118 ASTRONOMY. 
primary, expressed in geographical miles, 196,000, 254,000, 296,000, 339,600 
680,000, and 1,360,000. Their magnitudes, so difficult to determine, must 
be very great to make them visible at so immense a distance from the earth. 
' The plane of their orbits is almost perpendicular to that of the orbit of 
Uranus (fig. 8); and it is remarkable that these six moons have the unique 
motion from north to south. The inclination of the equator of Uranus to 
its orbit, or the obliquity of his ecliptic, is very nearly a right angle, whence 
all difference of zones must disappear ; while, on the other hand, that of sea- 
sons must be very great. When near one pole of Uranus, the sun stands 
during summer almost immovably in the zenith, and afterwards, for 
almost a year, describes a very small circle about the zenith; in like man- 
ner, the satellites present themselves a very long time in the first and last 
quarters. New moon and full moon only take place when a pole of Uranus 
has the sun in its horizon, and at this time alone can eclipses of the sun 
and moons occur. 
In conclusion, it may be remarked that jigs. 6, '7, and 8, pl. 10, represent 
somewhat in perspective, the systems of the moons of Jupiter, Saturn, and 
Uranus ; and fig. 9, the orbit of our moon. The proportional size of the 
orbits is represented as accurately as the small size of the scale would 
allow. 
Estimates of the proportional size of the Planets. 
36. To obtain a clear idea of the relative size of the planets (Astrea, 
Neptune, and Iris excepted) and the sun, we may compare the scale at the 
bottom of p/. 14, representing the sun’s radius, with the diameters of the 
circles representing the planets (figs 1 to 11). Taking the diameter of the 
sun, 2AB, at 770,944 geographical miles, that of the eleven planets in geo- 
graphical miles, and the ratio of the planets’ diameters to that of the sun, will 
be as follows :— 
Ratio of Diameters of Planets 
Figs Planets. True Diameter. to Sun’s Diameter. 
1 Saturn, 62,072 Le 123 
2 Jupiter, 77,228 Li 10 
3 Uranus, 29,888 Pe 254 
4 Earth, 6,880 Le) dae, 
5 Venus, 6,776 Te 6 T13s2 
6 Mars, wore ry. 2167 
vi | Mercury, 2,688 1: 2862 
8 | Pallas, 1,800 1: 428-3; 
g an Ceres, 1,360 | 1: 5663, 
10 Juno, 1,200 l sy sBeaee. 
11 | Vesta, 200 1 : 3,854,%, 
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