NATURE 



{Nov. I, 1877 



In the first are given the descriptions of the various phe- 

 nomena, in the words of the observers, with the Green- 

 wich sidereal times of the different phases, obtained from 

 accurate reduction of the ojjservatipns for longitude here 

 particularised ; where such longitudes depend upon lunar 

 observations the places of the Nautical Almanac were 

 carefully corrected by observations on nearly the same 

 days at Greenwich, Paris, Strasburg, and Konigsberg. 

 In studying these original descriptions, Sir George Airy 

 was led to infer that it was " possible to fix upon three 

 distinct phases for the Ingress and four for the Egress," 

 though it might have been supposed that Egress and 

 Ingress would exhibit the same number of distinct phases 

 in inverse order ; this was not the case in practice. The 

 first phase, a, utilised in the calculations is the appear- 

 ance of the planet just within the sun's disc, but the light 

 between the two limbs being very obscure. After an 

 interval of about twenty seconds " the light begins to 

 clear, and the observers generally think that the contact 

 is passed ;" this is phase /3. About twenty seconds later, 

 the light which at phase /3 was not equal to that of the 

 sun's limb, is free from all shadow, and the phase is 

 called y. Sir George Airy finds that of these phases /3 is 

 the most exact, observers, even in the presence of clouds 

 of moderate density, agreeing within three or four 

 seconds, though for other phases much greater discord- 

 ances are exhibited. Similarly at the Egress, the first 

 appearance of a fine line or faint shadow is called 8, 

 this becoming definite, or a " brown haze " appearing, is 

 called e. When most observers record "contact," the 

 shadow having reached a maximum intensity, the phase 

 is called f, and in this phase there is an agreement 

 amongst observers, much closer than in other phases at 

 Egress. The "circular" contact at Egress is called »;. 



In the second section of the Report, or Table II., these 

 " adopted phases are massed for each district in which 

 the parallax-factor is nearly identical," and several of the 

 details of reduction are included. With the longitudes 

 determined as above, the recorded times of the various 

 phases of the transit were converted into Greenwich 

 sidereal times. With the calculated apparent places of 

 the sun and Venus in the Nautical Almanac, as deduced 

 from Leverrier's Tables, an ephemeris was prepared ex- 

 hibiting the predicted geocentric places for every tenth 

 second of Greenwich sidereal time throughout the transit, 

 and from these numbers the apparent positions of sun 

 and planet at each station were computed. Calculations 

 were further made, showing how the predicted places 

 would be affected by alteration of the local longitude, by 

 change in the tabular places of the sun and Venus, and 

 by alteration of their tabular parallaxes ; the first two 

 alterations were not essential in these reductions, but the 

 determination of alterations of the third class, as it is 

 remarked, constituted " the special object of the expe- 

 dition." The form of the reductions was " entirely de- 

 termined by the consideration that such alterations must 

 be made in the parallaxes as will render the observations 

 of the same phenomena in different parts of the earth 

 consistent with each other." In Table III. we have 

 " the mean solar parallax deduced from every available 

 combination." Thus Ingress accelerated at the Sandwich 

 Islands is compared with Ingress retarded at Rodriguez 

 and with Ingress retarded at Kerguelen's Land ; Egress 



retarded at Mokattam and Suez with Egress retarded at 

 Rodriguez, and likewise with Egress accelerated at the 

 two stations in Kerguelen's ; and again the retarded 

 Egress at Thebes is compared with Egress retarded at 

 Rodriguez and with Egress accelerated at Kerguelen's. 

 The greatest separate value of the solar parallax re- 

 sulting from these different comparisons is 8'''"933 and the 

 least 8""407. Weights are given to the various deter- 

 minations depending, firstly, upon the number of observa- 

 tions and the magnitude of the parallax-factor ; and 

 secondly, upon the particular phase a, /3, ^y, S, e, and f 

 being included. Thus it is found that all the combinations 

 for Ingress give the mean solar parallax 8"739, weight 

 io"46, and all the combinations for Egress give 8"'847, 

 weight 2'53, whence the general result is 8"76o, from 

 which Sir George Airy finds the mean distance of the sun 

 equal to 93,300,000 miles. The New Zealand observa- 

 tions were not included in these calculations ; their mean 

 result is 8"'764, almost identical with the above. It is 

 remarked that many persons may perhaps consider that 

 the more closely-agreeing phases /3 and ^ should be em- 

 ployed in deducing the value of the parallax to the 

 exclusion of the others. If this be done we shall have 

 from the Ingress 8"748, and from the Egress 8"*905, or 

 with their due weights a mean value 8"773. 



In this outline of the details contained in the Astro- 

 nomer-Royal's first Report upon the observations of the 

 transit of Venus, and the conclusions to be drawn from 

 them we have adhered closely to his own words. Pending 

 the appearance of the deductions to be made from the 

 complete measuring of the photographs, the results before 

 us are perhaps to be regarded as provisional ones only, 

 or we have not yet learned all that may be done from the 

 work of .the British expeditions, so laboriously organised 

 by Sir George Airy. Many astronomers we can imagine 

 will regard with some 'suspicion so small a parallax as 

 8''76, which is a tenth of a second less than has been 

 given by the most reliable previous investigations, upon 

 different principles. In illustration we may quote the 

 separate results from which Prof. Newcomb obtained his 

 value of the parallax, now adopted in most of our 

 ephemerides : — 



8-855 

 8-842 

 8-838 

 8 •809 



From meridian observations of Mars, 1862 



From micrometric observations of Mars, 1862 



From parallactic inequality of the moon 



From the lunar equation of the earth 



From the transit of Venus, 1769 (Powalky's reduc- 

 tion) 8-86o , 



From Foucault's experiments on light 8 '860 



To these may be added Leverrier's value subsequently 

 deduced from the planetary theories, which is also 8"'86. 

 Newcomb's mean figure, taking account of weights cor- 

 respondirg to the probable errors is 8"'848, which, with ] 

 Capt. Clarke's measure of the earth's equator, implies that 

 the mean distance of the sun is 92,393,000 miles. Sir 

 George Airy's 8 "760 would similarly place the sun at a 

 mean distance of 93,321,000 miles. 



It is well known that some astronomers have not 

 expected our knowledge of the sun's distance to be greatly 

 improved from the observations of the transit of Venus, ^ 

 regarding such an opportunity as is presented by a close | 

 opposition of Mars as affording at least as favourable J 

 conditions, [and the result of Mr. Gill's expedition to 



