jupiter, or sixteen of the eleven-year sun- 

 spot periods. 



In the Astronomische Nachrichten, Nos. 2,123 

 and 2,124, Dr. Spoerer, of Potsdam, gives 

 some recent results of his solar observations. 

 A striking feature of the sun's surface in 1876 

 an epoch of spot minimum was the rela- 

 tively high chromosphere in the polar regions, 

 though large prominences were less numerous 

 than usual. One solar explosion was witnessed 

 a jet of flame which shot up suddenly and 

 remained visible seven minutes. Dr. Spoerer 

 twice observed prominences at points on the 

 limb where spots almost immediately after- 

 ward made their appearance. The spots were 

 generally observed within thirty degrees of the 

 equator, though found occasionally at higher 

 latitudes. 



Mass and Density of Mercury. As the peri- 

 helion of Encke's comet is in the vicinity of 

 Mercury's orbit, the occasional near approach 

 of the two bodies furnishes data for deter- 

 mining the disturbing influence of the planet. 

 A new and greatly reduced value of Mercury's 

 mass has thus been obtained by Dr. von Asten 

 after a thorough discussion of the comet's per- 

 turbations. According to this astronomer, the 

 mass of Mercury is to that of the sun in the 

 ratio of 1 to 7,600,000. The corresponding 

 density is 0.88, that of the earth being 1. This 

 density is slightly greater than that of Mars, 

 and less than that of Venus. 



The Late Transit of Venus. The reduction 

 of the transit observations of December 8, 

 1874, has been patiently looked for by the sci- 

 entific public. A preliminary estimate, based 

 on a few of the French observations, gave a 

 parallax of 8". 879, corresponding to a dis- 

 tance somewhat less than 92,000,000 miles. It 

 now appears, however, that this value (of the 

 parallax) must be materially diminished. In 

 October, 1877, the official report of Sir George 

 B. Airy, the astronomer-royal, on the obser- 

 vations made by the expeditions sent out by 

 the British Government was given to the pub- 

 lic. The general result, derived from a dis- 

 cussion of all the British observations, is a 

 parallax of 8".760, from which the astronomer- 

 royal finds the sun's distance to be 93,300,000 

 miles. This value of the parallax is consider- 

 ably less than was expected by astronomers 

 less by one-tenth of a second than that ob- 

 tained in various methods by the most trust- 

 worthy researches. Thus we had : 



Leverrler's value, deduced from his planetary theories 8". 860 



Powalky's, from the transit of 1769 S".860 



Foucault's, from the velocity of light 8".860 



Newcomb's, mean value by various methods 8".S48 



This discrepancy, it must be confessed, is 

 somewhat perplexing, and other reports will 

 be looked for with undiminished interest. It 

 is proper to add that Sir George Airy's discus- 

 sion takes no account of the photographs ob- 

 tained by the British observers. A large num- 

 ber of these sun-pictures are yet to be ex- 



amined, and the details of the process are 

 reserved for a separate report. 



New Determination of the Diameter of Ve- 

 nus. A. W. Downing, Esq., of Dublin, Ireland, 

 has recently communicated to the Royal Astro- 

 nomical Society the result of his discussion of 

 the observations of the vertical diameter of Ve- 

 nus made witli the Washington Transit Circle 

 during the years 1866-'72, inclusive. The true 

 diameter of the planet, derived from this dis- 

 cussion, is 7,469 miles less by 150 miles than 

 the value found by Prof. Peirce from obser- 

 vations made by the Washington Mural Circle 

 during the year 1846. The density correspond- 

 ing to this diameter is slightly greater than 

 that of the earth. 



The Mass of the Earth. It is well known 

 that a fixed relation exists between the solar 

 parallax and the mass of the earth. The value 

 of the latter, as derived from Sir George Airy'a 

 determination of the former, is ?B - 5 1 653 , the 

 mass of the sun being unity. 



The Satellites of Mars. On the night of Au- 

 gust 11, 1877, Prof. Asaph Hall, of the United 

 States Naval Observatory, noticed a small star 

 near the disk of Mars, and recorded its posi- 

 tion. Cloudy weather prevented any further 

 observations till the night of the 16th, when, 

 at ll h - 42 m- , a star of the thirteenth or four- 

 teenth magnitude was again observed very close 

 to the planet. The relative places of the two 

 bodies were carefully noted, and after an inter- 

 val of nearly two hours the planet and star 

 were a third time examined, when it was found 

 that the latter was moving with the former. 

 Prof. Hall at once concluded that he had discov- 

 ered a Martial satellite. On the morning of 

 the 17th the observations were submitted to 

 Prof. Simon Newcomb, who, from the data 

 furnished by Prof. Hall, calculated the period 

 of the satellite at nearly thirty-one hours. This 

 indicated that the body would pass .behind 

 Mars on or before the night of the 17th. That 

 evening it was accordingly looked for in vain. 

 About 1 o'clock, however, on the morning of 

 the 18th it reappeared on the opposite side of 

 the planet's disk. On the evening of the same 

 day it was again found very nearly in its pre- 

 dicted place, and its position was determined 

 by Profs. Hall, Newcomb, and Harkness. 



About 4 o'clock on the morning of the 18th 

 Prof. Hall discovered a second satellite, inte- 

 rior to the first, and of somewhat superior 

 brightness. On the same day these discover- 

 ies were communicated to Prof. Joseph Henry, 

 LL. D., Secretary of the Smithsonian Institu- 

 tion, by whom they were announced to the 

 principal astronomers both in Europe and 

 America. 



From a series of observations extending to 

 the morning of August 21st, the elements of the 

 orbits of both satellites were computed by Prof. 

 Newcomb. The distance of the inner one from 

 the centre of Mars is about 5,800 miles, its pe- 

 riod seven hours and thirty-eight minutes ; the 

 distance of the outer satellite is nearly 15,000 



