Proctor's Astronomical Lectures. 



25 



Day unto day uttoreth speech and night unto night 

 showeth knowledge. There is no speech nor language 

 but tlxelr voices are heard among them." 



TRANSIT of VENUS The MOON 



FOURTH LECTURE OF R. A. PROCTOR. 



HOW THE TRANSITS OF VENUS ARE OBSERVED THE 

 IMPORTANCE OF THE TRANSIT OF 1874 THE 

 MOON'S ASPECT AND MOTIONS HER SERVICES 

 TO THK EARTH LAPLACE'S IDEAL MOON LUNAR 

 PHOTOGRAPHY THE MOON DESTITUTE OF WATEK 

 AND AIR THE MOON'S PROBABLE HISTORY, 

 SHAPE, PRESENT CONDITION, AND APPAKEN.T 

 VOLCANIC PHENOMENA. 



Prof. Richard A. Proctor delivered the fourth of 

 his series of lectures on Astronomy at Association 

 Hall on Jan. 19, the subject treated being "The 

 Transits of Veti us and the Moon." Ingenious diagrams 

 were employed to depict the various appearances of 

 the moon as observed through the telescope, and de- 

 lineate the path, and points of view for the coming 

 transit of Venus, which occurs this year, and which 

 is at present a topic of greatest discussion and inter- 

 est to astronomers. Tlie audience was very large, 

 and the lecture was listened to throughout with tlie 

 closest attention. 



THE LECTURE. 

 It has seemed well to the gentlemen who 



have arranged this course tbat a few remarks should be 

 made upon a sublet at present attracting grout interest 

 among astronomers I mean the question of the ap- 

 proaching transit of the planet Venus. You know, of 

 course, that on this transit depend the best methods of 

 determining the distance of the sun, and that upon de- 

 termining the sun's distance depends our estimate of all 

 the dimensions of the planetary system. Therefore tliat 

 Is the fundamental problem of astronomy, because 

 to the stars also, our ideas 

 one fundamental measurement 

 it happens that the plaiiet 

 between the earth, aud the 

 Bim, enables us to measure that distance iu a manner 

 very easily explained. We know that the distance from 

 Venus to the euu is to the distance of the earth from the 

 Bun aa five to seven. If observers at the north and 

 at the south of the earth's globe look at Venus at 

 a time when she is directly towards the sun, the 

 southern observer will see Venus at the highest of those 

 Stations, and the northern observer will soe her at the 

 lowest of tho- : e stations; say they are 6,000 miles apart: 

 then we know the distance apart of ihe lines that Venus 

 appears to have traveled on the surface of the sun is 

 15,000 miles, and then we know the whole diameter of 

 the sun is so aud so. because we can compare it and 

 know it Is proportionately so much larger. We ascer- 

 tain the diameter of the sun, aud If we know the 

 size of any object aud know how large it 

 looks, we know how far away it is. You see that 

 having this distance of 6,000 miles we ascertain that 



when we pass on 

 depend on that 

 of distance. Now 

 Venus, by coming 



distance of 15,000, and afterwards, knowing the diam- 

 eter, we know the distance of the sun. But then it ia 

 not possible for observers to note the planets at two 

 points like that, because they are not in communication. 

 Those two observers at distant places cannot work at 

 the same moment. The southern observer watches 

 Venus crossing the suu's face on tho northern track, the 

 northern observer sees her on the lower track, and if 

 they note how long a time she takes, they determine how 

 long those two tracks are, and then it is a simple prob- 

 lem in geometry to tell tho distance of one from 

 the other. Tuat is Halley'a method. Again, you will 

 notice that the observer who sees Venus traveling this 

 longer course will see the transit begin earlier than the 

 observer who notices her traveling the shorter course. 

 Therefore, if one observer is placed where Venus begins 

 as early as possible, and another is placed on the earth 

 where Venus begins as late as possible, by comparing 

 those two moments it becomes a mere geometrical prob- 

 lem to tell where those chords are aud how far apart 

 they are. This is Delislo's method. Ilalley's method 

 requires only two ooservations of the length of time 

 Venus takes. If they have a very ordinary kind of 

 clock, so long as it. does not gain or lose during tho 

 time their observations are in progress, their result is 

 achieved. 



DELISLE'S METHOD OF OBSERVING THE TRANSIT. 



A, B, stations on nppudit sides (if the 'avth. A a. pflrt ot the enrtu'8 

 orbit. V, Veuug. C 1) and E I 1 ', aupareut paths of Venus on tLe tmu. 



SUN'S mK wrrn TRANSIT THUS OBSERVED. 



Upper ami dark ima e of Venus as sei'i) froTi Southern Hemisphere! 

 lower aud light image aa seen from Northern Hemisphere. 



