Why do we have seasons?
― It's not distance, it's "tilt"
"Summer is hot because Earth moves closer to the Sun" — you might have assumed that. In fact, when it's summer in Japan, Earth and the Sun are actually moving further apart. The seasons aren't caused by changing distance at all. They come from a completely different reason: Earth spins while tilted slightly on its axis.
Basking in strong midsummer sunshine, you might have vaguely thought, "It must be hot because Earth is getting closer to the Sun." Earth does trace an orbit around the Sun, and it's true that the distance changes slightly over the course of a year.
But actually, Earth and the Sun are closest together around early January — for Japan, that's the depths of winter. Conversely, Earth and the Sun are furthest apart around early July, right in the middle of Japan's summer.
If you go by distance alone, the story simply doesn't add up.
When it's summer in Japan, Earth is actually slightly further from the Sun. The explanation "closer means hotter" doesn't match the facts.
Earth's rotation axis stays tilted at an angle relative to its orbital plane as it circles the Sun. This tilt is the real source of the seasons.
Not "distance" but "tilt" creates the seasons — let's look at how this works, step by step.
The instinct "closer means hotter" doesn't match the facts
Earth doesn't orbit the Sun in a perfect circle — it follows a slightly stretched, near-elliptical path. So the Earth-Sun distance does change somewhat over the year. But the closest approach happens around early January, and the furthest point around early July.
If "closer means hotter" were the right explanation, January should be the hottest month of the year. In reality, it's exactly the opposite. This fact alone shows that distance isn't the main cause of the seasons.
There's another weakness in that explanation. When it's summer in the Northern Hemisphere, it's winter in the Southern Hemisphere. If distance were the only factor, the whole planet should heat up or cool down at the same time — but in reality, the two hemispheres experience completely opposite seasons.
What creates the seasons is "the tilt of the axis"
Earth's spin axis stays tilted about 23.4 degrees relative to its orbital plane as it circles the Sun. What's more, this tilt direction barely changes over the course of a year.
So as Earth travels around the Sun, at some point the Northern Hemisphere leans toward the Sun, and at another point the Southern Hemisphere leans toward it. When the Northern Hemisphere leans toward the Sun, it's summer there; when it leans away, it's winter there. In the Southern Hemisphere, it's exactly the reverse.
They come from the simple fact that Earth keeps spinning while tilted.
Two effects created by the tilt
When a hemisphere tilts toward the Sun, two effects happen at once. First, daylight hours get longer. In the hemisphere tilted toward the Sun, the fraction of each 24-hour rotation spent in sunlight increases.
Second, sunlight strikes the ground at a steeper, more direct angle. This second effect gets less attention, but it's actually considered a huge factor in how hot or cold a season feels.
The more "directly" sunlight hits, the stronger it is
Point a torch straight down at a wall, and you get a small, bright, round spot of light. Tilt the same torch at an angle against the wall, and the shape of the light stretches out and grows larger, while the brightness at any one spot weakens. The same amount of light is simply spread over a wider area.
Sunlight works the same way. When the Sun sits high in the sky (light arriving at a near-vertical angle), the same patch of ground receives more energy. When the Sun sits low (light arriving at a shallow angle), the same amount of light spreads over a wider area, so the energy per patch of ground is less. The reason the Sun climbs high in summer and stays low in winter is precisely this tilt of Earth's axis. In the next expandable section, we'll check exactly how big that difference is, using real numbers.
In countries south of the equator, like Australia, December and January are summer, and June and July are winter. Earth-Sun distance is the same for both hemispheres, yet the seasons run completely opposite — this is clear evidence that the cause of the seasons is the axis tilt, not distance.
By the way, the axis tilt determines "how long daylight lasts," but "what time it gets dark" turns out to be a slightly trickier story. See also Is it true that the earliest sunset isn't on the winter solstice?
Something you can check yourself
- In a dark room, get a torch (or a phone's flashlight) and a sheet of white paper
- Point the torch straight (perpendicular) at the paper and look at the shape and brightness of the light
- Keeping the same distance, tilt the torch at an angle and compare the shape and brightness
- Confirm that at an angle, the light shape stretches out and looks noticeably dimmer over the same area
This is exactly the same mechanism by which the Sun's height in the sky changes how much energy the ground receives.
Summary
The seasons exist not because of changes in Earth-Sun distance, but because Earth orbits the Sun while its axis stays tilted. In the hemisphere tilted toward the Sun, daylight lasts longer, and sunlight also strikes at a more direct angle, delivering more energy per unit area, so temperatures rise. The fact that the two hemispheres experience opposite seasons also fits this tilt-based explanation perfectly.
It isn't distance to the Sun that makes summer hot.
It's the tilted posture Earth faithfully keeps.
The fact that the Sun's height in the sky differs by location can even be used to measure the size of Earth itself. That story is told in How was the size of Earth measured?
