Below Zero And Closest To The Sun — How So?

Earth’s oval or elliptical orbit causes our distance from the sun and orbital speed to vary during the year. This Wednesday, Jan. 4, we’re both closest and moving fastest. Illustration: Bob King

Wednesday morning, the people in my town will wake up to strong winds and 4 below zero (–20°C). Funny that that very day the Earth will be closest to the sun for 2017. Do you think we’ll feel any warmer? Absolutely not.

The momentary proximity occurs at 8:17 a.m. Central time, when the distance from the sun’s center to the Earth’s center will be 91,404,322 miles (140,100,997 km). That’s about 1.6 million miles closer than average and 3.1 million miles closer than when the two bodies are at their most distant from each other.

How to draw an ellipse. The two toothpicks are called foci. In a real planet’s orbit, the sun is at one of the foci, with the other left empty.

We swing around the sun once a year on an elliptical orbit, the same type of orbit the moon follows around the Earth. The moon passes closest (perigee) and farthest (apogee) from our planet once every 27 days. When perigee coincides with full moon, we call it a supermoon. Last year, we had three supermoons!

Likewise, the Earth reaches its closest point to the sun every January at a point in its orbit called perihelion (peri=close, helion=sun). In July, it’s on the other side of its orbit at aphelion and most distant from the sun, 94.5 million miles (152 million km) on July 3. The difference between them amounts to 3.3%, small enough to pass unnoticed. Earth’s orbit is very close to a circle.

Mars has a more squished or eccentric orbit than Earth with more extreme seasons. Credit: NASA

Not so Mars, a planet with a much more eccentric orbit. Mars also runs around the sun in an elliptical orbit, but it’s ellipse is more flattened and elongated — more eccentric — than Earth’s. If you lived on Mars, you’d easily notice the 9.3% difference between perihelion at 128.5 million miles and aphelion at 155 million miles. Its seasons are more extreme. At perihelion, the extra heat raises vast dust storms; at aphelion, so much of the atmosphere freezes out that the planet’s atmospheric pressure drops 25%!

The Earth’s speed varies around its orbit, too. At perihelion on Wednesday, we’ll be ripping along 67,680 miles an hour (30.3 km/second), about 680 miles per hour faster than average, compared to 65,520 mph (29.3 km/second) in early July. How weird to think we’re always slowly and inexorably either accelerating or decelerating as we orbit the sun each year. There’s no constant speed of travel, no cruise control setting when it comes to planets. Or moons, comets and asteroids for that matter.

Earth’s northern hemisphere leans toward the sun in summer and away in winter while maintaining  a constant tilt in the same direction. In winter, the planet is at the close end of its elliptical orbit around the sun. Credit: Thomas G. Andrews / NOAA

Let’s revisit our changing distance. While it really does affect the amount of heat we receive from the sun, it’s not nearly enough to offset the much more dramatic temperature changes brought on by the seasons, which are caused by the 23.5° tilt of our planet’s axis. The northern hemisphere tilts toward the sun in summer. Long days and a high sun make for lots of sunlight and heat. Six months later, when the hemisphere is oriented away from the sun, short days make us shiver.

But our increased speed does benefit those who don’t care much for winter. It shortens the season! 89 days separate the start of winter from the spring equinox in March, while summer, when Earth is farthest from the sun and travels slowest, lasts 93 days. Summer sticks around, but it always seems like a long time getting there.