The moon's declination varies from year to year. A maximum (18°+) occurred in March 1932 and in 1941. The reason for the variation is the regression of the Moon's nodes. The ecliptic is inclined to the celestial equator by 23°27'. The moon's apparent path on the celestial sphere is inclined to the ecliptic on an average of 5°8', but the intersection points, the nodes, move relatively fast, covering 360° in about 19 years. When the Moon's ascending node lies at the Vernal equinox, the angle between the Moon's apparent path and the equator is at the greatest, for 23°27' must be added to 5°8' making 28°35'. Half a revolution later, or about 9½ years, the descending node is at the Vernal equinox, and the angle between the moon's path and the equator is at the least, and 5°8' is subtracted from 23°27', giving 18°19'. The more the moon's path is inclined to the equator, the greater is the declination.
ASTROLABE ENCYCLOPEDIA
