Friday, February 28, 2014
The Earth has four seasons—spring, summer, fall, and winter. Do other planets in the solar system have seasons? Before answering that question, let’s review what causes the Earth’s seasons.
We use yearly celestial events to mark the beginning and end of each season. For example, on March 20 at 9:57 a.m. on the West Coast, the winter season will end and spring will begin in the Northern Hemisphere. This is the spring equinox.
Because the Earth’s axis of rotation is tilted in relation to the plane of its orbit around the sun, the sun’s path in the sky appears to move north and south during the year. On the day of the spring equinox the sun rises due east and sets due west. Before the spring equinox, the sun was rising south of the due east point; after, it will rise north of due east. Each day the sun will rise farther north, until on June 21 it will reach its northernmost point. This is the summer solstice, the beginning of summer.
The sun will then move south until, on Sept. 22, it will again rise due east on the fall equinox, the beginning of fall. It will continue south until Dec. 21, when it will rise at its southernmost point on the winter solstice, the beginning of winter. After that it will move north again, rising due east on March 20, 2015, on the 2015 spring equinox, as the cycle starts again.
The farther the sun is to the south, the fewer hours it is above the horizon and the shorter the daylight hours are. This fact, combined with the low angle of the sunlight, causes winter days to be colder. Conversely, the northern sun spends more time above the horizon with the angle of the sunlight closer to overhead, causing the hotter days of summer.
The orbit of the Earth around the sun is not a circle but an ellipse, so the distance between the Earth and the sun changes during the year. The Earth is closest to the sun in January and farthest in June. This change makes only a small difference in the temperature on Earth, but it does make the winters a little warmer and the summers a little cooler than they would otherwise be, giving us a more stable climate.
Another effect of this elliptical orbit is that the Earth moves faster in its orbit when it is closer to the sun, making winter a little shorter than summer. The difference in the length of each of the seasons on Earth is small because the orbit of the Earth is nearly circular.
This year winter is about 89 days long, whereas summer will be 93 days long. (Winter only seems longer!)
Now back to our earlier question: Do other planets in the solar system have seasons?
The answer is yes and no, it all depends on the tilt of the axis of rotation of the planet.
If the Earth’s axis of rotation were perpendicular to the plane of its orbit, the sun would not appear to move north and south during the year. It would always rise due east and set due west, and the number of daylight hours would be the same every day of the year. There would be no seasons. This is the case for the planet Venus, which has no seasons because its axis of rotation is almost perpendicular.
Mars, on the other hand, has a tilt of 25 degrees, which is almost the same as the 23.5-degree tilt of the Earth. Mars therefore has seasons like the Earth, but the seasons are longer and of different lengths. It takes Mars 687 Earth days (1.88 years) to orbit the sun, which means that seasons on Mars are almost twice as long as Earth’s.
Mars is also in an elliptical orbit around the sun, but its orbit is less circular than the Earth’s, so the orbital speed difference is greater. This means that there is a greater variance in the length of each season on Mars than on Earth.
For example, this year on Feb. 15 Mars was at its summer solstice; on Aug. 17 it will reach its fall equinox, after a summer of 183 Earth days. The winter solstice on Mars will occur Jan. 11, 2015, and the spring equinox on June 18, 2015, making Mars’ winter just 158 Earth days. On Earth the seasons are almost the same length: 93 days of summer and 89 days of winter.
There is a great interactive website where you can set different tilts (inclinations) and see their effects on the path of the sun and the sunlight angle. It also predicts the average daily temperature at the observation site at latitude 45 degrees north. Check it out.
I hear that our forecast calls for cold weather and snow this weekend, but remember that our winter is short compared to a Martian winter, and spring is almost here!
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.