Astro Bob: Seasonal variations — why winter gets the short end of the stick
Both Earth's speed and distance from the sun changes around its orbit which affects the length of the seasons.
On January 2 at 6:51 a.m. Central Time the Earth reached its closest point to the sun called perihelion. The word comes from joining the Greek "peri", meaning near, and "Helios," the Greek god of the sun. At that moment, Earth stood 91,399,453 miles (147,093,162 km) from the sun, more than 3 million miles closer than on July 5 at aphelion, its most distant point. "Ap" means away or apart. It's also the root of the word apology .
If the Earth's orbit were a perfect circle then both its distance from the sun and orbital speed would be the same around its entire orbit. Perfect circles are nearly impossible to achieve because they would require constant tweaking from outside forces. When it can, nature prefers to take the easy way out. That's why objects orbit one another in ellipses instead of circles.
An ellipse is a slightly-out-of-round circle with the sun centered a little to one side. Consider it the path of least resistance. That said, the orbits of the planets are very close to circles. If you could float above the plane of the solar system, look down like some Greek god and speed up time, their orbits would look like circles to your eyes.
But careful measurements show every one of them, including every asteroid, comet and even Elon Musk's 2008 Tesla Roadster , have varying elliptical orbits. Some are nearly circular while others are so stretched out (astronomers use the term "eccentric") that they resemble the shape of a cigar. The Roadster, launched into orbit in 2018, has a perihelion of 0.99 astronomical units (a.u.), where one a.u. equals the average Earth-sun distance, and an aphelion of 1.7 a.u.
Earth's elliptical orbit brings it closest to the sun during northern hemisphere winter and farthest in the summer. The slight change in distance, a difference of 3.3 percent at the summer and winter extremes, has little effect on the seasons, which are caused instead by the tilt of the Earth's axis.
In winter, the northern hemisphere is tipped away from the sun, and in summer, toward the sun. This changes the sun's altitude in the sky which affects not only the intensity of its rays but also the day length. Long days make for lots of heating, and short ones mean cranking up the thermostat.
The Earth zips along its elliptical orbit at an average speed of 66,616 miles per hour (107,208 km/hr) but more slowly at aphelion — when it's farthest from the sun — and more rapidly at perihelion. Right now the planet is dashing along at 0.6 miles per second (1 km/sec) faster than it will be in July. That's why winter is shorter than summer.
Count the days. From Dec. 21, 2020 to March 20, 2021 (first day of spring) there are 88 days. Now count the days from June 20, the first day of summer, to the Sept. 22 fall equinox, and you'll tally up 93.
Winter is 5 days shorter than summer if you live in the northern hemisphere and 5 days longer for southern hemisphere dwellers. So if you don't like winter, don't go living below the equator.
The changing apparent size of the sun makes for another perihelic peculiarity. In December and January the sun appears slightly larger than in summer when it's most distant. The difference amounts to 1/30th of its diameter or one arc-minute. Not much, but if you took photos on the first days of summer and winter and placed them side by side you'd see the difference.
So enjoy this week's big sun and our pedal-to-the-metal ride as we hurtle toward winter's end.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob .