The Earth’s Rotation, RevolutionAround the Sun, and the Seasons
I. Earth’s Rotation•Earth spins around on animaginary axis– This iscalledrotation.•Theaxis of rotationis animaginary straight linethrough Earth Between theNorth and South Poles.•The axis is currentlyinclined at23.5from theplane of Earth’s orbit.
II. Earth’s Cycle of Precession•Like a spinning top thatrotates rapidly on its ownaxis while that axis slowlyrevolves about thevertical, Earth's axischanges its direction overthe course of time; This iscalledprecession.•During a complete cycleof precession, which takesabout 26,000 years,Earth's axis traces out acone.Earth's axis currently points nearly towardthe star Polaris. Some 12,000 years fromnow—nearly half-way through one cycleof precession—Earth's axis will pointtoward a star called Vega, which will thenbe the "North Star." Five thousand yearsago, the North Star was a star namedThuban in the constellation Draco.
III. Parallelism of Earth’s AxisThroughout the year, Earth’s axis has always the same tiltand points in the same direction.Thisparallelism of theaxisis important in the change of seasons and in thelength of daylight and night.