Earth's axis is tilted about 23.4°, and it keeps pointing the same way in space (toward the North Star) all year. As Earth travels around the Sun, first one hemisphere and then the other leans toward the Sun. This tilt, not our distance from the Sun, makes the days longer or shorter and the Sun higher or lower.
- Around June 21 the Northern Hemisphere leans toward the Sun: long days and a high noon Sun there (summer), short days in the south (winter).
- Around December 21 it is the other way around.
- Around March 20 and September 22 (the equinoxes) neither hemisphere leans toward the Sun, and day and night are close to 12 hours everywhere.
Key idea
Seasons come from the tilt of Earth's axis. The hemisphere tilted toward the Sun gets a higher Sun and longer days, so it has summer, while the other hemisphere has winter.
Why it works
A high Sun is strong sunlight. Shine a flashlight straight down and you get a small, bright spot; tilt it and the same light spreads over a bigger, dimmer patch. When the Sun is high, each square meter of ground gets more energy, and long days give it more hours to collect it. Low Sun and short days add up to winter.
Worked example: The noon Sun at an equinox
How high is the noon Sun above the horizon at 40° N on March 20?
- At an equinox the Sun is overhead at the equator (0°).
- Each degree you go away from the equator, the noon Sun is a degree lower: $90 - 40 = 50$.
Answer: 50° above the horizon
Worked example: The noon Sun at a solstice
On June 21 the Sun is overhead at 23.4° N. How high is the noon Sun at 40° N then? And on December 21, when it is overhead at 23.4° S?
- June: the place is $40 - 23.4 = 16.6$ degrees from where the Sun is overhead, so the Sun is $90 - 16.6 = 73.4$° high.
- December: the place is $40 + 23.4 = 63.4$ degrees away, so the Sun is $90 - 63.4 = 26.6$° high.
Answer: 73.4° in June and 26.6° in December
Common mistake
Saying summer comes when Earth is closest to the Sun. Earth is closest in early January (about 147 million km) and farthest in early July (about 152 million km), only about 3% different. If distance made the seasons, both hemispheres would have summer at the same time, but Australia has summer in January while the United States has winter.