 Greetings and welcome to the Introduction to Astronomy. In this lesson we are going to talk about the seasons and what causes seasons here on the Earth and how might that apply elsewhere in our solar system. So the seasons, of course, we know those, winter, spring, summer and fall, are associated with changes in the weather, but they are also very astronomical events and are strongly related to various properties of the Earth that we will discuss. So let's look at a summary here. First of all, what is the cause of the seasons? Well there is an underlying cause, and that is the tilt of the Earth's axis. So it's the fact that the Earth is tilted, not going straight up and down like this, having a north pole and a south pole, but being tilted instead at an angle and having a north pole and a south pole. And what that means is that there are times when the north pole is leaning towards the sun and times when the north pole is leaning away from the sun. And that is what gives us the seasons, and that is what I call the underlying cause of the seasons, the 23.5 degree tilt of the Earth's axis. If this were not tilted, then we would have no seasons. So if the tilt were 0 degrees, it wouldn't matter the seasons would be the same all year round. If it were tilted significantly more, the seasons would get more and more extreme. And we see this elsewhere in the solar system, a planet that goes almost straight up and down would have no seasons, and something that's tilted almost on its side, like Uranus for example, has very extreme seasons because of that tilt. Now we know it's the tilt, but what about the changing distance? Often one of the misconceptions that people have is that the seasons are caused by the changing distance between the Earth and the sun. Well we know that the Earth does have an elliptical orbit, and that means that it is sometimes closer to the sun and sometimes further away. It turns out this is about a 3% variation, much less than what we would need to explain the seasons. So while we are closer and further away from the sun, it is not the reason that causes the seasons to occur. Now if we want to think about this scientifically, we can think about the scientific method. And if we go to the scientific method and say, well the reason that we have different seasons here on the Earth is that the Earth is closer or further from the sun. Well that makes a prediction, and that does make it a good scientific theory in that it makes a prediction, and the prediction is that when we are closer to the sun we would have summer, and further away we would have winter. It wouldn't matter where you are on the Earth so that seasons would be the same in the Northern and Southern Hemisphere. So when it is summer here in the Northern Hemisphere, it would also be summer in the Southern Hemisphere. Now what we know is that this is not correct, the seasons are opposite. When it is summer in the North it is winter in the South and the other way around. So winter in the Northern Hemisphere means that it is summer time in the Southern Hemisphere. So this prediction that it would make if the Earth's changing distance were the reason that the seasons change is incorrect because it does make that prediction. And in actuality what we find out is that in January we are closest to the sun and in July we are furthest from the sun. So if you live in the Northern Hemisphere that would definitely not explain the seasons because we are further away in July when it's hotter and we are closer to the sun in January when it is colder. So this could explain the seasons in the Southern Hemisphere perhaps but would not explain the seasons in the Northern Hemisphere. So in reality we know that the seasons are caused by the tilt of the Earth's axis. Now let's look at that here and let's look at a little image here. So here is the Earth's axis that is tilted. It is causing two things to change. So what is really changing? The Earth's axis isn't changing but what happens as we get, as the Earth moves around the sun is that the directness of the sunlight changes. It gets hit more or less directly. And the length of the daylight changes. Not the length of the day, that's always 24 hours, but the length of daylight, the amount of daylight that we get does change over the course of a year. And you know that in a nice hot summer day the sun rises very early and sets very late and is in the sky for more than 12 hours giving it extra heating time. In the winter the sun rises very late and sets very early giving it far less time to heat up the Earth. So these are the two things that are actually changing. It is how direct the sunlight is hitting us. So here we look at this, this would be an example of Northern Hemisphere summer. The North Pole is tilted towards the direction of the sun and that means that the sun's rays are hitting very directly in Northern latitudes. In Southern latitudes the rays are much more spread out so that the same amount of energy coming in is spread over a wider area and doesn't heat things up near as much. So the length of the daylight changes and sorry the directness of sunlight changes but also the length of the daylight changes. Up here in the North you can see these, if you look at the lines going across here, here your north of the equator and more of it is in sunlight. There's a longer time that that portion of the Earth will spend in sunlight. In fact if you get far enough North the entire circle will be in sunlight and the sun would never set. So that's what we define as the Arctic Circle and the Arctic Circle is actually the furthest southerly portion where the sun will never set at least on one day of the year. So if you are at the Arctic Circle or further North there will be days where the sun never sets. It remains above the horizon all day long and all night long. If you go down into the tropics, we have the tropic of Capricorn and the tropic of Cancer which are labeled here, this is the tropic of Cancer in the Northern Hemisphere, tropic of Capricorn in the Southern Hemisphere. Those are the points at which the sun would be directly overhead one day a year. If you're further North than that then the sun would never be directly overhead. If you are further South of that somewhere in this range between the tropical regions then the sun would be overhead twice a year at the tropics one day a year and in between that and down to the equator twice a year the sun would be directly overhead. Now if you're actually at the equator itself the length of the daylight now no longer matters. If you look at the equator here even in summertime the equator is always half light here and half dark here. So the length of the day on the equator is always 12 hours. So let's look at a couple of these examples here and review some of this. What we have is that during summer what do we know about it? The sun is more is directly overhead or closer to directly overhead it is in the sky for more than 12 hours leaving it more time to heat up. If you're at the Arctic Circle or further North the sun will be up for 24 hours during some times of the year. If you're near the tropics the sun will be at the zenith exactly one day of the year and that would be the day of the solstice. That would be the day at the tropic of cancer the summer solstice the sun would be directly overhead and at the winter solstice the tropic of Capricorn the sun would be directly overhead. If you're in between those then it will be overhead two days a year but it will always be near the zenith. So why is the equator hotter? Because no matter what you're always have the sun relatively close to being straight overhead beating straight down on you with those direct rays and up for a good amount of time up for 12 hours never having short days down at the equator. Now look at one thing here is a discussion on the solstice which is June 21st the summer solstice in the northern hemisphere the sun is highest in the sky but why is this not the hottest day of the year usually it's hotter in August and why does that occur well it takes time to warm the earth so the earth has just been in a freeze for the northern hemisphere during winter and it takes time to warm it up so it takes lots of those long days with the sun up high in the sky to warm up the earth. The similar thing happens in the winter time December 21st would be when the sun is lowest in the sky and is the shortest daylight of the year but it's not the coldest day because the earth has been warm from summer and it takes time to cool it off. So let's review here what we've covered in this lesson and that would be just to summarize a little bit here that we had a couple of things. The underlying cause of the seasons is this 23.5 degree tilt of the earth's axis it has nothing to do with the changing distance between the earth and the sun. Now that could be different for an object that has a very elliptical orbit something that has a very highly elliptical orbit perhaps a planet around another star or some objects in the solar system that have very elliptical orbits not the planets but other objects could have the distance being a factor but for the planets it is the tilt that really makes the difference because the orbits are so close to being circular. What does this tilt do? It changes two things it changes the directness of the sunlight how directly it hits different parts of the earth and it changes how long the sun is in the sky the longer it's there to warm things up and the more directly it's striking the earth the warmer it's going to get. The lower the sun is in the sky and the shorter the length of the daylight the less warm it is going to get and the colder it will be. So that concludes our discussion here on the seasons here on the earth. We'll be back again next time for the next lecture so until then have a great day everyone and I will see you in class.