Seasons

Why does Earth have seasons when its distance from the Sun barely changes?

The tempting answer is that summer is when we are nearest the Sun. Test it before trusting it. Earth's orbit is slightly off-round: about 147.1 million kilometres out in early January, about 152.1 million in early July, which makes January sunlight roughly 7% more intense. So the distance answer does not merely fail — it predicts the wrong season for half the planet.

There is a sharper objection too. At this instant it is summer somewhere and winter elsewhere. Whatever causes seasons cannot be a property of the whole Earth at once, because the whole Earth is at one distance. What sort of cause could differ between the top and the bottom of the same planet on the same day?

Something not shared by both hemispheres. Earth spins on an axis tipped about 23.4 degrees from the perpendicular to its orbit, and — this is the part that matters — that axis keeps pointing the same way in space all year. It does not swing to follow the Sun. So for half the orbit the northern end leans sunward, and half an orbit later the same fixed tilt leans away.

What does leaning change? Think of a beam of light meeting the ground. Perpendicular, it makes a small bright patch; tilt the ground and the identical beam smears into a longer, dimmer ellipse — same energy, more square metres to share it. At 50 degrees north the noon Sun stands 63 degrees high in June and 17 degrees in December, and energy per square metre scales with the sine of that angle, so December noon delivers roughly a third of June's before a single cloud is considered.

Is that all? Notice what else the tilt controls. Lean toward the Sun and your latitude spends more of each rotation in daylight: at that same 50 degrees north, about 16 hours of geometric daylight in June against under 8 in December. The second channel is simply how long the collecting goes on.

Here is what deserves a pause. These two are not competitors. Both follow from the same lean, so both peak in the same week and both bottom out in the same week. They multiply rather than average — which is why summer and winter differ far more than either effect alone would suggest.

One puzzle remains: the tilt maximises in June, yet the hottest weeks are usually late July and August. Why the lag? Because ground, and far more so ocean, takes weeks to warm. What we feel is an accumulated balance, and a slow reservoir always peaks after its inflow does.

Distance is not nothing — it is small and mistimed. That 7% January excess falls on southern summer, so the southern hemisphere does receive somewhat more solar energy at the top of its atmosphere in its summer than the north does in its. That southern summers are not correspondingly fiercer is largely because that hemisphere is mostly ocean, and water swallows heat with little change of temperature.

Nor is "seasons" one phenomenon everywhere. Near the equator the noon Sun is high all year and days barely vary, so the strong cycle is rain rather than temperature. Near the poles the day-length channel dominates so completely that it ends in months of unbroken light or dark.

• Why the hottest and coldest weeks lag the solstices by about a month, and why coastal lag is longer.
• How slow changes in tilt, orbital shape and axial wobble — the Milankovitch cycles — pace the ice ages.
• Why the tropics have wet and dry seasons instead of hot and cold ones.

Every term the collection defines is gathered in the glossary.