PLANET EARTH (WHTM) — Perihelion Day is the date when the Earth makes its closest approach to the sun. The sun is as large in the sky as it’s going to be.
The orbit of any planet (not just the Earth) is not a perfect circle. It is, in fact, an ellipse, shaped by the sun’s gravitational pull on a world. (The helion part of perihelion comes from the Greek word for sun.)
As a planet, such as Earth, travels away from the sun, the speed of its orbit slows. As it reaches its farthest point from the sun — the “aphelion” — it’s traveling at its slowest. Then the sun’s gravity pulls the planet back. As it starts to fall back towards the sun, it speeds up. When it makes its closest approach to the sun — the perihelion — it’s racing along at top speed — fast enough, in fact, to overcome the gravitational pull of the sun and start heading out to space again. The sun’s pull asserts itself, bending the planet’s orbit so it eventually starts falling back, and the whole process repeats itself.
But the process never repeats itself exactly. The gravitation pull of other planets, especially Jupiter, can nudge the orbit of a planet, and in the case of Earth, the moon inserts an additional wobble as it circles the Earth. (Which isn’t precisely what happens, but close enough. We’ll talk about barycenters some other time.)
There are also what are called Milankovitch cycles, changes in Earth’s orbit that take place over hundreds, even thousands of years.
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What this all means is that Perihelion Day changes from year to year. Centuries ago, it was on or around the winter solstice (which was Dec. 21 last year) but now happens about two weeks later. Over the last decade, it’s happened anywhere from Jan. 2 to Jan. 5.
But wait, you might say, if the sun’s at its closest approach, why is it so cold? Shouldn’t it be warm?
Well, it is warm — if you go south of the equator. The difference between the perihelion distance and aphelion distance is only about 3%. (Our graphic, obviously, is not to scale.) This means the amount of sunlight and warmth from the sun really doesn’t change.
The important thing is how many hours of sunlight a location on Earth gets in a day. Right now, because of the tilt of Earth’s axis, the northern hemisphere has more hours of darkness than light, which means we spend more time losing heat than gaining it. Meanwhile, down in the southern hemisphere, they are having more hours of heating than cooling. This will reverse after the spring equinox, and we’ll start getting the warm weather.
Just be patient.