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Means which have been adopted for ascertaining the Velocity of 

 Light and the Sun's Distance, with especial reference to the forth- 

 comiiuj transit of Venus over the Sun's disc, in 1874 and 1882. 



[Read by FRANCIS Abbott, F.R.A.S,, 8th March, 1864.] 



By M. Foucault's recent experiments on the velocity of light, astronomy has 

 received a new accession, especially as it applies to the great question concern- 

 ing the Sun's distance, a correct knowledge of which enables astronomers to 

 mete out the exact distance and dimensions of every planet and satellite, and 

 the distance of those fixed stars whose parallaxes are approximately known. 

 Any error, therefore, in the Sun's distance entails a proportionate error in the 

 distance of all the other heavenly bodies. 



I have been induced to bring this subject before the meeting as astronomers 

 have already begun to talk of preparing means and adopting situations for 

 making unremitted observations on the next transit of Venus over the Sun's 

 disc, in 1874 and 1882. It is essential to remark that the transits mil take 

 place in the month of December, and at that time the earth's South Pole will 

 be turned towards the sun, and those regions of the earth included between 

 the South Pole, and the southern limit of illumination will be carried by 

 rotation, opposite to the direction of all the northern parts of the earth. 

 Only two transits of this planet over the sun have occurred since Dr. Halley 

 invoked the attention of astronomers to these rare astronomical events, viz., 

 in 17(51 and 1769. The transit of 1769 was most favorable, and afforded an 

 opportunity which can only occur once in a lifetime. 



Mr. Breen, Assistant Astronomer to the Eoj^al Observatory at Greenwich, 

 has made for the Astronomer Royal drawings both of the ingress and egress, 

 together with the illuminated portion of the earth during the time of each 

 transit. In these drawings, Tasmania for the first transit, 1874, is illuminated 

 from beginning to end. This will appear clear by inspection, from the time of 

 conjunction given on the diagram. At the second transit, 1882, the first 

 contact is at 2h. 5m. 54s. a.m., on that day, 7th December, the sun does not rise 

 at Hobart Town until 4h. 28m. a.m., the ingi-ess therefore will not be visible. 



So important have these obsei-vations appeared to astronomers that at the 

 last transit of Venus, in 1769, expeditions were fitted out on the most efficient 

 scale by the British, French, Russian, and other Governments to the remotest 

 corners of the globe. The celebrated expedition of Captain Cook to Otaheite 

 was one of them ; and it is not more certain that this phenomenon will recur 

 than that every provision will be made for observing it in every inhabited 

 quarter of the globe. With the improved instrumental means, and the more 

 accurate methods of observing, - should other things jDrove favorable — it is ex- 

 pected that the results may furnish an universal standard of astronomical measure. 



" In the transit of 1861," says the Astronomer Royal,* "the result depended 

 almost entirely upon an accurate knowledge of the differences of longitude of 

 very distant stations, which are undoubtedly subject to great uncertainty. In 

 the transit of 1769 it happened that the result depended almost entirely upon 

 the observations made by Father Hell, at Wardhoe ; and to these great 

 suspicion has attached, many astronomers having, without hesitation, designated 

 them as foigeries. It is evidently desirable to repeat the practical investigation 

 when opportunity shall iJresent itseK." 



The solar parallax from the observations of 1769 — which Encke deduced 

 from an elaborate discussion fifty yeai-s after they were made — is 8''. 57116. 

 This corresponds with a solar distance of 95,^360,000 statue miles ; and should 

 the forthcoming transit of Venus prove unfavorable, any reduction of the 

 possible error in the sun's parallax within the limit of one hundredth of a 

 second will be hopeless for near two centuries to come, t 



In glancing first at the two astronomical methods that have been adopted for 

 measuiing the velocity of hght, and from which some appai-ent difficulties 

 arose, owing to the extent of space through which the ray is transmitted, 

 which, in stellar astronomy, surpasses in magnitude even the velocity of light, 

 by which the luminous ray appears to be retarded in its passage, so that a 

 distinction exists between the actual and the apparent interval of each suc- 

 cessive transit. For example, the first satellite of Jupiter resolves round its 

 primary in about 42g hours, and as it takes light more than 40 minutes to pass 

 over the average distance of Jupiter, the eclipse is not seen until so many 

 minutes, on the average, after it has happened. Now if this delay were con- 



* Monthly Notices, R.A S. 



+ No other transits of Venus over the sun's disc will take place until June, 2004, and 

 June, 2012. 



