November—Pisces
There is in Pisces not a single bright star, and its only point of interest is to be found in the fact that it contains the point, marked by the cross and letter V in the diagram, that is known variously as "the vernal equinox," "the equinoctial point" and "The First Point in Aries." This is a very important point of reference in the heavens, just as the meridian of Greenwich is for the earth, and it marks the point where the sun crosses the equator going north in the spring. Owing to the Precession of the Equinoxes, as it is called, this point is gradually shifting its position westward through the zodiacal constellations at a rate that will carry it completely around the heavens through the twelve zodiacal groups in a period of 25,800 years. Since the beginning of the Christian era, this point has backed from the constellation of Aries, which lies just east of Pisces, into Pisces, though it still retains its name of "The First Point in Aries."
[XIII]
DECEMBER
The eastern half of the sky on early December evenings is adorned with some of the finest star-groups in the heavens; but as we are considering for each month only the constellations that lie on or near the meridian in the early evening hours, we must turn our eyes for the present from the sparkling brilliants in the east to the stars in the less conspicuous groups of Aries, The Ram, and Cetus, The Whale. We will also become acquainted this month with the beautiful and interesting constellation of Perseus, the hero of mythical fame to whom we referred last month in connection with the legend of Cepheus and Cassiopeia. Cetus, you will recall, represents the sea-monster sent to devour Andromeda, the daughter of Cepheus and Cassiopeia. We have included the constellation of Andromeda in our diagram for this month, since it is so closely associated in legend with the constellations of Perseus and Cetus, though we also showed it last month.
The brightest star in Perseus, known as Alpha Persei, is at the center of a curved line of stars that is concave or hollow toward the northeast. This line of stars is called the Segment of Perseus, and it lies along the path of the Milky Way, which passes from this point in a northwesterly direction into Cassiopeia. According to the legend, Perseus, in his great haste to rescue the maiden from Cetus, the monster, stirred up a great dust, which is represented by the numberless faint stars in the Milky Way at this point. The star Alpha is in the midst of one of the finest regions of the heavens for the possessor of a good field-glass or small telescope.
A short distance to the southwest of Alpha is one of the most interesting objects in the heavens. To the ancients, it represented the baleful, winking demon-eye in the head of the snaky-locked Gorgon, Medusa, whom Perseus vanquished in one of his earliest exploits and whose head he carried in his hand at the time of the rescue of Andromeda. To the astronomers, however, Algol is known as Beta Persei, a star that has been found to consist of two stars revolving about each other and separated by a distance not much greater than their own diameters. One of the stars is so faint that we speak of it as a dark star, though it does emit a faint light. Once in every revolution the faint star passes directly between us and the bright star and partly eclipses it, shutting off five-sixths of its light. This happens with great regularity once in a little less than three days. It is for this reason that Algol varies in brightness in this period. There are a number of stars that vary in brightness in a similar manner. Their periods of light-change are all very short, and the astronomers call them eclipsing variables. At its brightest, Algol is slightly brighter than the star nearest to it in Andromeda, which is an excellent star with which to compare it.
December—Perseus, Aries and Cetus