Next applet shows the earth's orbit around the sun. Apparently it's a circle with the sun in the centre. But when dragging the earth on its orbit you'll see there's more...
Select the checkbox to show more details on the earth's orbit.

The earth's orbit is not a circle but an ellips with two foci. When the distance between these two increases, the ellips has got a greater eccentricity and its form gets flatter. When the two foci coincide, the eccentricity is 0 and the form becomes a circle.
The sun is placed in one of the two foci. This is the reason that the distance between earth and sun is not constant. The point on the orbit with the shortest distance is called the perhelion, the point with the greatest distance is called aphelion. You can hardly distinguish the earth's orbit from a circle since the eccentricity is very small. Move the sun to the right and see how the earth's orbit changes with it.

properties of an ellips

You can read more about the properties of an ellips in next worksheet.