68 SCENERY OF THE HEAVENS. 



the same time in the same direction, though at a slower pace. The planet accomplishes 

 an angular motion of 1 36' per day, while the earth follows at the rate of 59', and is thus 

 gained upon by only 37' daily. But the two planets will obviously appear to keep on the 

 same side of the sun, until Venus has gained half her orbit in advance, or 180. This it 

 will require about two hundred and ninety days to effect, as the difference of their daily 

 rate, 37' x 290= 10730' 180 nearly. It will be seen from the diagram, that at the 

 time of the inferior conjunction, the unenlightened half of the orb is turned towards us. 

 Did she present in that position her illuminated side, we should see the planet as a small 

 brilliant moon, shining with twenty times her ordinary lustre, as she is then a hundred 

 and sixty millions of miles nearer the earth than when at the opposite point. As it is, 

 however, she is our best friend among the stars, the most radiant of the host, and has 

 been observed to cast a clearly defined shadow. 



This planet, like Mercury, transits the sun's disk, and then appears shorn of her beauty, 

 under the form of a dull dark spot. These events are of great interest, being remarkably 

 rare, and of singular importance to physical astronomy. They can occur only twice in a 

 century, because it is only twice in that time that any number of complete revolutions of 

 Venus are just or nearly equal to a certain number of the earth's revolutions, and she 

 passes her inferior conjunction in a direct line between the sun and the earth, and in 

 that position is in the plane of the earth's orbit. A transit is a phenomenon of precisely 

 the same kind as a solar eclipse, and occurs under the same conditions, the different 

 appearances of the two arising simply from the different distances from the earth of the 

 interposing bodies between it and the sun. The passage of Venus across the sun's disk 

 is of importance, as furnishing the best means of ascertaining his distance and volume, 

 which supplies data for determining the distances and magnitudes of the planets, and is 

 in fact a universal standard of astronomical measurement. It was the solution of this 

 problem that led the European governments to fit out expensive expeditions, at the last 

 transit of Venus in 1 769, for the purpose of observing it at different stations ; among 

 which was the celebrated voyage of Captain Cook to Otaheite. The next transits will be 

 in 1874 and 1882. They will be anticipated and provided for, be observed with better 

 instruments than on the last occasion, and be watched with a solicitude more wide-spread 

 than has ever been excited by any natural phenomenon since the world began, in order to 

 verify or correct former results. For more than a hundred times in the lapse of ages, 

 the planet accomplished her passage across the solar disk, undetected by mortal eye. The 

 first transit ever known to have been seen by any human being occurred December 4th, 

 1 639. The place of observation was an obscure village in the neighbourhood of Liverpool. 

 The observer was a young scholar of the name of Horrocks, who died at the early age 

 of twenty -two, and left memorials behind him, which sufficiently indicate, that had his 

 life been prolonged, he would have become one of the most eminent men of his time. 

 Without much assistance from books and instruments, he found that a transit might be 

 expected at the time mentioned ; and confident of the result, he prepared himself to watch 

 it, with all the carefulness and enthusiasm of a scholar ambitious of being the first to 

 predict and witness a celestial appearance that had never been observed by man. He 

 waited for the issue with ardent anticipation, and the event realised his expectations, 

 of which he wrote an account entitled, " Venus in Sole visa," which was afterwards 

 printed by Hevelius. The planet appeared on the sun at the time calculated. But it 

 was Sunday ! The interesting remark occurs concerning the season : "I observed 

 from sunrise till nine o'clock, again a little before ten, and lastly at noon, and from ong 

 to two o'clock, the rest of the day being devoted to higher duties, which might not 

 be neglected for these pastimes." 



After the present century, the planet will not transit the sun's disk during the century 



