Just the Right Seasoning

© 2000 by Robert C. Moler

One of the misconceptions that most folks share about the world around them is the reason we have seasons. This month we start the season of summer with the summer solstice on June 21st. I shall tackle this challenge by using new and different diagrams and charts. Maybe this will give a different slant on the subject.

The common misconception is that the earth is closer to the sun in the summer than during the winter. The diagram left was created by a computer fed with the mathematical description of the earth's orbit by plotting the earth every 5 days. (Note at the 4 o'clock position the overlap caused by the quarter day difference in the actual length of the year.) The points at the quarters are VE-vernal equinox, SS-summer solstice, AE-autumnal equinox, and WS-winter solstice. Noted too are perihelion, the earth's closest point to the sun, and aphelion, the earth's farthest point from the sun. The diagram, though accurate for distances, shows the earth and sun as being way too large. The sun would be a tiny dot, and the earth an invisible point.

The earth's orbit seems quite circular. It isn't exactly. Out of the earth's mean distance of 92.9 million miles from the sun, the earth gets as close as 91.3 million miles to the sun around January 2nd and as far as 94.5 million miles around July 4th. So the earth is actually closer to the sun in winter than in summer. But I've left out a small detail, when we in the northern hemisphere have summer, those in the southern hemisphere have winter.

Since the earth has opposite seasons at the same time, the earth's distance from the sun isn't the reason for the seasons. The immediate cause for the seasons is the tilt of the earth's axis. It is 23.5 degrees from the perpendicular to the earth's orbit. The axis tends to point in the same direction, but as the earth orbits the sun, the north pole and then the south pole are alternately tilted toward the sun bringing summer to that hemisphere. At the midpoints between these extremes are the equinoxes where the sun appears above the equator, and the entire earth receives 12 hours of sunlight and 12 hours of night.

When a hemisphere is tilted towards the sun the sun gets higher in the sky at noon plus it also stays out longer. When the sun is high in the sky it's rays don't spread out as much as when the sun is low. It's heat is more concentrated. To the right is a diagram showing the intensity and duration of sunlight on the winter solstice, December 21st, an equinox, March 20th or September 23rd, and the summer solstice, June 21st for the latitude of Traverse City. In the vertical scale of the graph an intensity of 1 equals the energy delivered to the earth, when directly overhead. The area under the curve represents the total energy applied to the surface of the earth at our latitude. On an equinox Traverse City receives 77% of the total energy of the summer solstice, while on the winter solstice the total energy drops to only 30% of the summer solstice value.

The actual effect of the earth being closer in the northern hemisphere winter increases the sun's heat by 3%, that's 103% of 30%. It also diminishes the summer intensity by about the same value. The true effect of the earth's elliptical; orbit is to change the number of days in each seasons. The winter (in the northern hemisphere) perihelion makes winter the shortest season at 88.99 days, summer's aphelion makes it the longest season at 93.65 days. By the way, spring has 92.76 days and autumn has 89.84 days. The reason for the differences is that at perihelion, earth's closest point to the sun, the earth moves the fastest, and zips through that season quicker, than when the sun is farther from the sun.

So despite everything its still the tilt that causes the seasons.

Questions? Comments? Send Email to me at bob@bjmoler.org

Uploaded: 05/28/00