Seasons

One important concept about the year is the idea of seasons. In this lecture, we're going to learn exactly why the Earth has seasons, and also learn about some important dates connected to the seasons.

The seasons are probably one of the most misunderstood concepts in astronomy. If you ask people on the street, "Why do we have seasons?" what do you think the most common answer is? Well, overwhelmingly, the most common answer people get for why we have seasons is that in summertime, the Earth is closer to the Sun, and in wintertime, the Earth is farther from the Sun. And at first, this makes sense. Distance, you know, you think about a campfire. The closer you get to the campfire, the warmer you feel.

It turns out, though, that this is 100 percent not the reason why we have seasons. How can we know this for sure? Well, for one thing, yes, the Earth does get a little closer to the Sun at some times in the year than others. But you know what? The day of the year that the Sun, the Earth gets the closest to the Sun is actually in early January, and it's farthest from the Sun in early July. So, for those who would say, "Oh, well, it's summertime because we're close to the Sun," well, that doesn't work. Because when it's summer here in the Northern Hemisphere, June, July, we're actually farther from the Sun.

So, what is the real reason? What is the real reason why we have seasons on the Earth? Well, it has to do with the fact that the Earth's rotation axis is tilted by about twenty-three and a half degrees. What does that mean? That means the Earth doesn't stand up straight like this and spin around. Instead, it's tilted twenty-three and a half degrees, and then it spins around like this. You may have noticed that tilt if you look at any globe, um, Earth globe. You might see, you know, it's often tilted to the side. That's to mimic this twenty-three and a half degree tilt.

So, how does that make a difference? How does that give us seasons? Well, because of the tilt of the Earth, sometimes the Sun hits the Earth more directly than others. So, check out this picture here. The Sun is over here to the right. Right now, the light is coming in, and it's hitting this part of the Earth pretty much full force. You're getting the full beam, falling directly on this part of the Sun... this part of the Earth. The Sun is high in the sky. You're getting very direct sunlight. This is what you call summertime. But you notice down here, the sunlight is still coming in, but this part of the Earth is now tilted away from the Sun, and the light is spread out over a much larger area. The Sun is low in the sky. You get less direct sunlight. We have wintertime.

So, here's an important fact: When we have summer up here, say in the Northern Hemisphere, the season is exactly opposite down there in the winter in the Southern Hemisphere. They're having winter. Seasons are opposite from north to south. You might have known this already. Maybe it's the first time you're hearing it, but pay attention to this now. When you think about, when you read news stories, for instance, it's January. You might be reading a news story about how hot it is, the heat waves going through Australia. Why is that? Well, when we were in January, in the middle of winter here in the North, they're going through the middle of their summertime.

So, let's zoom out a little bit here. We are sort of a zoomed-out version of the previous picture. The Northern Hemisphere here's tilted toward the Sun. We're getting direct sunlight. We're getting summertime. But notice something: As the Earth goes around the Sun, the tilt stays the same direction. So, six months later, when the Earth is on the other side of the Sun, like we're here, you see the Earth. The now the Northern Hemisphere is tilted away from the Sun. What does that mean? It means that we have wintertime here, and the Southern Hemisphere, we have summertime.

What about spring and fall? Well, spring and fall occur when the Sun and the Earth is not totally either towards or away from the Sun, but sort of side to side. And now light is hitting all the parts of the Earth pretty equally. We have average weather, not too hot, not too cold. Fall and spring. Now, think about it for a second. What do you think? What would happen if the Earth didn't move? What if the Earth didn't move around the Sun? We already saw in a previous lecture that if the Earth didn't move around the Sun, stars would rise the same time every day. That's one thing. But also, if the Earth didn't move around the Sun, if it always stayed like here, for instance, would we have seasons? No, we wouldn't. We'd always have the exact same weather year-round because the same amount of sunlight would be hitting a given part of the Earth every single day of the year. It's only the Earth going around the Sun and the tilt of the Earth causing different amounts of sunlight to hit the Earth at different times during the year that's what causes seasons.

As the Earth goes around the Sun, because of that tilt, the location of the Sun in the sky changes a lot. In the summertime, the Sun can be quite high in the sky. In wintertime, the Sun is quite much, much lower in the sky. We might not notice this much in our daily life because that how much it changes from one day to the next is very small. When you compare where the Sun is in the sky in, say, the beginning of summer to where it is in the sky the first day of winter, it's actually about 47 degrees difference in altitude. That's about halfway up the sky. Check out this picture here. Over here on this side, you see where the Sun is at its highest point at transit around the winter solstice, the beginning of the wintertime. This is, this is for somewhere, give or take, around England. So, a little bit farther north than us. Here's where the sky is, where the Sun is in the sky at the first day of summer. Notice the huge difference here.

Have one question for you. Let's imagine that the Earth had no seasons. Sorry, let's imagine the Earth had no tilt. If the Earth had no tilt, would we have seasons? Well, the answer is no. If the Earth had no tilt, the same sunlight would be hitting the Earth every single day of the year as it went around the Sun. We'd have the same weather year-round. There would be no seasons. The tilt is what gives us the seasons. The bigger the tilt is, the more extreme the seasons are, say, from summertime to wintertime.

