What is the difference between rotation and revolution of a planetary body?
ANSWER Although the Moon is nearly a sphere, one side always facing Earth, inspection of photographs taken on different dates discloses an apparent shift of major landmarks relative to reference point at the lunar limbs. Mapping demonstrates that almost 60% of the total surface (front and back) of the lunar sphere has been seen from Earth through telescopes. Because this seems to be a “wobbling” or “rocking back and forth”, the term libration (from a Latin word for “balance”) is applied to the phenomenon. Librations result in part from apparent (optical) displacements that bring more of both equatorial and polar regions into view. The motions involve variations in angular velocity of the Moon’s revolution in elliptical orbit, tilt of its rotational axis relative to its orbital plane (inclined at 5°09′ to the ecliptic), and parallax effects for different observation points on Earth. There is also a real libration owing to varying gravitational pull by Earth as the Earth-Moon distance va