Take egg. Stand on its end. The Bad Astronomer has more details.

Since the standing an egg on end myth has something to do with celestial mechanics, and this post would be pretty short if I stopped here, I’ll talk about Milankovitch cycles as well. Weeeee.

(Note: I am stealing borrowing a few images from Wikipedia since I am a horrible artist. The ones they use are pretty good. If I made them, you would be tempted to jump off a cliff.)

What is the shape of Earth’s orbit?
For this to make any sense, you need to know that the Earth orbits around the sun. If you didn’t know that, I probably won’t be able to help you. You should take a remedial elementary school level Earth sciences course.

The Earth does not orbit around the sun in a perfect circle. Instead, the shape of its orbit is an ellipse. An ellipse is like an oval shape, or a squished circle. In fact, a circle is an ellipse. The equation of an ellipse is:

(x - h)² / a² + (y - k)² / b² = 1

You don’t need to be a math geek to know what that means. How to make an ellipse. An ellipse is the shape obtained when you set two points, known as foci, and then take every point around the foci where the distance from one foci added to the distance from the other foci is a constant. If the foci are spread far apart, you naturally end up with a long, narrow ellipse. If the foci are at the same point, you end up with a circle. The degree of “flatness” is called the eccentricity. An ellipse that has zero eccentricity is a circle. The higher the value of the eccentricity, the more elongated the ellipse will be.

Does the eccentricity cause the seasons?
Since the Earth’s orbit is not circular, there must be some point during its orbit when it is closer to the Sun. It would make sense that during this time it would be warmer. However, this does not cause the seasons. As it turns out, the Earth is closest to the Sun during Northern Hemisphere winter. If the eccentricity caused the seasons, it should be summer when the Earth was closest to the sun. However, it is Southern Hemisphere summer when the Earth is closest to the sun. The fact that the Northern Hemisphere has the opposite seasons as the Southern Hemisphere tells us that it cannot be the eccentricity in the Earths orbit that causes the seasons.

What the heck is obliquity
Obliquity is the degree of tilt with respect to the plane of the Earth’s orbit.

To visualize this, take a grapefruit (or any other roughly spherical object) and put it in the center of a table. Now take a ping-pong ball and mark a black dot on it. This is the North Pole. On exactly the opposite side of the ping-pong ball, make an X. This is the South Pole. Place the ball on the table so the North Pole is facing directly upward. In this case, the obliquity is zero. Now, rotate the ping-pong ball so the North Pole dot is halfway between being vertical and being horizontal relative to the table top. This represents an obliquity of 45 degrees.

Now, make your ping-pong ball revolve around the grapefruit with an obliquity of 45 degrees. Make sure that the obliquity doesn’t change! The dot should not point always point towards the grapefruit in the center of the table. Make one side of the table with an object such as a spoon. The North Pole dot should always be facing the spoon. In this way, at one point in the orbit, the North Pole dot will be pointing towards the grapefruit sun, and at the opposite point in its orbit it will be pointing away from the grapefruit sun.

I should also point out that the ping-pong ball should be rotating around its North Pole dot to South Pole X axis. This is hard to simulate with this tabletop demonstration, so use your imagination. It is this rotation that causes the days.

What does obliquity have to do with the seasons?
Hopefully when you did the grapefruit and ping-pong ball demonstration you noticed that at one part of the orbit the North Pole dot was pointed in such a way that light from the sun would not illuminate it during the entire day. And at the other side of the orbit the North Pole dot would always be illuminated throughout the day. Obviously, when the sun is illuminating the dot it will be warmer than when it receives no sunlight.

To generalize this point, consider the following diagram.

If the Sun’s rays hit the Earth’s perpendicular to the surface, the area illuminated is represented by X1. Now, let’s tilt the surface to a 45 degree angle. The Sun hasn’t changed, so the rays emanating from it are the same. This time however, the rays illuminate an area represented by X2. It is a little difficult to see, but X2 is larger than X1. Since the amount of light emitted by the sun is the same in both cases, and the area has increased in the bottom case. Therefore the intensity, or amount of sunlight recieved per area, has decreased in the bottom case.

This is called the sun angle effect. It is this that causes the seasons. In the summer, the rays from the sun hit the surface of the Earth at an angle that is closer to perpendicular. In the winter, the sun’s rays are more parallel to the surface. When the Northern hemisphere is pointed towards the sun, the Southern hemisphere is pointed away from the sun. This explains why the different hemisphere are on opposite seasons.

What does this have to do with Milankovitch cycles?
The Earth also has something called precession, or wobble. This is the angle of the axis of rotation relative to the fixed stars. I won’t go into detail about precession here.

The eccentricity, obliquity, and precession are not constant; they are always changing. For instance, the obliquity varies between 22.1 and 24.5 degrees. It is currently about 23.44 degrees. The eccentricity, obliquity, and precession do not vary on the same time scales.

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