112 ASTRONOMY. 
the sun can be obtained. According to Encke, who has fully and most 
accurately carried out the calculation, the mean horizontal parallax of the 
sun at the equator amounts to 85,778, seconds, and consequently the mean 
distance of the earth from the sun to 95,103,000 (English) miles. That the 
transits of Mercury are not available in determining the sun’s parallax, is 
readily intelligible, when we know that the parallax of Mercury at its inferior 
conjunction is not very different from that of the sun, while the parallax of 
Venus at inferior conjunction is almost four times as great as that of the 
sun. 
Pl. 9, figs. 7 and 8, represents the individual phenomena of the transits 
of Mercury for the whole earth, as they occurred May 4, 1786. The 
obscure portions in figs.'7 and 8 cover those regions in which the transit 
was visible, the bright portions answer to the countries in which it was in- 
visible. Those places lying in the boundary of the obscure and light parts, 
observed, as indicated in the figures, either an entrance or emersion at sun- 
tise only, or an entrance or emersion at sunset only. 
Additional Remarks on the Course of the Moon. 
30. We have already said all that is essential with respect to the origin 
of the moon’s phases, the inclination of the moon’s orbit to the ecliptic, the 
retrograde motion of the moon’s nodes, as also the causes of solar and lunar 
eclipses. There still remain a few additional considerations respecting the 
moon’s course. The principal figure on pl. 10, fig. 5, contains the phases 
of the moon, or the various aspects under which she presents herself to us. 
The earth being in the centre of the external circle, the moon’s orbit is so 
placed, with reference to this circle, as readily to exhibit its eccentricity. 
The moon revolves in this orbit from west to east around the earth, and the 
figure represents her in her proper proportion to the earth and in the eight 
principal points of her course. The sun may be supposed to be stationed 
at a distance to the right of the earth in pl. 10, exceeding 410 times that of 
the moon from the earth. In order to exhibit the phases of the moon in 
their fullest conditions, they are represented within the moon’s orbit in much 
larger proportion. The orbit of the moon is properly an ellipse, the earth 
standing in one of the foci. Its eccentricity amounts to 0.0548442; the 
greatest distance of the moon from the earth (that is at time of the apogee), 
to 63.842 semi-diameters of the earth, and the least, or that at time of 
perigee, to 55.916 semi-diameters. Both perigee and apogee retrograde 
from evening to morning about 40° 42’ annually. The moon appears to 
revolve in 24 hours from east to west around the earth, which, with the 
apparent daily motion of the heavens, is produced by the rotation of the 
earth on her axis. Again, the moon, ag full moon during the nights of sum- 
mer, appears to describe a very small arc, and during the nights of winter a 
very large arc above the horizon, which is explained in the following man- 
ner: Let us suppose the night to be that of the winter solstice (pi. 9, fig. 1), 
on December 21, consequently the longest night in the year. Let now the 
112 
