What causes the seasons?
Seasons are caused by the tilt of Earth's axis, about 23.4 degrees. As Earth orbits the Sun, each hemisphere takes turns leaning toward the Sun and getting more direct sunlight.
When the Northern Hemisphere tilts toward the Sun, sunlight hits it more directly and days are longer, so it has summer. At the same time, the Southern Hemisphere tilts away and has winter. Six months later, the situation reverses. That is why Australia celebrates Christmas in summer.
Many people think seasons happen because Earth is closer to the Sun in summer. In fact, Earth is closest to the Sun in early January, during northern winter. The change in distance is small and has little effect compared with the tilt.
Solstices and equinoxes
The solstices, around June 21 and December 21, mark the longest and shortest days of the year. At the equinoxes, around March 20 and September 22, day and night are roughly equal everywhere on Earth. Near the equator, seasons are mild, while near the poles they bring months of daylight or darkness.
Did You Know?
Other planets have seasons too. Uranus is tilted so far that each pole gets about 42 years of continuous sunlight followed by 42 years of darkness.