January 2 – Perihelion (Perhaps)

Today’s Factismal: Winter happens when the Earth is closest to the Sun.

Quick! If you live in the Northern Hemisphere, what was the weather like today? Odds are that today was a little chilly where you live. Why is that?

Ask that question of most people and they would tell you that it is because it is Winter (and possibly add a “duh” for good measure). But that answer isn’t good enough for scientists, because it simply substitutes the name of a thing (“Winter”) for the cause of it. So why is it cold in the Northern Hemisphere during Winter?

Most people assume that the reason that is is cold outside during the Winter is because the Earth is farther from the Sun. But that isn’t right. As a matter of fact, today the Earth was as close to the Sun as it gets during the year; it was a mere 147,098,290 km away from the Sun. During the Summer in the Northern Hemisphere, the Earth is actually about 3% farther away at 152,098,232 km. Overall, the difference is virtually negligible. If the Earths orbit were a wedding ring, it would be rounder than the ones found on most people’s fingers. So why would the Northern Hemisphere be colder even when the Earth is closer to the Sun? For the same reason that you lie down to get a tan.

The Earth's orbit. The Sun is to scale; the Earth has been magnified 100 times to make it visible.

The Earth’s orbit. The Sun is to scale; the Earth has been magnified 100 times to make it visible.

When you lie on a beach, you are putting a maximum amount of your skin perpendicular to the Sun’s rays. That means that you can intercept more of the Sun’s light, which then makes you warmer and improves your tan. The Earth does the same thing (though it doesn’t tan well). The Earth spins like a marble around an invisible, intangible pole known as the spin axis. This spin axis extends from the North Pole to the South Pole (both of which are also invisible and intangible). And – here’s the cool part – that spin axis doesn’t stick straight up and down. Instead, it is tilted at 23° relative to the Earth’s orbit at perihelion (that’s today).

At our closest approach to the Sun, he Sun's light comes in more directly in the Southern Hemisphere (giving them Summer) and more obliquely in the Northern Hemisphere (giving us Winter).

At our closest approach to the Sun, he Sun’s light comes in more directly in the Southern Hemisphere (giving them Summer) and more obliquely in the Northern Hemisphere (giving us Winter).

At our farthest distance from the Sun, the situation is reversed,

At our farthest distance from the Sun, the situation is reversed,

That tilt means that the Sun’s rays come in more obliquely, which then means that the same amount of light gets spread out over a larger area. And that means that it gets cooler in the Northern Hemisphere. But while we freeze, the Southern Hemisphere swelters because that same tilt means that the Sun’s light is coming in more directly there and so putting more light into less area. Come six months form now, we’ll be at the other end of our orbit and the situation will be reversed (because the tilt stays pointed in the same direction). So you can thank the same tilt that is to blame for Winter’s chill for Summer’s warmth.

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