The Seasons

The seasons are caused by the tilt of Earth's axis, not by changes in Earth's distance from the Sun.

Earth's Axial Tilt

Earth's axis is tilted 23.5° from perpendicular to its orbital plane. This tilt remains fixed in space as Earth orbits the Sun, causing the Sun's apparent path (the ecliptic) to move north and south across the celestial equator throughout the year.

The Four Seasons

• Summer Solstice (June 21) - The Sun is at its northernmost point. The Northern Hemisphere has its longest day; the Southern Hemisphere has its shortest.
• Winter Solstice (December 21) - The Sun is at its southernmost point. The Northern Hemisphere has its shortest day; the Southern Hemisphere has its longest.
• Vernal Equinox (March 21) - The Sun crosses the celestial equator moving northward. Day and night are equal length worldwide.
• Autumnal Equinox (September 21) - The Sun crosses the celestial equator moving southward. Day and night are equal length worldwide.

Why It's Warmer in Summer

In summer, the Sun is higher in the sky, so sunlight strikes the Earth more directly and spreads over a smaller area. Days are also longer, giving more time for heating.