Is Earth closest to the Sun in summer?

It seems obvious. Summer is hot, the Sun feels close, so Earth must be nearer to it. It is such a reasonable guess that plenty of adults never question it. But if you live in London, New York or Tokyo, here is a fact to enjoy while scraping ice off your windscreen: in early January, Earth is at its closest to the Sun all year.

The short answer

Earth’s orbit is a slightly stretched circle, so our distance from the Sun changes a little over the year. The closest point, called perihelion, comes in early January, when we are about 147 million kilometres away. The farthest point, aphelion, comes in early July, at roughly 152 million kilometres. Seasons are not caused by that change in distance. They come from the tilt of Earth’s axis.

The real story

Earth spins on an axis tilted about 23.5 degrees. As we orbit, that tilt stays pointing the same way in space. For part of the year the Northern Hemisphere leans towards the Sun: days are longer, the Sun climbs higher in the sky, and its light hits the ground more directly, so each patch of land receives more energy. That is northern summer. Six months later, the Northern Hemisphere leans away, the days shorten, the light comes in at a low slant, and winter arrives. The Southern Hemisphere gets the reverse, which is why Australians eat Christmas lunch at the beach.

Distance does have a small effect. According to NASA, sunlight reaching Earth in January is, averaged over the globe, about 7 percent more intense than in July. But how land and oceans are spread around the planet softens how much that shows up in actual temperatures.

The changing distance has another quirky side effect. Earth travels a little more slowly when it is farther from the Sun, a rule described by Johannes Kepler centuries ago. Because we are farthest away in July, Earth dawdles through that part of its orbit. As a result, according to EarthSky, summer in the Northern Hemisphere lasts nearly five days longer than winter.

Why people get it wrong

The distance idea matches everyday experience. Stand closer to a campfire and you feel hotter, so it seems natural that Earth would warm up by moving closer to the Sun. Diagrams in books often exaggerate the oval shape of Earth’s orbit too, making the distance change look far bigger than it really is. In reality, Earth’s orbit is so close to a circle that the difference between nearest and farthest is only about 3 percent.

The giveaway is simple: if distance caused the seasons, the whole planet would have summer at the same time. Instead, when it is summer in Sydney, it is winter in Stockholm. Only a tilt can explain opposite seasons in opposite hemispheres.

“Down here in Australia we get our summer when Earth’s closest to the Sun, so I reckoned I had this one sorted. Turns out it’s the tilt, mate. I’m just leaning the right way.”