Friday, December 21, 2007

Axial Tilt: The Reason for the Season

[Added 2012: this picture does not belong to me, but the source website went defunct at some point]
This picture has been going around the blogosphere. Forget discussions about how this fits into the "War on Christmas"--instead, I'm going to use this opportunity to explain the basic science of seasons.

The Earth moves around the Sun in a roughly circular orbit. The plane that includes Earth's orbit is called the ecliptic plane. While the earth is orbiting around the sun, it is also spinning around itself. When it is spinning, exactly two points on the surface, known as the north and south poles, do not move. The imaginary line going through the two poles is called the axis of rotation.

Seasons are caused by the fact that the axis of rotation is not perpendicular to the ecliptic plane. If they were perpendicular, the Earth's spin and orbit would go in the exact same directions. Instead, they are off by 23.5 degrees. The axis of rotation is tilted towards the sun during summer and away from the sun during winter. Why is the Earth apparently tilted in different directions at different times of year? It is in fact tilted in the same direction all year, it's just that the direction towards the sun changes throughout the year.

[Image from]

The day when the axis of rotation is tilted furthest away from the sun is called the winter solstice. This year, the winter solstice is December 22. The day when the axis is tilted closest towards the sun is called the summer solstice, which is somewhere around June 21 or 22.

[Image from Penn State University - I'm not sure what's up with the smiley faces.]

The reason axial tilt causes seasons is twofold.

First, the power from the sun is not evenly distributed on the Earth's surface. If you look at the above picture, there is about an equal amount of power between each of the yellow lines. However, towards the top and bottom of the Earth, that same amount of power is spread over a larger area of Earth. Therefore, the power per unit area is smaller closer to the poles, and larger at the equator. The power peaks at the Tropic of Capricorn during the winter solstice and at the Tropic of Cancer during the summer solstice.

Second, the length of the day grows shorter in winter, and longer in summer. In the above picture, you might notice that most of the tropic of cancer is in the dark. This indicates a shorter day. Also note that the entire arctic circle is completely in the dark. This indicates that it is perpetually nighttime. During the summer solstice, the arctic circle is in perpetual daylight. If you stand at the north pole, there is exactly one night and one day every year.

Both the length of the day and amount of power from the sun affect the temperature and climate. Around December, the northern hemisphere has shorter days and less power from the sun. However, the southern hemisphere has longer days, and more power from the sun. The southern hemisphere has summer at the same time that we in the northern hemisphere have winter. And vice versa. The southern hemisphere, I imagine, doesn't get many white Christmases.

Now, some people mistakenly think that the reason for the season is the distance from the sun. The Earth's orbit is not exactly circular, meaning that there are times of year when the Earth is closer to the sun, and times when it is further from the sun. This does have a very small effect on the climate, but it is not nearly as large as the effect of axial tilt. In fact, the day when the Earth is furthest from the sun (called the aphelion) is around July 7--in the middle of the northern hemisphere's summer. The Earth is closest to the sun (this point is called the perihelion) around January 3.

So there you have it--third grade science, as explained by me. Next time, I'll talk about more advanced stuff like precession or something.


Anonymous said...

Something that they never explained to me in third grade: If you are at the equator, the sun passes directly overhead twice in a year, and also proceeds to a point of most indirect radiation twice in a year. Does that mean that at the equator there are two summers and two winters in one year?

miller said...

It is true that the sun only passes overhead twice a year. Theoretically, there should be a small decrease in power at the solstices. However, daytime remains twelve hours long all year. This apparently doesn't have a significant effect on climate, since it does not actually cause seasons.

I looked it up, and it seems many tropical regions distinguish a "wet" and "dry" season. These seasons are related to monsoon cycles. There was also a study that shows that historically, seasons near the equator cycled in one anomalystic year. That is, they are correlated with the perihelion and aphelion.

Anonymous said...

Every explanation I have seen treats the axial tilt as a given, but what causes the tilt to begin with?

miller said...

You mean, what caused the origin of the Earth's axial tilt? I'm not sure what the specific answer to that question is, or if anyone knows the answer. Most likely, the answer is in planetary formation theory, and in moon formation theory. The moon is thought to have been formed in a giant collision, which might have affected Earth's tilt.

But clearly, Earth's axial tilt is an "accident" of history, because if you look at all the other planets, they all have different axial tilts.

Anonymous said...

Ah last time I looked Mars has a 24 degree tilt. Might want to read Velikovsky for a possible theory of axial tilt.

Bill said...

Has anybody measured the tilt lately? Could be the cause of the inclement weather!

Joe said...

I think the smiley faces represent magnetic north and south poles, which moves and is not the same as True North and True South