10 REPORT—1868. 
The visibility of objects generally is affected in a much greater degree by 
the morning and evening illuminations. Shortly after sunrise the slopes of 
mountains facing the W. are so illuminated that their detail may be well and 
satisfactorily made out. It is only a little before sunset that the E. slopes are 
similarly illuminated. Most of the apparent changes in the appearance of 
objects are due to these illuminations. It is, however, manifest that Jow and 
flat objects are less affected than high and sloping ones. We have already 
indicated (letterpress, areas IV A*, IV AS, p. 7, and Report Brit. Assoc. 1866, 
p- 243) the mode of finding epochs of similar illumination ; still the changes 
which some objects undergo as the sun rises above their horizons are so re- 
markable that it may in some measure assist the observer in endeavouring to 
refer these changes to their legitimate sources, if we give for the limits of 
each zone of 5° the sun’s altitude for intervals of 12 hours during the period 
elapsing from sunrise to meridian passage, which will also be applicable to 
the decline from meridian passage to sunset. The epoch of sunrise at any 
particular spot with © — § =0° or 180° is manifestly that at which the lon- 
gitudes of the spot and terminator are equal; with © — & =270° the sun 
is slightly above the horizon, and with © — g& =90° as slightly below in the 
southern hemisphere. 
The longitude of the terminator may be found by the following formule :— 
Calling the longitude of terminator /,, the moon’s mean longitude /,, the sun’s 
true longitude ©, west longitude on the moon’s equator W, and E longi- 
tude E, then for the morning terminator we have 
W1=-(270°+1,), EL =@70°+1,)-0, 
and for the evening terminator 
W 1,.=O—(90°-+1,), E1,=(90°+1,)—O. 
TABLE OF SOLAR ALTITUDES AT THE Moon. 
Equator. 
Int. Hour-angle. | Winter Solstice. Equinoxes. |Summer Solstice. 
h fe) ] a ° 1 u“ oO 1 u“ ° 1 a“ 
0 JOR On ae Or) 0 0 0 0 0 O 
12 83 54 15 Co SA 6 5 46 6 5 37 
24 77 48 30 12 10 34 12) 150) 12 10 34 
36 71 42 45 18 16 49 18 16 18 16 49 
17 
48 65 37 0 24 22 27 24 23 0 24 22 27 
60 59 31 15 30 28 0 30 28 48 30 28 0 
72 53 25 30 36 33 35 36 34 29 36 33 35 
84 47 20 0 42 38 52 42 40 0 42 38 52 
96 41 14 15 48 44 19 48 45 44 48 44 19 
108 35 8 30 54 49 46 54 51 30 54 49 46 
120 29 2 45 60 55 4 60 57 13 60 55 4 
132 22 57 0 67 0 5 67 3 0 67 0 56 
144 16 51 15 73 4 40 73 8 40 73 4 40 
156 10 45 30 79 7 50 79 14 25 79 7 50 
168 4 39 45 85 5 20 85 20 20 85 5 20 
Mer, Osh OrO 88 27 51 909 0 0 88 27 51 
