There are two opposite points on the celes tial sphere at which the equator and the eclip ' tic intersect. These are called equinoxes be cause, when the sun crosses them, the days and nights are equal all over the earth. That equinox which the sun passes toward the north is called the vernal equinox, because the crossing marks spring in the northern hemisphere. The other is called the autumnal equinox for a similar reason. Observations continued through many centuries show that•the equinoxes are not fixed, but travel slowly along the ecliptic at such a rate that they make a complete revolution from the east toward the west in about 26,000 years. This motion is called the precession of the equinoxes. Its existence shows that the direc tion of the earth's axis is slowly changing, and hence the position of the celestial pole is chang ing also. Since the equator is defined by the condition that it spans the heavens midway between the celestial poles, this change in the poles causes a corresponding change in the equator.
The actual motion of the pole is at the rate of about 20" per year. The smallest visible object that can be seen with the naked eye to be anything else than a point of light subtends an angle of about 1' or 60". It follows that the pole moves through this smallest visible space in three years. In a long life of 90 years the change would be about equal to the diameter of the sun or moon. The centre of the motion is the pole of the ecliptic which is distant from that of the equator by about Owing to the smallness of the obliquity, the equinox travels along the ecliptic at more than twice the rate of the pole, or about 50" per year. It has therefore changed about 30° since its motion was first noticed, about 2,000 years ago. It is found that the planets describe their course around the sphere in circles which do not de viate greatly from the ecliptic. A belt of the heavens extending 8° on each side of the eclip tic will include all the planets visible to the naked eye. This belt is called the zodiac. Be ginning at the vernal equinox it is divided into 12 portions, of 30° each, known as the signs of the zodiac. In former times great stress was laid upon the entrance of the sun into these several signs, which entrances occurred about a month apart. They now occur about the 20th of every month. In our times, when the super
stitions connected with this subject have van ished, the entrance of the sun into the signs is no longer of importance (see Zornac). There are also 12 constellations, beginning with Aries, and ending with Pisces, which have the same names as the signs of the zodiac, and are scattered along its course. Two thousand years ago these constellations coincided pretty closely with the signs. But now, in consequence of the precession of the equinoxes, the two no longer correspond. The sign Aries is now lo cated in the constellation Pisces; the sign Tau rus in the constellation Aries, etc.
The ecliptic, together with circles drawn parallel to it on the celestial sphere, is fre quently used as a reference circle to indicate the positions of stars and planets, the circles drawn through the poles of the ecliptic then taking the place of the hour circles already described. The co-ordinates of the object in this system are known as Longitude and Latitude, respectively, but it is to be noticed that these are not analogous to latitude and longitude on the earth since the equator is no longer the fun damental reference circle. This system is em-. ployed almost exclusively in orbit work and in celestial mechanics, but it is inconvenient for the purposes of practical astronomy.
The Time of Day It is in its relation to times and seasons that the results of astro nomical science come into every household. Our daily round of activity and rest is deter mined by the earth's rotation on its axis, alter nately bringing us under the sun, and then carrying us around until it is hidden from our sight. A century ago people used to set their docks at 12 when the sun crossed the meridian. This moment, being the middle of the day, is noon properly so-called. But if a good clock is exactly regulated, and kept going all the time, it will not show noon at the true time. The reason is that the intervals of time between one noon and the next are not exactly the same. See Tim!.
Bibliography.— The most extended gen eral treatise on astronomy for the use of the general reader is Chambers'