Seasons: Why Weather Changes

Axial Tilt and Solar Angle

Seasons are the repeating changes in weather and daylight that happen as Earth moves around the Sun during the year. They matter because they affect temperature, plant growth, animal behavior, and human activities such as farming and energy use. Many students think seasons happen because Earth is closer to the Sun in summer, but that is not the main cause. The real cause is the tilt of Earth's axis.

Earth's axis is tilted by about 23.5° compared with its orbit around the Sun. As Earth travels around the Sun, this tilt makes each hemisphere receive different angles and lengths of sunlight during the year. When a hemisphere is tilted toward the Sun, sunlight is more direct and days are longer, so temperatures are usually warmer. When a hemisphere is tilted away, sunlight is less direct and days are shorter, so temperatures are usually cooler.

Key Facts

Earth's axis is tilted by 23.5° relative to the plane of its orbit.
Seasons are caused by axial tilt, not mainly by changes in Earth-Sun distance.
More direct sunlight means more energy per unit area and warmer temperatures.
Longer daylight hours increase total solar energy received in a day.
Northern and Southern Hemisphere seasons are opposite at the same time of year.
Solar energy spread over area increases as sunlight arrives at a lower angle: intensity is proportional to cos(theta).

Vocabulary

axial tilt: Axial tilt is the 23.5° angle between Earth's rotation axis and a line perpendicular to its orbital plane.
orbit: An orbit is the path Earth follows as it moves around the Sun.
hemisphere: A hemisphere is one half of Earth, usually the Northern or Southern Hemisphere.
solstice: A solstice is the time of year when one hemisphere is tilted most toward or away from the Sun, giving the longest or shortest day.
equinox: An equinox is the time of year when both hemispheres receive nearly equal daylight and night.

Common Mistakes to Avoid

Thinking summer happens because Earth is much closer to the Sun. This is wrong because Earth's distance changes only a little, while axial tilt changes sunlight angle and day length much more.
Assuming both hemispheres have the same season at the same time. This is wrong because when the Northern Hemisphere tilts toward the Sun, the Southern Hemisphere tilts away.
Believing hotter weather comes only from longer days. This is wrong because both longer daylight and more direct sunlight work together to increase heating.
Confusing weather with seasons. This is wrong because weather is short term day to day change, while seasons are long term patterns caused by Earth's position and tilt.

Practice Questions

1 Earth's axis is tilted by 23.5°. If the axis were tilted by 0°, what major seasonal effect would change for most places on Earth?
2 A location gets 10 hours of daylight in winter and 14 hours in summer. How many more hours of daylight does it receive in summer than in winter?
3 Explain why Australia has summer during December while Canada has winter during December.