The hour circle which passes through this point is known as the equinoctial colure, and may be considered the prime meridian of the heavens, for from it is measured the right ascension of all bodies. Right ascension of a body is the angle at the celestial pole between this meridian of reference and the hour circle passing through the body. It is always measured eastward through 24 hours of sidereal time (360°). The angle is measured by the arc intercepted on the celestial equator. For example, a star 15° east of the equinoctial colure has a right ascension of 1 hour or 15°, but, if the star is 15° west, its right ascension is 23 hours or 345°.
The positions of heavenly bodies are defined by right ascension and declination exactly as positions upon the earth are expressed by longitude and latitude, right ascension corresponding to longitude and declination to latitude.
In the discussion of Time, to follow, more facts concerning right ascension will be found.
Precession of the Equinoxes
A comparison of the present positions of the fixed stars with their places as recorded in ancient times shows a great discrepancy. The celestial latitudes, which were reckoned from the ecliptic, show no appreciable change; but in the declinations and right ascensions there is a great departure from the old positions. The error of right ascension was found by the old Greek astronomer, Hipparchus, to appear as a uniform eastward movement of all the stars, which led him to reason that, instead of the stars themselves changing, their point of reference was moving westward, thus lengthening all right ascensions.
The famous astronomer after further reasoning decided that the position of the celestial pole was changing, in fact that the line of the earth’s axis was describing a circle on the heavens, which was left-handed or against the hands of a watch as viewed from the north pole of the earth. This movement was found to be extremely slow, requiring 25,800 years to complete the circle which has as its radius the amount of the inclination of the earth’s axis—23° 28´.
If a match is put through a piece of cardboard about the size of a half dollar to the distance of ¼ inch, and spun, the motion of the cardboard just as it staggers through loss of speed, gives some idea, although exaggerated, of the precession movement of the plane of the equator, which is of course infinitely slower. The movement of the top of the match is a semblance of the corresponding motion of the vanishing point of the axis on the celestial sphere.
The earth, as already explained, points its axis at practically the same spot in the heavens throughout the year, and if it were not for this annual precession of 50´´ it would for all intents and purposes hold a permanent direction. About December 21, the winter solstice, while the earth is still some degrees from the perihelion, its northern axis, is inclined directly away from the sun. Each year this distance from the perihelion is becoming greater, widening this angle between the direction of the axis, projected on the plane of the orbit, and the major diameter of the orbit, until in time the north pole will be headed directly away from the sun in that part of the orbit which the earth now occupies in September, and so on.
This diagram shows the successive positions of the earth at the Vernal Equinox (March 21st) due to the revolution of the axis and the consequent westward movement of the First Point of Aries.
Fig. 1.