An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

136''iTf roox.
this period, consistinlg of about 1.8 years and -10 days, under
thle naime of the Saos, was used by the Chaldeans and othellcr
ancient 3nations ill predicting eclipses.
234. The A.ietonic Gycle is not the same with the Saros,
lbut colisists of 19 tropical years. l)Durling this period the moonl
makes very nearly 235 synodical revoluttions, and hence tihe
new anft(tll moontS) if r'CekonlC1d by periods of 19 years, rccur
at the salme dates. If, for example, a new moon fell on tho
ifitictll d(ay of one cycle, it Nwould also fall on the fiftietl day
of eaclh succeeding cycle; and, since the regulation of gamles,
feasts, and fasts, lhas bccn made very extensively according to
new or full moons, hence thi s lunar cycle has been much used
both in ancient andl modern tines. The Atlhenians adopted it,
433 years before the Christian era, for the regulation of tleir
calendar, andl had it inscribed in letters of gold on thle walls
of the temple of:MinervIa. H:nctce the term Golden )iYumber,
whllic dcnotce tlhe year of the lutnar cycle.
235'. 7twe liae of- thew asdes f1 the moon's orbit n'e volves
fr'onm west to east though her whole o-rbit in about qnine years.
If, in any revolution of tlhe moon, we should accurately mark
tlec pllace in the heavens where the moon comes to its perigee
(Art. 230), we shlould find that at the next revolution it would
collme to its perigcc at a point a little furtlher castward than
before, and so oil at every revolution, until, after 9 years, it
Woulll come to its perigee at nearly the samtte point as at first.
This fact is expressed by) syillg that thle perigcc, and of course
the apogee, revolves, and that tihe line which joins these two
points, or the line of thle apsides, also revolves.
I The place of the perligc may be found by observing whlen
hllc moon hlas the greatest apparent diamieter. Blut as thle
magnlitud of the moon varies slowly at this point, a lbetter
method of ascertaining the position of the apsides, is to take
two points in the orbit whlere tlhe variations in apparent diallcter are most rapid, and to find wh1ere they are equal on opp1)osite sides of tlhe orbit. The middle point betwcceen tlhe two Nwill
give the place of the perigee.
tlhe angular distatllce of the moon from h]er perigee ill ally
part of her revolution, is called the MAlon's Atnomaly.