 Greetings and welcome to the Introduction to Astronomy. In this week's Misconception in Astronomy, we are going to discuss the cause of the seasons and how that is often considered incorrectly. Well, some think that the seasonal changes are caused by the changing distance between Earth and Sun. And you can consider that this makes logical sense. If you are close to the Sun, it should be warmer. And this does make some kind of sense, because if you are closer to the Sun, you would expect it to be warmer. And if you are farther from the Sun, you would expect it to be colder. And we see here an elliptical orbit, which is what the Sun has. However, this is greatly exaggerated. The Earth's orbit is almost a perfect circle. And there is a very slight change, but it's only a few percent. And there would be no significant seasons if the cause were simply due to this changing distance. Now, as we look at these, we can see here what is the difference in the distance? Well, we have perihelion when we are closest to the Sun, which occurs in January. And there we are about a little over 91 million miles from the Sun. At aphelion, when we are the farthest, around July 5, then we are about 94 million miles from the Sun. So it's about a 3 million mile difference, but 3 million miles out of nearly 100 million is really a very, very small change. Now, you also may notice one thing that would be wrong here is that we are farthest from the Sun in July and closest in January. For those in the northern hemisphere, these are completely opposite to when we get the seasons. The other thing that we can do is if we think about this through the scientific method, it would make a prediction. If the changing seasons are caused by the distance of the Earth and Sun between those two, then the seasons would be the same everywhere, because we would be closest to the Sun, and everywhere would have summer at the same time, and then six months or so later, we would have winter all at the same time. But we know that seasons are opposite in the northern and southern hemispheres. So, when we look at what actually happens here, the seasons are caused by the tilt of Earth's axis. So in June, which would be summer in the northern hemisphere and winter in the southern, the northern hemisphere is tilted toward the Sun and the southern hemisphere tilted away. It makes a couple of different changes, and in fact it changes how directly the sunlight strikes, and it changes how long the Sun is in the sky. So let's take a look at another sketch here, and we can see that when the Sun is here, now we're looking at winter in the northern hemisphere and summer in the southern, that the southern hemisphere is directly getting its light on it. So the sun is in the northern hemisphere, it's very spread out. Now we also know from experience that during the summer the Sun rises early and sets late, and during winter it sets late, sorry, it rises late and sets early, so you get a much shorter amount of time that our Sun is actually in the sky. So the combination of these two things is what is causing our seasons. How long the Sun is in the sky, exposing us to its warmth, and how directly the Sun's rays are striking us. You'll note on a summer day at noon the Sun is very high in the sky. Depending on your location it may be straight overhead or it may be close to overhead. But you'll tend to see it very high in the sky. In winter at the same time of day you will see it very low over the horizon. So those combination of those two things are what are causing our seasons and it has really nothing to do with the tilt of, sorry, nothing to do with the changing distance between Earth and Sun. So what we've looked at is that seasons are not caused by the changing distance between Earth and Sun. They are caused by the tilt of the Earth's axis. Were Earth not tilted there would be no seasons. And this causes changes in how directly sunlight strikes the surface and in how long the Sun is in the sky each day. So that concludes this misconception on the cause of the seasons. We'll be back again next time for another misconception in astronomy. So until then, have a great day everyone and I will see you in class.