1875.} The Illuminated Disc of the Moon. 3 
To an observer at 45° north latitude the line passing through 
a,b, and c would not be a straight line, or would not appear 
as a straight line. If the stars, @ and c, be joined by a 
straight line, and this straight line were produced in the 
direction of and beyond the meridian, r M, this line would 
not pass through o. In order to pass through o this line 
must be curved and not straight, and no straight line joining 
a and cand produced would pass through the star o, if the 
observer were in 45 north latitude. Having examined 
these facts, we will now note how the same laws apply to 
the illuminated disc of the moon. 
The moon shines and is visible to us in consequence of 
the light of the sun and because of the moon reflecting this 
light. The moon being a sphere, it follows that half the 
moon must at all times be illuminated by the sun. When 
the illuminated portion of the moon is turned directly 
towards the earth the moon is of course full; when this is 
the case the moon and the sun as seen from the earth are 
180° apart. 
When the moon and the sun as seen from the earth are 
go apart the moon will appear half illuminated, the illumi- 
nated hemisphere visible to us being turned exactly towards 
the sun. 
Let us take for illustration the following conditions :— 
The moon and the sun both on the equino¢tial, the moon 
apparently half illuminated, the sun therefore go° from the 
moon. 
Under the above conditions, we will suppose an observer 
situated at the North Pole of the earth, and we will examine 
the appearance of the moon and sun as seen by him. 
To an observer at the North Pole the equinoctial would 
coincide with the horizon ; consequently, the moon and the 
sun would both appear on the horizon, and go’ apart. The 
line separating the dark part from the light part of the 
moon would appear at right angles to the horizon; conse- 
quently a line drawn at right angles to the line separating 
Fig. 2. 
MM s 
TL R 
the light part from the dark part of the moon, and produced, 
would pass through the sun’s centre. The appearance of 
the half moon and the sun to an observer at the North Pole 
would be, as shown in the sketch above (Fig. 2), where 
M represents the moon, s the sun, and the line 1 rR the 
horizon. 
