Astronomy: Motions of the Planets

The ecliptic plane is coplanar with Earth’s orbit around the Sun, and the orbital planes of the Moon and the planets lie within a few degrees of it. Viewed from above (looking south), the planets orbit the Sun in an anticlockwise direction and this is called prograde or direct motion. Most of the planets rotate about their axis in the same sense, but the spin axes of Venus, Uranus and Pluto are tilted at more than 90º and therefore rotate in the opposite, retrograde sense.

Orbital inclination is the angle between a planet’s orbital plane and the ecliptic plane; axial tilt is that between a planet’s axis and the normal to its orbital plane; obliquity of the ecliptic (≈23.45°) is that between Earth’s orbital plane (coplanar with ecliptic) and Earth’s equator (=Earth’s axial tilt).