84 ASTRONOMY. 
about midnight, and crosses the meridian at about 6 o’clock in the morning. 
Her semicircular disk now begins to become narrower, the straight edge 
concave, and the moon again assumes, as she approaches the sun, a crescent 
Shape, which is smaller as the interval of time between the rise of the moon 
and that of the sun diminishes. About seven days after the last quarter, the 
moon entirely vanishes, again reaching NSya, her position about four weeks 
before. She now becomes new moon afresh, rising and setting with the 
sun, and her phases follow in the same succession. 
8. Since our earth is also an opaque globe illuminated by the sun, we 
must present the same alternation of phases to an observer at the moon, that 
the moon presents to us, only in an inverted order. At the time, therefore, of 
new moon, first quarter, full, and last quarter, our earth must be full earth, 
last quarter, new earth, and first quarter. 
The moon, during a revolution around the earth, describes an orbit, 
cutting the apparent path of the sun (the ecliptic), or the orbit of the earth, 
to which it is inclined about 5° 8’ 48’, in two points called the moon’s 
nodes (pl. 6, fig. 20). The point in which the centre of the moon cuts the 
ecliptic in passing from the south to the north, is called the moon’s ascending , 
node, &, or the head of the dragon; and the one produced in passing from 
north to south, the moon’s descending mode, ¢3, or the dragon’s tail. The 
distance of the moon from the ecliptic is called the latitude, which may be 
north or south. At the nodes. the latitude is 0°;-hence it follows that the 
sun, moon, and earth (the sun and earth being in the same plane), must lie 
at all times nearly in the same plane ( fig. 20), and when the moon is in one 
of her nodes, that is in the ecliptic itself, then the three bodies—sun, moon, 
and earth—-have their centres in the same straight line. 
The ancients observed that the moon’s nodes did not remain in the same 
part of the ecliptic, but continually moved backwards, or from east to west 
(indicated in fig. 20 by the dotted lines). This retrogression takes place in 
the following manner: one of the nodes of the moon’s orbit—the ascending, 
for instance—retrogrades in such a manner that if it had coincided with the 
new moon at starting, this coincidence would again happen after an interval 
of 18 years and 11 days. The moon’s node will then be still 11 degrees 
distant from the position assumed 18 years and 11 days before. But the 
sun has in the meantime advanced 11 degrees, and consequently stands 
again in the node; therefore, since the moon is again in the new moon, an 
eclipse of the sun must occur, just as it did 18 years, 11 days before. How- 
ever, the coincidence of the new moon with the node does not take place 
exactly in the same manner. These periods of 18 years and 11 days, 
supposed to have been called Saros by the Chaldzan astronomers, are 
known as the periods of Halley, and were employed by the ancients in the 
prediction of solar and lunar eclipses. The retrogression of the moon’s 
nodes is a consequence of the secular perturbations of the moon’s orbit, 
and at a mean, with reference to the fixed stars, amounts annually to 
19° 20 29” (pl. 6, fig. 20). 
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