Dante and the early astronomers . n continued, we shouldsee Spica, for instance, sink slowly lower in thesouthern sky and after ages rise again northwards,but there would be very little preceptible movementeast or west. The movement observed by Hipparchus, then, wasnot a movement of Spica and other stars, but amovement of the equinox. For the celestial equatoris simply a reflection of Earths equator in the skies,and as it keeps changing the direction of its tilt inthe way described, it changes the point at which itcuts the ecliptic. This may best be seen by takingtwo rings or hoops (two large
Dante and the early astronomers . n continued, we shouldsee Spica, for instance, sink slowly lower in thesouthern sky and after ages rise again northwards,but there would be very little preceptible movementeast or west. The movement observed by Hipparchus, then, wasnot a movement of Spica and other stars, but amovement of the equinox. For the celestial equatoris simply a reflection of Earths equator in the skies,and as it keeps changing the direction of its tilt inthe way described, it changes the point at which itcuts the ecliptic. This may best be seen by takingtwo rings or hoops (two large curtain rings, forinstance), one of which just fits inside the other. Tiltthe inner ring, so that half of it is above and halfbelow the other ring, and they touch at two points,1 and 2 (Fig. 27). The outer ring is the Ecliptic, theinner the Equator, and where they touch each other arethe Equinoxes. Now move the inner ring, not slidingit round, nor making any difference in the anglebetween the two, but simply so that they touch at. Fig. 2ti. The moveuieut of Earths axis, which is the truecause of Precession. V is the vernal equinox, at the intersection of the equator and plane of theecliptic: APX the earths axis, which always preserves the same inclination(2;U%) to the plane of the ecliptic. As AFX slowly revolves round 2 in the direc-tion of the arrow, the vernal equinox is gradually shifted to V^, and so on. (From Youngs Manual of Astronomy, 1903.) HIFPARGHUS. 133 fresh points, 1 and 2. In this way you may makethe points of contact revolve entirely round. Thisis what the real equinoxes are doing: while the equatoropposite the group of stars in figure 25 rises andfalls, the equinox travels on, and finally returns tothe same place. The phenomenon is called precession of theequinoxes, because they thus move on to meet thesun in his yearly course. The discovery of precession is what has chiefly madeHipparchus famous, but the invention of the astrolabe Fig. 27. Precession of the eq
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