This Demonstration shows features of the orbit of the Earth.
Position angle: Construct the polar coordinate system with the Sun at the origin and the major axis along the Earth's elliptical orbit, shown here as an orange line. Then the position of the Earth can be expressed in terms of the distance from the origin and the position angle from the major axis.
Incidence angle: This is the angle at which the Sun shines upon the equator of the Earth. It is defined to be positive toward the north.
Tilt angle: This is the angle at which the self-rotation axis of the Earth skews away from the positively oriented normal of its orbit. This value is 23.44° for the Earth.
Eccentricity: This is a constant that determines the shape of the orbit. For an ellipse, it is between 0 and 1. The value for the Earth is currently 0.0167, which makes its orbit almost a circle; the eccentricity has varied between 0.0034 and 0.058.