A Revision of Young's Manual Of Astronomy

Fig. 128. Secular Perturbations of the Earth’s Orbit

The calculated motions, for 200,000 years, of the center of the earth’s orbit, in its own plane, are shown in A; and those of the pole of the ecliptic, on the celestial sphere, in B The positions at intervals of 10,000 years are marked by short lines, and those at the present time by small circles. The black dot in A represents the sun, and the isolated circle in B represents the pole of the invariable plane. The earth’s orbit itself, on the scale of A, would be about 40 inches in diameter. The eccentricity of the orbit is now 0.016 but 1s diminishing and will reach a minimum value of 0.003 about 24,000 years hence. The inclination to the invariable plane is now 1°35' and is also dimmhing. It will reach a minimum value of 47' in about 20,000 years.