THEORETICAL ASTRONOMY. Pat 
Apparent Size and Diameter of the Sun, as seen from the Planets. 
Figs Apparent Size in Order. | Apparent Diameter. 
so Mercury, 1° 22" 49" 
37 Venus, 0 40 82 
38 | Earth, Ote3Bou 
BOw sip; Mars, DipnZdte: 2 
40 | Vesta, OD gleh je) 
41 Juno, j, 12 
42 Ceres and | Og gan 
Pallas, OP Milit'"S3 
43 Jupiter, horeguly GLO 
44 Saturn, Oo. heSu 122 
A5 yy ae Uranus, 0 1 40 
The figures above are also constructed on the scale AB, only the greatest 
divisions amount to 1500, the smaller to 150, and the smallest to 15 seconds. 
Hence it follows that to Uranus the sun appears 493 times smaller than to 
Mercury, and to Jupiter 54 times smaller than to the earth. 
Finally, figs. 46-54 show the true diameter of the earth’s moon, and of 
these planets, compared with the true diameter of the earth. If in pl. 14, 
jig. 46, the diameter of the earth be taken as equal to 100 parts. then will 
the other diameters be as follows :— 
Moon of the earth, fig. 47, 26 parts. 
Venus, 48, 98 
Mars, 49) 52 
Mercury, 50, 39 
Pallas, 51, 26 
Ceres, 52, 20 
Juno, 53, 18 
Vesta, BaD 
Principal Phases of a Transit of Mercury or Venus over the Sun’s Disk, 
and manner of observing them. 
40. The general theory of the transits of Venus and Mercury has been 
ditcany explained ‘in section 2eyeby Tetererice to~ yess "G6; 7) 8) ple: 
It still remains to show by fig. 55, pl. 14, the mode of observing such a 
phenomenon. When, as is generally the case, an astronomical telescope is 
employed, which represents all objects inverted, the attention of the ob- 
server, before the commencement of the transit, is to be directed to the 
right border of the sun’s disk. The straight line, passing through the figure, 
represents the course that Mercury, for instance, takes across the sun. 
When arrived at d, the first external contact of the limbs of Mercury and 
the sun, or the so-called external immersion, takes place; at this time the 
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