L2. Seasons: It’s the Tilt, Not the Distance

Unit 1 · Earth in Space

R-report

L2. Seasons: It’s the Tilt, Not the Distance

Almost everyone guesses that summer happens when Earth is closer to the Sun. But Earth is actually nearest the Sun in early January — the middle of winter in Korea, Japan, and Europe. So what really makes the seasons?

A planet leaning to one side

Earth’s axis is not upright. It leans over by about 23.5 degrees, and — this is the important part — it keeps pointing in the same direction all year, like a spinning top frozen mid-lean. Earth also takes 365 and a quarter days to travel once around the Sun. Because the lean stays fixed while the planet moves around its orbit, each half of the planet takes a turn leaning toward the Sun. - Around June, the northern half leans sunward. The north gets summer while the southern half, leaning away, gets winter. - Around December, the situation is exactly reversed. - In March and September neither half leans toward the Sun, and day and night are close to equal everywhere. This is why the seasons in Australia are the opposite of those in Korea. If distance from the Sun caused the seasons, the whole planet would have summer at the same time. It does not.

Two effects, working together

Leaning toward the Sun changes two things at once, and both add heat. - Angle of the sunlight. When the Sun is high, a beam of light lands on a small patch of ground and heats it strongly. When the Sun is low, the same beam smears across a much wider patch, so each square metre gets less energy. Try it with a torch: shine it straight down on a table, then tilt it. The bright spot spreads out and fades. - Length of the day. The tilted half in summer spends more of each rotation in the lit part of Earth. A midsummer day in Seoul is about 14 and a half hours long; a midwinter day is under 10. More hours of sunlight means more hours of heating. Combine a steeper Sun with a longer day and you get summer. Combine a low Sun with a short day and you get winter.

Checking it against what you know

Once you understand the tilt, several everyday facts stop being random. - Near the equator the Sun is always high, so there is no strong summer or winter — just wet and dry seasons. - Near the poles the tilt is so extreme that the Sun stays up for weeks in summer and never rises for weeks in winter. - The hottest weather comes a month or two after midsummer, not on the longest day. Land and oceans take time to warm up, the same way a pot of water keeps heating after you turn the burner to full. - Shadows at noon are short in summer and long in winter, because the Sun is higher or lower in the sky. You can measure this yourself: stand a stick upright and mark the length of its noon shadow once a month for a year.

Putting the investigation together

Seasons come from geometry, not distance. Earth’s axis leans 23.5 degrees and keeps that lean fixed while the planet orbits, so each hemisphere spends part of the year tilted toward the Sun — receiving steeper light and longer days — and part tilted away.

This lesson is a good example of testing an idea instead of trusting it. "Summer means we are closer" sounds sensible until you check two facts: Earth is closest to the Sun in January, and the two hemispheres have opposite seasons at the same moment. A single well-chosen fact can rule out a popular explanation.