Why the seasons happen

Here is a puzzle. In early January it is freezing in Chicago, yet Earth is closer to the Sun than at any other time of year. If distance made the seasons, January would feel like summer. In this lesson we will find the real cause, work out how high the Sun climbs in June and December, and use those numbers to explain why days grow long and short.

Earth passes closest to the Sun in early January, at 147.1 million kilometres, and farthest in early July, at 152.1 million. That is a difference of just 3.4%. Worse for the distance idea, the northern hemisphere is coldest when we are closest and warmest when we are farthest. Distance cannot be what drives the seasons.

The seasons are caused by tilt, not distance. Earth’s spin axis leans 23.4 degrees away from upright, and it keeps pointing the same way all year as we circle the Sun. That lean decides which half of the planet is aimed more directly at the Sun, and so when each half has summer.

Here is Earth at four points on its orbit. Its axis leans 23.4 degrees, and the lean points the same way in space all year. In June the northern half leans toward the Sun, in December it leans away, and in March and September the lean is sideways, so neither half is favoured. The planet moves, but the tilt never turns.

Look at two identical beams of sunlight. In June, at noon in Chicago, the Sun is high, so the beam lands on a small patch of ground and heats it strongly. In December the Sun is low, so the very same beam is stretched over a patch more than twice as long, and each square metre receives less energy. Slanted light is spread thin.

To find how high the Sun climbs at noon, start with 90 degrees, which is straight overhead. Subtract your latitude, 42 degrees for Chicago, because the farther north you live the lower the Sun sits. Then add the tilt, 23.4 degrees, because in June the north leans toward the Sun. The answer is 71.4 degrees. In December you subtract the tilt instead and get 24.6.

Put the two Sun heights together. Flat ground at noon receives the sine of the Sun’s height times the full overhead sunlight. In June that is about 95%, in December about 42%. Divide them and a square metre of Chicago gets 2.3 times as much sunlight in June as in December. The atmosphere absorbs more of a low Sun, so the real gap is even bigger.

Tilt also changes how long the Sun stays up. At Chicago’s latitude the longest day, on 21 June, has 15.1 hours of daylight. The shortest, on 21 December, has just 8.9 hours. That makes June days 1.7 times as long as December days. Longer days and higher Sun both add heat in summer, and both shrink in winter.

Now it is your turn. Predict what would happen if Earth’s axis stood straight up. Explain why Sydney is in summer while Chicago is in winter. And calculate how high the Sun climbs at noon on 21 December where you live, using 90 degrees minus your latitude minus 23.4. Work in pairs for two minutes, then compare.

To recap: the seasons come from Earth’s 23.4 degree tilt, not from our distance from the Sun. Tilt changes how high the Sun climbs and how long the days last. For homework, look up today’s sunrise and sunset times where you live, look them up again in a month, and work out how many minutes of daylight you gain or lose each day.