In this video, I'm going to get you prepared for the seasons lecture tutorial. The causes of the seasons on the earth is one of the most most misunderstood concepts in astronomy. And so I'm hoping to give you a brief overview of seasons and the lecture tutorial will help you uh process all of it. One of the most misunderstood ideas concerning the seasons has to do with the Earth's orbit around the Sun. Many people know that the Earth's orbit is elliptical, but it's not that elliptical. We the Earth doesn't get so close to the Sun and so far away for that distance to make much difference on the Earth. And so a lot of figures in books uh that you'll find for the Earth's orbit make it look like the Earth gets very close to the sun and then very far away when in fact Earth's orbit is very near to being a circular path. The image that I'm showing you right now comes from NASA and this gives you the orbits of the inner pl inner planets, the asteroid belt and Jupiter. And you can see here that the orbits for nearly all of these planets is very circular. The only ones that deviate uh very much from that would be Mercury and Mars. But if we look at Earth, which is the green circle here, uh I'm sorry, the green orbit, it's very close to being a circle. And so the Earth doesn't get that uh much farther away than it is when it is closer to the sun. And so the idea that distance has something to do with seasons is uh it doesn't hold a it it doesn't hold up very well under this uh line under this line of evidence. Let me show you a uh table that gives the uh variation in distance over the course of the year. In December, the Earth is 91.5 million miles from the Sun. That is, in fact, the closest that we get to the sun during the course of the year. In June, we're 94.4 million miles away from the sun. And that is in fact the furthest that we get from the sun during the course of the year. In September and March, we're about uh 93 million miles from the sun. On average, the Earth is about 93 million miles from the Sun. A lot of people think that uh and they misunderstand uh this idea that distance doesn't affect the uh uh seasons on the Earth. And so we're not having summer when we're closer to the sun and we're not having winter when we're further away. In fact, for people living in the northern hemisphere on the earth, we are actually closer to the sun during the beginning of winter on the earth. And if you know anybody in one of the other hemispheres on the earth or if you've ever traveled there over the course of a year, you'd know that when it's summer in the northern hemisphere, it's winter in the southern hemisphere. And so the two hemispheres experience opposite seasons uh at the same time. This would not be the case if the change if a changing distance of the sun throughout the year was the cause of the seasons. So what are the causes? Well, there are a few. One of them has to do with the number of daylight hours over the course of the year. And this is determined by where on the earth you are at. Remember, I am focusing on people who live in the continental US in the northern hemisphere. So, for those of us living in New York, uh, at 41 degrees north, I'm showing you a graph that comes from the University of Nebraska simulations. And, uh, this graph has the number of daylight hours versus day of the year. Notice that um on average it's about 12 hours uh uh 12 hours of daylight, but you get the most hours of daylight right at the end of June, beginning of July. And that coincides with the summer solstice. And you get the least number of hours of daylight right at the end of Janu uh end of December, beginning of January. And that coincides with the winter solstice. On the equinox, autumal or vernal, that would be the uh the uh fall or spring equinox, you get 12 hours of daylight. And so this variation uh goes up and goes down for somebody living in the northern hemisphere. You'd get something very similar if I showed you a graph for somebody living at 41 degrees south in the southern hemisphere, except the uh max and min would be reversed on those dates of the year. Let me show you what that looks like. So, here we are. We have it set for 41 degrees north. And let me put the day of the year to roughly today, beginning of uh February. And so, we're we're only at about 10 hours of daylight. But as we head on towards the uh spring and towards the summer, we'll get uh greater hours of daylight as we uh uh as we move each successive day uh forward. And so when I move to say March, let's look at the uh uh uh spring equinox, March 21st, thereabouts, we get about 12 hours of daylight. But as we get closer to summer, we're getting greater hours of daylight. And then we can we'll move back down finally somewhere around the winter. Here I have it marked for December 25th, Christmas Day, only about 9 hours of daylight. These number of hours of daylight correlate to the temperature that you experience at your location on the Earth. On average, there are higher temperatures in the summer and lower temperatures in the winter for those of us in the mid latitudes on the Earth. What about the equator? If you lived at the equator, you'd get pretty much 11 I'm sorry, you'd get pretty much 12 hours of daylight throughout the year. It is fairly even. There's not a lot of variation. And if you've ever lived in the tropics or on the equator or have visited there at different times of the year, you'd know that the temperature doesn't show a lot of variation throughout the year. There's not really a change of four seasons living in those areas. At best, you might get a rainy season and a dry season. And for somebody living in the mid latitudes in the southern hemisphere, the uh maximum and minimum number of hours of daylight are just reversed compared to what we had in the northern hemisphere. The other