A Little Tilt Is All It Takes

I hope the first couple days of spring have lived up to seasonal expectations where you live. They have here, but now a howling snowstorm is on its way, threatening to repave all the lovely, exposed ground with a foot of wet, white concrete.

The sun is shown at noon on the first day of spring (left) and 10 days later. The arrow shows the direction it's moving. Notice that the sun's path, called the ecliptic, intersects the imaginary arc of the celestial equator on the first day of spring. By the 30th, the sun has moved noticeably north of the equator and is higher in the sky. Created with Stellarium

In Sunday’s blog we discussed that the reason for the seasons has to do with the tip or inclination of Earth’s axis, rather than our planet’s distance from the sun. The tip causes first one hemisphere and then the other to alternatively face toward and away from the sun. This makes the sun’s apparent path across the sun swing from low to high to low in the year-long cycle of four seasons. Spring began when the sun crossed the imaginary projection of Earth’s equator – called the celestial equator – on its march northward.

This diagram shows the various tilts of the planets and dwarf planet Pluto. Credit: Image courtesy of Courtney Seligman / cseligman.com

What about the other planets in the solar system? Do they have seasons too? Well, to have seasons, you need to be tipped on your side. You can see that Mercury, Venus and Jupiter have little to no axial tip, so it follows they don’t experience the seasons. The sun follows the same path across the sky, neither dipping low nor ascending to the heights, the entire year round on these planets.

On Earth, and sometimes in Duluth, liquid water returns as winter transitions to spring. Photo: Bob King

Venus is a bit of an oddball. It’s tipped so far over – 177 degrees – that its north pole is where the south pole should be. This extreme tip means Venus is actually rotating backwards or clockwise as seen from above the plane of the solar system. From the same perspective, most of the other planets rotate counterclockwise.

Because Earth, Mars, Saturn and Neptune have similar inclinations, they all experience similar seasons. For instance, right now it’s late spring in Mars’ northern hemisphere and early spring in Saturn’s northern hemisphere.

Keep in mind that seasons on these outer planets last longer than those on Earth, because they take longer to orbit the sun. A typical season on Mars last twice as long as the equivalent on Earth. Neptune takes takes 165 years to journey once around the sun, making spring a tedious 41 years in length. Well, maybe you like spring more than I do.

Pluto and Uranus also have seasons, but surprising things happen when you’re rolling around the sun on your side.

The Hubble Space Telescope photographed Uranus and its rings between 2003 and 2007. The rings, which lie above the planet's equator, went from open presentation in 2003 to edge-on in 2007. Credit: NASA/ESA/M. Showalter

Uranus orbits the sun every 84 years. As the north polar regions face the sun for 21 years, the sun hugs the horizon at the equator and the south pole is in complete darkness. During the next 21 years, the equator faces the sun and all parts of the planet experience sunlight. The poles see sunlight too, but just barely, as the sun circles the horizon for years on end. This is followed by 21 years of sunlight at the south pole and an equally long night at the north pole. The Uranian year wraps up with the equator once again facing the sun with sunlight across the whole planet. Just weird.

Seasons on Earth keep life interesting, but when you add in the rest of the planets, you really get a feel for how much variation is possible from a simple little thing like tilt.