Causes Of The Seasons (1979)

[Music] Oh yeah. [Music] It's mid January, winter in parts of the Earth well north of the equator. Even at noon, the sun is fairly low in the sky, and there are only about 9 hours of sunlight each day. [Music] But on the same January day, in places well south of the equator, it's the middle of summer. At noon, the sun is high in the sky, and there are about 15 hours of sunlight. Now, obviously, the same sun is shining on the Earth both north and south of the equator. So, it's not a change in the sun that causes the difference in the seasons. The reasons for the changing seasons must have something to do with the earth. If we look at the way the earth moves, we can find a clue to the causes of the seasons. For one thing, the earth turns or rotates on an imaginary line called an axis. The north end of the earth's axis is called the north pole and the south end is called the south pole. Now our earth also revolves in a regular path or orbit around the sun. As we move from one position to another along this orbit, the axis of the earth is not straight up and down. It's tilted. And that tilt is part of the cause of the changing seasons. To measure the amount of the tilt, imagine a line that is straight up and down and compare it to the line of the real axis. The difference is an angle of 23 1/2°. At every position along this orbit, the north end of the tilted axis points toward the north star. Now, notice that in one of these four positions, the northern end of the axis is tipped toward the sun. In another position, the northern end of the axis is tipped away from the sun and the southern end of the axis is tipped toward the sun. In the other two positions, neither end of the tilted axis is tipped toward the sun. To see how the tilted axis helps to cause the seasons, we'll follow the Earth through its orbit for one full year and notice the tilt of its axis on the first day of each of the four seasons in the northern hemisphere. On the first day of spring, about March 21st, the Earth is about here in its orbit around the sun. In northern North America, the weather has begun to turn warmer after the cold of winter. On this first day of spring, we can look at the Earth from the point of view of the sun. You can see that the north end of the tilted axis is not tipped toward the sun, but the southern end of the axis is not tipped toward the sun either. Now suppose we look down on the earth from above the north pole. We can see that exactly half of the earth is in daylight and the other half in darkness. So on this first day of spring there are 12 hours of daylight at any given point on the rotating earth and then 12 hours of night as that part rotates through the dark side. Day and night are exactly equal in length everywhere on the Earth on the first day of spring. It's the day of the spring equinox. Now, notice the way the sun's rays strike the Earth on this first day of spring. First, look at the rays that fall on or near the equator. At noon on this day, a person standing on the equator would find that the sun's rays are coming from directly overhead. [Music] But on the same day, look at the way the sun's rays strike a part of the Earth well north of the equator. Because the Earth is curved, a person standing here at noon would find that the Sun's rays come at an angle and not from directly above. Places on Earth that get more direct rays of sunlight are brighter and also warmer than places where the sun's rays fall at an angle. To see why, look at this flat surface lighted by direct rays from a spotlight. If we tilt the surface back so the light rays fall at an angle, the light spreads out and it's not as bright or as warm as when the rays are more direct. [Music] And notice something else. Between the Earth and the Sun is the Earth's atmosphere. The air, water vapor, and dust in the atmosphere take up or absorb some of the light and heat of the sun's rays. Even direct rays lose some of their light and heat as they pass through the atmosphere. But slanted rays pass through more of the atmosphere, and so they lose even more light and heat. You can see how much farther the slanted rays travel in the atmosphere compared to the direct rays. Well, that helps to explain why the sun heats some places on Earth more than others. And we'll see that it also helps to explain the changing seasons. As the Earth continues in its orbit, it reaches this position on the first day of summer, about June 22nd. On this day, the Earth's axis is still tilted just as much as before, but now the northern end of the axis is tilted toward the sun. The first day of summer is the day of the summer solstice. At noon on this day, for a person in the northern hemisphere, the sun will appear to be higher in the sky than on any other day. This means that the sun's rays are more direct than at any other time and more direct rays give more heat. Now, if we look down on the Earth from above the North Pole on the same day, we'll see that the days have been growing longer and the nights growing shorter in the northern hemisphere. The first day of summer is the longest day of the year. In the northern United States and Canada, there are about 15 hours of daylight and about 9 hours of night. Naturally, with more hours of sunlight, we also get more heat from the sun. And so, this is the warmest time of the year, summer, in the northern hemisphere. After the summer solstice, as the Earth continues in its orbit, the northern end of the axis gradually begins to be tipped away from the sun again. [Music] On the first day of autumn, about September 22nd, neither end of the Earth's tilted axis is tipped toward the sun. And on the first day of autumn, just as on the first day of spring, there are 12 hours of daylight and 12 hours of night, exactly equal. This is the day of the autumnal equinox. Now, the northern hemisphere is receiving less heat from the sun than it did in summer because there are fewer hours of daylight and the sun's rays are not as direct. [Music] As the Earth continues in its orbit around the sun, the fact that its axis is always tilted at the same angle means that the northern end of the axis will gradually be tipped farther and farther away from the sun. On the first day of winter, about December 22nd, the Earth is at this point in its orbit. On this day, the north end of the axis is tilted farthest away from the Sun. This day is exactly the opposite of the first day of summer when the north end of the axis was tilted farthest toward the sun. The first day of winter is the day of the winter solstice. During this season of the year, parts of the Earth well north of the equator get the least amount of heat from the sun. One reason is that the sun's rays are very indirect in winter. At noon, the sun appears to be much lower in the sky than in June when the rays were more direct. Another reason is that there are fewer hours of daylight, so there's less time each day for the sun to heat this part of the world. If we look down from above the North Pole on the first day of winter and follow the Earth's rotation, we'll see that the northern United States and southern Canada have about 15 hours of night and only about 9 hours of daylight. But now, as the Earth continues in its orbit around the Sun, days in the northern hemisphere will gradually become longer and nights shorter. When the Earth reaches the point of the spring equinox about March 21st, day and night are once again equal in length. And once again, in the northern hemisphere, the weather is turning warmer. We've been following the seasons in lands well north of the equator. Meanwhile, in lands the same distance south of the equator, the seasons have been reversed. On about June 22nd, the northern hemisphere is tilted toward the sun and it's summer there, but the southern hemisphere is tilted away from the sun. And so it's winter here. And when the northern hemisphere tilts away from the sun in winter, the southern hemisphere tilts toward the sun. And here it's summer. But even though the seasons are reversed, the full cycle is the same. So the pattern of the changing seasons is caused by the tilt of the axis of the earth and the changing direction of the tilt in relation to the sun as the earth revolves in its orbit around the sun once each year. [Music]