Jimmy Westlake: How far is the sun?
June 30, 2014
Steamboat Springs — Let me pose a simple question and see if you can do better than some recent Harvard graduates did. Why is it hot in the summer and cold in the winter?
After four years of attending Harvard University, some graduates were asked this question as they walked off of the stage after being handed their diplomas. While I do not have the specific breakdown of the numbers, many of these graduates linked the varying temperatures of Earth's seasons to its distance from the sun. They explained that, because of the Earth's elliptical orbit around the sun, Earth is closer to the sun in the summer, and therefore hotter than it is in the winter, when Earth is farther from the sun. Seems logical.
If this was your answer, then you are in good company. Unfortunately, that answer is completely wrong. Even a Harvard education, it seems, cannot undo this common misconception that the seasons are caused by the Earth's varying distance from the sun.
It is true that the Earth's orbit is not a perfect circle, as was believed up until the early 17th century, when German astronomer Johannes Kepler demonstrated otherwise. Planets orbit the sun in elliptical paths, swinging in closer and then farther from the sun. But, Earth's orbit is only slightly out of round, and here's the kicker: Earth is farthest from the sun in early July each year, as the Northern Hemisphere is sweltering in the summer heat. This point in Earth's orbit is called aphelion and literally means "farthest from the sun."
Earth reaches its aphelion this Thursday at a distance of 94.6 million miles. Earth reaches its closest point to the sun, or perihelion, in early January every year, during the dead of Northern Hemisphere winter. Earth's perihelion distance this past Jan. 3 was 91.5 million miles.
I know – this seems counterintuitive. Clearly, the hotness of summer and the coldness of winter are caused by something other than Earth's slight variance in solar distance. What is that something?
Earth's seasonal changes are due to its 23.5-degree tilt on its side. When that tilt aims the Northern Hemisphere toward the sun, we experience more direct solar rays and the sun is up in the sky for more than 12 hours a day. It gets hot.
When the tilt aims the Northern Hemisphere away from the sun, we get very glancing solar rays, and the sun is up in the sky for fewer than 12 hours. It gets cold. The slight 3 percent variance in Earth's solar distance has almost no effect on our seasons — much to the chagrin of many Harvard graduates.
If you would like to see for yourself a brief video of the Harvard study, visit this website: http://www.youtube.com/watch?v=p0wk4qG2mIg.
Happy Fourth of July and happy aphelion.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. His “Celestial News” column appears weekly in the Steamboat Today newspaper and his “Cosmic Moment” radio spots can be heard on local radio station KFMU. Check out Jimmy's astrophotography website at http://www.jwestlake.com.