Earth's Motion and the Seasons

The Earth has two main motions: rotation on its axis and revolution around the Sun.

Earth's motions

• It rotates on its axis from west to east in 24 hours. As a result, day and night alternate.
• Moving objects are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• It revolves around the Sun in 365 days 6 hours. This is one year.
• Every 4 years there is a leap year: 366 days, February has 29 days.
• The Earth's axis is tilted relative to the plane of its orbit (about 66,5°). This is why the seasons change.

The seasons

Date	Event	Characteristics
21 March	Spring equinox	day and night are equal, the Sun is at its zenith at the equator
22 June	Summer solstice	longest day in the Northern Hemisphere, the Sun is at its zenith at the Tropic of Cancer (23,5° N)
23 September	Autumn equinox	day and night are equal, the Sun is at its zenith at the equator
22 December	Winter solstice	shortest day in the Northern Hemisphere, the Sun is at its zenith at the Tropic of Capricorn (23,5° S)

• 22 June: the longest day (in the Northern Hemisphere). 22 December: the shortest day. 21 March and 23 September: the equinoxes.
• Above the polar circles (66,5°), polar days and polar nights occur.
• Rotation on its axis takes 24 hours, revolution around the Sun takes 365 days. Do not confuse these two.