Elliptical Orbits

Elliptical Orbit
Credit: Horvath and Rubber Duck
You might think that the Earth orbits the Sun in a perfect circle. But actually it's more like an oval. We call the shape of the Earth's orbit, elliptical. This means it is shaped like an ellipse. An ellipse is a circle which has been squashed or flattened a bit.
All 8 planets in our Solar System travel around the Sun in elliptical orbits. Not all ellipses are the same. The 'eccentricity' of an ellipse tells us how flattened (or how elliptical) it is. The more flattened an ellipse is, the closer the eccentricity is to 1. A perfectly circular obit has an eccentricity of 0, which is not at all flattened. So, all ellipses have an eccentricity between 0 and 1. Earth's orbit has an eccentricity of 0.0167, which is very close to 0. This is why it's easy to mistake it for a perfect circle. Mercury, with an eccentricity of 0.2056, is the planet with the most elliptical orbit. 
Axes of an Ellipse
Credit: Sae1962

Use the electric orrery to view orbits of the planets in our Solar System.

An ellipse has 2 focus points or foci. At any point in its orbit, a planet's total distance from these 2 focus points stays the same. An ellipse also has 2 lines of symmetry. The longer line is the major axis. The shorter line is the minor axis. Half of the major axis is the semi-major axis. Likewise, half of the minor axis is the semi-minor axis.

Johannes Kepler wrote 3 laws to describe the motions of planets. This useful video from the European Space Agency describes Kepler's 3 Laws. You can investigate this science using our Kepler's Laws workshop.