The Earth’s orbit around the Sun has many interesting characteristics. First, the speed of our orbit is 108,000 km/h. The planet travels 940 million km during one orbit. The Earth completes one orbit every 365.242199 mean solar days(that might help explain the need for a leap year). The planet’s distance from the Sun varies as it orbits. Actually, the Earth is never the same distance from the Sun from day to day. When the Earth is closest to the Sun it is said to be at perihelion. This occurs around January 3rd at a distance of 147,098,074 km. When it is at its furthest distance from the Sun, Earth is said to be at aphelion. That happens around July 4th at a distance of 152,097,701 km.
The seasons are caused by a combination of two factor: the Earth’s axial tilt and its distance from the Sun during the orbital period. The planet is tilted 23.4° offset of the axis from a direction perpendicular to the Earth’s orbital plane. This puts a different hemisphere towards the Sun at different times of the year. When the Earth is at a certain place in its orbit, the northern hemisphere is tilted toward the Sun and experiences summer. Six months later, when the Earth is on the opposite side of the Sun, the northern hemisphere is tilted away from the Sun and experiences winter.
The shape of Earth’s orbit isn’t quite a perfect circle. It is more like a “stretched out” circle or an oval. Mathematicians and astronomers call this shape an “ellipse”. An ellipse can be long and skinny or it can be very round. Scientists need a way to describe how round or “stretched out” an ellipse is. They use a number to describe this, and call it the eccentricity of the ellipse. Eccentricity is always between zero and one for an ellipse. If it is close to zero, the ellipse is nearly a circle. If it is close to one, the ellipse is long and skinny. Earth’s orbit is almost a circle, it has an eccentricity of less than 0.02. That is why the distance from the Sun at perihelion and aphelion are very close.
Here are a couple of links that give you some more information about Earth’s orbit: one is about the ellipse and the other goes to NASA. Here on Universe Today we have a great article that gives you 10 interesting facts about Earth. Astronomy Cast offers a good episode about black holes unbalancing the Earth.