Earth's orbit around the sun is due to the gravitational attraction between the earth and the sun. It follows an elliptical path, similar to an oval, but if viewed from space would look almost circular because the distances between the nearest and most distant points from the sun are not very different. Currently the earth is closest to the sun in the Northern Hemisphere winter. However, the earth's tilt of its rotational axis causes bigger changes in incoming sunlight than the distance from the sun and so it has a bigger effect on the seasons than the distance from the sun itself does.
The earthâ€™s orbit around the sun doesnâ€™t always stay the same. Sometimes the shape of the orbit changes, called the eccentricity. Sometimes the tilt of the axis changes, called the obliquity. Sometimes the earth wobbles as it turns, called the precession. All of these have a different effect on climate.
Eccentricity is the shape of the earthâ€™s orbit. Over a time period of 100,000 years, the orbit ranges from being a nearly perfect circle to being an oval and back to a near-circle again. Right now, the orbit is almost a perfect circle. This causes the earth to be a little closer to the sun in January than it is in July, which leads to more solar energy reaching the earth in January than in July. But this effect is small compared to the variation in incoming sunlight caused by the tilt of the earth, and so at this point in time the eccentricity has very little effect on the climate over the year. If the orbit became a pronounced oval, it would be warmer when the earth was closer to the sun regardless of tilt, and the length of the seasons would be different.
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Obliquity is the earthâ€™s tilt relative to the earth's orbit around the sun. The earthâ€™s tilt causes the seasons (see Tilt and Latitude under Background and Basics). The tilt away from the axis changes from 22.1Â° to 24.5Â° over a period of 41,000 years. The current tilt is 23.5Â° and is slowly decreasing. When the tilt becomes larger, the seasons are more extreme, with more severe winter and summer weather. When the tilt is smaller, the seasons are milder and less different from each other.
|Image from NASA|
The precession is how much the earth wobbles on its axis. The earth wobbles like a top that is slowing down. The result is that the North Pole on earth changes where it points to the sky. At present it is pointing at what we call Polaris, the Northern Star. However, 13,000 years ago it was pointing somewhat away from Polaris. The position of the North Pole on the sky forms a circle that is traced out every 26,000 years. The combination of the precession with whether the earth is nearer or farther from the sun can affect the severity of the seasons in one hemisphere compared to the other.
|Image from NASA|