4 The Illuminated Disc of the Moon. (January, 
The point, however, to which particular attention is 
directed is that the line drawn at right angles to the line of 
light and shade on the moon, and produced towards the 
sun, will pass exactly through the sun’s centre. Such a 
Hine 1S’ HR. 
The moon and the sun, as here shown, are go’ distant 
from each other, and the moon consequently is half illu- 
minated. 
We will now suppose exactly the same conditions to 
prevail as regards the sun and the moon’s position in the 
heavens. ‘They will still be supposed go’ distant from each 
other, and both situated on the equinoctial, but we will now 
examine the appearance of these two objects as seen by an 
observer on the equator. 
We will suppose an observer to be so situated on the 
equator that his zenith is equidistant from the sun and the 
moon. ‘The sun and the moon being go distant from each 
other, it follows that to this individual the moon would 
have an altitude of 45° above one part of the horizon (say 
the east) where the equinoctial cuts it; the sun therefore 
would be 45 above the west horizon. The line drawn at 
right angles to the line of light and shade on the moon 
would be directed past the zenith and towards the sun’s 
centre. 
From an examination of this and of the preceding de- 
monstration, in which the appearance of the sun and moon 
is described as they would appear to an observer at the 
pole, it will be evident that the line drawn from the moon 
to the sun must coincide with the equino¢tial, and that 
under the conditions named, viz., both the moon and the 
sun being on the equinoctial, the line of light and shade on 
the moon will always be at right angles to the equi- 
noctial. 
We will now consider the appearances presented by the 
sun and moon when seen by an observer in 45° north 
latitude, and under the conditions of these two celestial 
bodies being go apart, both on the equinoétial, and both 
equally distant from the meridian. 
From the demonstration already given as regards the 
position of the equinoctial on the sphere of the heavens, we 
can at once place the sun and the moon in the positions 
they would occupy under the above conditions. 
Thus, let E H W (Fig. 3) represent the horizon, H R that 
part of the meridian intercepted between the horizon and 
the equinoctial, E M RS W the position of the equinoctial 
on the sphere of the heavens, M the position of the moon 
