The Seasons The topic of today’s misconception podcast is what causes the earth’s seasons. There are actually two misconceptions embedded in this one topic. The first misconception is that the seasons are caused by earth’s highly eccentric or highly elliptical orbit. The misconception that is embedded within that misconception is that the earth’s orbit is highly elliptical or highly eccentric. So let’s talk about that one first. The eccentricity of an ellipse is a measurement or comparison of the diameter and the short direction to the diameter in the long direction. The earth’s orbit is only 2% shorter in the short direction than it is in the long. The reason that students have this misconception is because of the side view often shown in textbooks which makes the earth’s orbit look like it is highly elliptical. Now you might be thinking 2% is highly elliptical, but if I went to your basketball court and I drew a scale model of the earth’s orbit on the court a group of people would walk out and look at it and say “wow, what a perfect circle!” It would be almost indistinguishable from a perfect circle. In fact I will upload a word document along with this podcast where there will be 6 or 7 circles on a piece of paper and then mixed in with those circles will be two ellipses that are compressed one direction or another by 2% and I challenge you to try to find the 2 that are compressed without using any measuring devices. It is very difficult to do. We did this before with a group of 30 people, and only a few were able to pick one of the compressed circles. Very few were able to find both of them. So the first misconception is that the earth’s orbit is highly eccentric or highly elliptical and it is not it’s very nearly a perfect circle. The other part of the misconception is that even though we now know that the earth’s orbit is very nearly a perfect circle students believe that the seasons are caused by that elliptical shape and when the earth is closer to the sun we have summertime and when the earth is farther from the sun we get wintertime. This isn’t logical because we know that in the northern hemisphere when it’s summer in the southern hemisphere it’s winter, and the northern hemisphere can’t be closer to the sun at the same time the southern hemisphere is farther from the sun. That’s not physically possible considering they are both part of the same celestial body. What really causes the seasons is the tilt of the earth. At one part of the year one part of the earth’s orbit around the sun the top half of the earth is tilted towards the sun and at htat time of year it’s summer in the northern hemisphere where we’re getting direct sunlight from the sun. The sun is hitting almost perpendicularly in the northern hemisphere and it is hitting at an angle in the southern hemisphere causing that sunlight to be spread over a large area and causing wintertime in the southern hemisphere. Then once the earth has gone 180 degrees around its orbit now the bottom of the earth is facing towards the sun and we get summer in the southern hemisphere and winter in the northern hemisphere. It turns out that even if you consider the small amount of eccentricity in the earth’s orbit in the northern hemisphere in the summer we’re actually farther from the sun than in the winter and in the southern hemisphere then it’s the reverse they are closer to the sun slightly in the summer and they’re farther from the sun slightly in the winter. So we covered 2 misconceptions today what causes the seasons which is the tilt of the earth as it orbits the sun and also that the earth’s orbit is highly elliptical which we now know that it’s not.