Well, let's say we had no tilt. If we had no tilt, what would the average weather be like? Would it be like summertime year-round for us in Los Angeles? Would it be wintertime year-round? Would it be fall or spring year-round? What season, what weather would we have if we had no tilt? Think about it for a second, and then we'll give you the answer.

Alright, which may have come across is if there is no tilt, that means the sunlight is hitting every part of the Earth pretty much the same. And that's sort of what's going on here at the beginning of spring and fall, when the Earth is not tilted towards the Sun, like there's not tilted away from the Sun light here, but is sort of tilted to the side. We have fall, we have spring. There's really no difference in this and there being no tilt. So, if there was no tilt, we would say that the weather will be very spring-like or fall-like year-round. Here in Los Angeles, definitely not a bad weather. I can live with that.

Alright, there's some important days and we need to think about days in terms that are related to the seasons. The first one here is called the summer solstice. Summer solstice is June 21st. It is the start of summer for us in the Northern Hemisphere. This also marks the day when the Sun is highest in the sky for us in the North. So, for instance, you know, on June 21st, the summer solstice, that is the highest the Sun is ever going to get here in Los Angeles. It turns out it never gets directly overhead. The Sun never goes to Zenith. We're not close enough to the equator. We're not in what's called the tropics. But the highest the Sun does get is about 80 degrees altitude. It occurs on June 21st. It's also the longest day of the year. You know this already from daily life. In the summertime, the days are longer. In the wintertime, the days are shorter. Turns out for us in the summertime, we have about 14 hours of daylight. Interesting thing about this is as you go farther north, as you go closer and closer to the North Pole, the amount of daylight you get on the summer solstice goes up dramatically. So, for instance, if you're in England, you might have 20 hours of daylight on the summer solstice. I was in northern Scotland, which is pretty far north, and I was up in northern Scotland on the summer solstice. The sun set right about 11 p.m., and the Sun rose at 1 a.m. There was two hours when the Sun wasn't in the sky on the summer solstice. Incredibly long days.

Now, the winter solstice in exactly the opposite. Because winter solstice six months later, December 21st, the start of winter. It is the shortest day of the year in the North. In Los Angeles, it's maybe ten hours. So, 14 hours the summer solstice, 10 hours in the winter solstice. Winter solstice, it's also the day when the Sun is lowest in the sky at a given place in the Northern Hemisphere. You can imagine as you go farther north on the winter solstice, the shortest day of the year becomes a big thing again. We might have 20 hours of daylight on the summer solstice, say in England, but on the winter solstice, you might only have four hours of daylight. I was in England on the winter solstice one time, and the Sun rose about 10 a.m., and by 2 p.m., the time the Sun had gone down. Four hours of daylight. As you went further north in the wintertime. So, in LA, you know, it doesn't change that much from summer to winter, but the further north you go, the more extreme it gets. Really long summers, and really long winters. It's hard for a body to adjust to that.

See, two final days down here, the equinoxes. Vernal equinox and autumnal. There's two equinoxes. The vernal one is right around March 21st. That is the start of spring. The autumnal equinox is right on September 21st, the start of autumn or fall. These are special because there are 12 hours of daylight everywhere on the Earth on the equinoxes. So, for near the vernal equinox, near the autumnal equinox, it doesn't matter whether you're at the equator, whether you're at the North Pole, whether you're in England or Los Angeles. There's gonna be 12 hours of daylight on the equinoxes.

At first, it may seem like these are a lot of dates to remember. But maybe a few hints with this. First of all, even though I have the 21st listed for all these, it does vary by a few days depending on where we are in the leap year cycle. The equinoxes or solstices could be on the 20th, they could be as late as the 23rd. Also, notice how these are all three months apart. The summer solstice will start there, June 21st. Three months later, September 21st, the autumnal equinox. Three months after that, December 21st, the winter solstice. Three months after that, March 21st, the vernal equinox. All spaced about three months apart from each other.

One other thing that's interesting is that where the Sun rises in the sky changes during the year. In the summertime, the Sun rises more toward the Northeast and sets more toward the Northwest. In the wintertime, the Sun rises more towards the Southeast and sets more towards the Southwest. And on the equinoxes, both in March and September, the vernal and autumnal equinoxes, the Sun rises exactly East, due East, we say, and sets exactly West, due West. The only time a year that does that.

This idea, the sunrise and setting in different places, this is where we get things like Stonehenge, calendars that mark different dates by the rising of the Sun. Don't believe me about the sun rising and setting in different places? Drive east or west on the 10 freeway, either near sunrise or sunset, near the equinoxes. You hit it on a traffic why? Because the Sun is the rising or the Sun is setting right in your eyes, 'cause the 10 freeway is exactly east-west. Do the same thing driving the 10 freeway during sunset, you know, in June or in December. You're not going to have traffic problems. The Sun is gonna be setting, but it's not straight in your eyes like it is during the equinoxes. So, equinoxes is sun rises/sets that exactly East, exactly West.

One last thing is the hottest time of the year, normally June 21st? No, it's normally in July or August. Same thing, coolest time in a year, not the winter solstice. It's normally in February, late January. Why the difference? It's called the lag of seasons. The weather is about six months, six weeks behind what's predicted by the seasons. Why is that? It takes time for our atmosphere, for the water on the Earth to heat up and cool down. Because of that, we don't match up perfectly with the seasons. But if we had no atmosphere, no water, then the hottest day of the year would be June 21st, the coldest day, December 21st.