For those who want to know more — terms, numbers, and textbook connectionsFrom junior-high science to topics still being researched, each level is clearly labelled
- JHSCovered in junior-high school science
- High SchoolCovered in high-school "Earth Science Basics"
- High School+Covered in high-school "Earth Science," or treated as advanced/column content in textbooks
- UniversityNot covered in high school — content from a specialized university course (astronomy)
- ResearchNot yet taught even at university as settled fact — something researchers are actively investigating
JHSTerms: words used to describe the seasons
- Orbit (revolution): Earth's motion as it circles the Sun.
- Axis: The line around which Earth rotates.
- Solar altitude at noon: The angle, as seen from the ground, at which the Sun sits when it's highest in the sky.
High SchoolChecking with a formula: how much does sunlight strength change with the Sun's height?
Let's actually calculate the relationship between the Sun's noon altitude (its offset from directly overhead) and the strength of sunlight the ground receives.
For a location at about latitude 35.7 degrees (roughly Tokyo), we calculate the offset from directly overhead at the summer and winter solstices. Earth's axial tilt is about 23.4 degrees.
| Offset from overhead at summer solstice | 35.7 − 23.4 ＝ 12.3 |
| Summer solstice offset | about 12.3° |
| Offset from overhead at winter solstice | 35.7 ＋ 23.4 ＝ 59.1 |
| Winter solstice offset | about 59.1° |
This shows that at the summer solstice the Sun climbs nearly overhead, whereas at the winter solstice it only rises to a much lower position.
It's known that the sunlight the ground receives follows a relationship where the larger the offset from overhead, the weaker it gets (proportional to the cosine of the angle). Looking up trigonometric values, the cosine of 12.3° is about 0.977, and the cosine of 59.1° is about 0.514.
| Ratio of sunlight strength, summer to winter solstice | 0.977 ÷ 0.514 ≒ 1.90 |
| Ratio of sunlight strength | about 1.90 times |
Based purely on the angle difference, sunlight delivers to the ground about 1.9 times more strength at the summer solstice than at the winter solstice. Add in the difference in daylight length, and the total gap in sunlight received between summer and winter grows even larger.
※ The latitude and angle figures here are representative estimates. Actual noon altitude and sunlight amounts vary by location and atmospheric conditions.
High School+The phenomenon of seasonal lag
It's known that the actual hottest period of the year lags slightly behind the summer solstice, when sunlight is strongest. This happens because oceans and land take time to store and release heat, a phenomenon called seasonal lag. This is thought to be why Japan's hottest period comes around August, later than the summer solstice (around June).
UniversityThe idea of Milankovitch cycles
It's known that the angle of Earth's axial tilt, the shape of its orbit (how elliptical it is), and the slow wobble of the axis's direction (precession) all change gradually over tens of thousands of years. These periodic changes are called Milankovitch cycles, and they're thought to be linked to large-scale periodic climate shifts such as glacial and interglacial periods.
ResearchWhat's still unclear
- Exactly how much, and at what specific timing, Milankovitch cycles influence the start and end of glacial periods is still debated and researched in the field of paleoclimatology. This is because multiple factors are intertwined, not just axial tilt and orbital changes, but also shifts in atmospheric greenhouse gas concentrations.
- Research also continues into more precisely predicting how the angle of Earth's axial tilt itself will change in the future. This tilt change is an extremely slow phenomenon measured in tens of thousands of years, but it's considered an important research theme for improving the accuracy of long-term climate models.
Behind the everyday phenomenon of "the seasons" lies a grand research theme that stretches all the way to celestial motions spanning tens of thousands of years.
Textbook connections (by level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| JHS | Science: Earth and Space | Basic terms: orbit, axis, noon altitude |
| High School | Earth Science Basics: motion of Sun and Earth | Calculating noon altitude and sunlight strength |
| High School+ | Earth Science: climate systems | The phenomenon of seasonal lag |
| University | Astronomy / Paleoclimatology | Milankovitch cycles |
| Research | Paleoclimatology (ongoing research) | Relationship between glacial cycles and Milankovitch cycles |
- Explanatory materials from the National Astronomical Observatory of Japan (国立天文台) "Calendar Computation Office" (暦計算室) on Earth's orbit and perihelion/aphelion.
- Explanation of the relationship between axial tilt and the seasons in Earth Science Basics textbooks.
- Explanatory material on "seasonal lag" from the Japan Meteorological Agency (気象庁).
- Explanation of Milankovitch cycles in paleoclimatology-related textbooks.
- Explanation of the relationship between noon altitude and sunlight amount (cosine law) in astronomy-related textbooks.
※ Figures for latitude, angle, and ratio are representative estimates; actual values vary somewhat by location and year.
※ This article is a general-audience science explainer. For precise figures on celestial motion and calendars, please check official sources such as the National Astronomical Observatory of Japan.