factor that uh works into the cause of the seasons has to do with the uh directness of the sunlight that we receive at our location on the earth and that is controlled by the axis tilt of the earth. The axis of the earth is tilted about 23 and a half degrees offset from the uh plane of the earth's orbit around the sun. that tilt points pretty much at the same place in the sky throughout the year, at least for human lifetimes. And so that axis tilt remains constant, but as the earth goes around the sun, sometimes of the year, the northern part of the earth will be tilted towards the sun and the southern part will be tilted away. And uh 6 months later, it will be just the opposite. The southern part will be tilted towards the sun and the northern part will be tilted away. The diagram that I have here comes from the seasons lecture tutorial. The uh earth position in its orbit around the sun is shown with the northern hemisphere of the earth pointed towards the sun. This would be northern hemisphere summer. And that's because when the uh uh when this part of the earth is tilted towards the sun, it's going it's going to get more direct sunlight. When it is tilted away 6 months later, it will get less direct sunlight. In a moment, I'll show you what I mean by more direct and less direct. On the equinoxes, on the uh fall or the spring equinox, neither hemisphere is tilted towards or away from the sun, and you would get equal amounts of daylight everywhere on the earth on those two days. And you would get uh uh a fairly uh constant amount of uh solar uh uh solar radiation, a fairly constant amount of the directness of that light on those days. I'm going to switch over to a season simulator from the University of Nebraska and show you both the Earth's orbit, which would be which will be on the left side of the screen. I'll show you uh the uh what the rays of light are like as they are coming towards the earth and then on the surface of the earth we'll look at uh the angle of the rays of those uh of the sunlight uh on the surface and uh the greater the angle uh closer to 90° that would be very direct and the more shallow angles are going to be uh less direct sunlight. Right. So, here we've got the orbit of the Earth around the Sun. And keep in mind that this is not the scale. The Earth and the Sun are not similar in size, but we're just trying to fit it all onto this one screen. I'm looking at it in a kind of perspective view here, but I can angle this so that we're looking at it top down. So, there's the sun, there's the Earth, we have the daytime side of the Earth and the nighttime side. In the top right screen, we've got a person standing at the Earth. And I'm going to move them up to 41 degrees north. And then we have the surface of the Earth. So what I'm going to do is start the animation so that we have the Earth orbiting the Sun. And as that occurs, we go to different times of the year. Right now, we've just moved past the spring equinox and we're moving towards the summer solstice. And what happens here is that the angle of the sunlight becomes more direct. So on June 21st just about we'll get the uh greatest amount of direct sunlight that we will get all year and that is the summer solstice, the most number of hours of daylight and the most direct light. So by direct light I mean light that is coming with a very direct angle from the sun. This light is more concentrated in a smaller area and so it's more potent in terms of the amount of solar radiation and heating ability that that light has. And so at the beginning of summer, you've got increased temperatures and you're able to sustain that going into the fall. I'll start the animation again and we'll head towards the fall equinox at our location on the earth 41° the uh angle and directness of that light is going to be decreased till finally we get to the equinox and we head towards the winter solstice. In the winter solstice, we'll get the least amount of direct light and we'll also have the uh lowest number of daylight hours. And so when we get to December 21st, I'll stop it. And notice how on this day, the angle of that sunlight is very shallow. That light is not concentrated. It is spread out over a big area on the surface of the Earth. And so it is not as potent in terms of uh its heating ability from the solar radiation. And so if you lived in the northern hemisphere on the earth, the sun's light is coming at you very indirectly. It's spread out over a bigger area on the earth and you're going to have lower temperatures as a consequence. You'll also have fewer hours of daylight with which to heat the surface. And so that contributes to the lower temperatures that you have in the winter. And so those are the two main factors that contribute to seasons on the earth. The uh directness of the light and the number of hours of daylight. Those two things correlate together throughout the course of the year for a person in the mid latitudes on the earth. If you lived at the equator, remember that the uh uh angle of that light doesn't change very much. We can even take a look at what that looks like. So, what I'm going to do is move the person to the equator and we'll watch what occurs over the course of the year. Look at the angle of the sunlight on the surface. It can get very direct. On the equinoxes, you get uh completely direct light. But over the course of the rest of the year, the shallowest angle is not very shallow compared to everywhere else on the Earth. And so living at the equator is a special location. It's one of those places where uh you don't get a great variation in the directness of the uh sunlight throughout the year. And so you don't experience a great deal of temperature change throughout the year at the equator. It's only when you're in the mid latitudes on the earth that you'll experience uh a great deal of