SPHERICAL ASTRONOMY. 83 
The Moon; her Revolution around the Earth; Phases of the Moon: the 
Moon’s Nodes. | 
7. Our earth, in her yearly course about the sun, is accompanied by a 
satellite, the moon, which revolves around the earth in about a month, and 
in little more than four weeks wanders through the whole zodiac. We 
need only observe the moon for a few hours, on several successive clear 
evenings, to satisfy ourselves that while, with the other stars, she follows 
the diurnal motion from morning till night, she has a peculiar motion of her 
own, from west to east, advancing daily a little over 13 degrees among the 
fixed stars of the zodiac. This peculiar motion of the moon is the result of 
her revolution about the earth; and for the same reason, being an opaque 
body illuminated by the sun, she is exhibited in all possible shapes (phases). 
The four principal of these are, the new moon, the first quarter, the full 
moon, and the last quarter. The new and full moon are known as the 
syzigtes ; the first and last quarters as the quadratures. Pl. 6, fig. 19, 
offers an intelligible illustration of the various phases of the moon, depending 
on her different position with respect tothe sun and earth. Let abcd be the 
earth placed in the middle of the moon’s orbit, NEVLN, and 8 the sun, 
whose distance, Sa, from the earth is supposed to be so great that all his 
rays are parallel to the line SNV. Let the moon be at Nye, or between 
the earth and sun, her dark side, Sya, will then be turned to the illuminated 
side (dac) of the earth. At this time the moon is new, and being above the 
horizon in the day time, is invisible. Compare pl. 10, fig. 5. Twe 
or three days after this time, the moon. moving in her orbit in the direction 
of the arrow (fig. 19), is seen soon after sunset, in the evening sky, as a 
narrow crescent, which soon sets. This crescentic shape of the moon 
becomes broader, and she sets later every day, and removing constantly 
from the sun, she shines through the first hours of the night. In about 
seven days the moon will have reached Lrvy, or the first quarter, and to an 
observer at d, or the boundary between day and night, will be seen to the 
left of the sun, as a semicircular disk, with the straight edge to the left or 
east: the moon now culminates about six o’clock in the evening (see pl. 10, 
jig. 5), and sets about midnight. From this time, the outline which from 
new moon to the first quarter was concave, becomes convex; the moon shines 
longer, and sets after midnight. In about seven days she will have reached 
Vaxn, and become full, standing directly opposite the sun, behind the earth ;- 
her illuminated half, Ax, consequently, to the dark side, dcb, of the earth, 
appears as a full circle, which rises in the east as the sun sets in the west,. 
culminates at midnight, and sets in the west at sunrise. The moon now 
rises about an hour later each night, and gradually loses the illumination of 
her right or westerly side, so that the circular disk becomes oval, until, in 
seven days after full moon, she will have arrived at EZd, or the last quarter. 
To the observer at c, the boundary between night and day, her illuminated 
side, Eed, appears as a half disk, to the left or west of the sun, and the line 
separating the dark and the illuminated portion will be on the west side, 
while in the first quarter it was on the east. At the last quarter, she rises 
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