Why are days longer in summer than in winter?
The answer lies in the tilt of the Earth.Earth's axis of rotation is tilted about 23.4°, and Earth travels around the Sun once a year while its axis keeps pointing the same way. In summer, when the Northern Hemisphere tilts toward the Sun, the Sun travels high across the sky, so days are long. In winter, when it tilts away, the Sun travels low, so days are short. Distance from the Sun is not the cause.
Earth's axis is tilted about 23.4°
Earth spins once a day like a top (rotation) and travels around the Sun once a year (revolution). Its axis does not stand straight up compared with the flat path it travels around the Sun; it is tilted about 23.4°. What's more, as Earth goes around the Sun, the axis always keeps pointing in the same direction.
The height of the Sun's path changes
Around June, the North Pole side faces the Sun, so in the Northern Hemisphere, including Japan, the Sun travels high across the sky. The Sun's height at its highest point of the day is called its noon altitude. The higher it is, the longer the time from sunrise to sunset, in other words, the longer the day. Around December, the South Pole side faces the Sun, and in the Northern Hemisphere the Sun travels low, so days are short. The longest day comes around the summer solstice, and the shortest around the winter solstice. Around the spring and autumn equinoxes, day and night are about the same length everywhere.
The shapes of the paths are simplified to show how it works.
It changes differently from place to place
How much day length changes depends greatly on latitude (how far north or south a place is from the equator). Near the equator, day and night stay about the same length all year. The higher the latitude, the bigger the change. Around the summer solstice, places near the North Pole have the midnight sun, when the Sun does not set all day, and places near the South Pole have polar night, when the Sun does not rise all day. The seasons and the changes in day length are opposite in the Northern and Southern Hemispheres.
Distance from the Sun is not the reason
Many people think it is summer because Earth is closer to the Sun, but that is not true. Earth's path is very close to a circle, so its distance from the Sun does not change much. In fact, Earth is closest to the Sun around January and farthest around early July. So for the Northern Hemisphere, Earth is closer in winter and farther in summer. If distance were the cause, both hemispheres would have the same season at the same time.
Want to know more?
- The earliest sunrise comes before the summer solstice, and the latest sunset comes after it. The earliest sunset comes about 10 days before the winter solstice.
- If the axis were not tilted at all, there would be no change of seasons. On the other hand, a planet tipped almost on its side, like Uranus, has summers with very long days and winters with very long nights.
- Earth's axis is thought to be tilted because a large body crashed into Earth not long after it formed.
Quick quiz: test yourself
Why are days longer in summer than in winter?
Show the answer
- Our distance from the Sun changes
- Earth's axis is tilted
- The Sun's brightness changes
Earth's axis is tilted Earth circles the Sun with its axis tilted. In summer the Sun takes a high path and days are long; in winter it takes a low path and days are short.
Earth circles the Sun with its axis tilted. In summer the Sun takes a high path and days are long; in winter it takes a low path and days are short.
Next quiz ›Sources
- 暦Wiki 季節はなぜ変化するのか？国立天文台 暦計算室 (in Japanese)
- 暦Wiki ところ変われば季節も変わる国立天文台 暦計算室 (in Japanese)
- 暦Wiki 昼や夜の長さの季節変化国立天文台 暦計算室 (in Japanese)
- What Causes the Seasons?NASA Space Place
This article was written after checking the sources above as of 2026-10-03. Explanations may change as research progresses. Photo credits are shown below each photo.
