The spring equinox falls on slightly different days during different years, ranging from March 19 to 21. That’s the case for a couple of reasons. Our human calendar doesn’t perfectly match with Mother Nature. And the Earth’s trip around the sun isn’t exactly the same each year, in part because we actually get pulled and pushed around a tiny bit by other planets.
The “reason for the seasons” lies in the fact that the north pole of the Earth is not at 90 degrees to the plane in which our planet moves around the sun each year. In the summer, the tilt of the north pole is toward the sun. Six months later, we’ve moved half way around our orbit, making the tilt of the north pole away from the sun.
Most citizens don’t understand that, so let’s make it more clear.
Imagine you take a blue ball and stick a toothpick in it. The ball represents the Earth (it’s blue because so much of our planet is covered by the seas). The toothpick you stuck in the blue ball represents the North Pole. Set the blue ball near the edge of a table with the toothpick nearly upright but pointed a bit at the center of the table.
Now put an orange in the middle of the table to represent the golden sun. What you have is a model for summertime. Our part of the Earth (the northern half) is tilted toward the sun. Life is good. We get more daylight than darkness each 24 hours.
If you keep the blue ball and the toothpick in the same orientation, but move the ball around the orange to the far side of the table, you have a model for winter. The northern half of Earth is pointed away from the sun. Life is dark and dreary.
One of my favorite lines of poetry was inspired by the condition shown on the tabletop in this second condition. The verse is: “The sun that brief December day/ rose cheerless over hills of gray.” At least when I recite that line, I can feel the icy darkness of winter, when the northern hemisphere of Earth is pointed away from the sun.
But for the next half year from where we are in March, sunlight will be blessing us northern peoples. And the farther north on the Earth you go, the more daylight you get. That means folks in Fairbanks get more light than we in the Lower 48. That sounds grand, at least if you like sunlight as much as I do.
Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. Follow her on the web at http://www.rockdoc.wsu.edu and on Twitter @RockDocWSU. This column is a service of the College of Agricultural, Human and Natural Resources Sciences at Washington State University.