According to the Kepler's first law, the Earth's trajectory around the Sun is an ellipse with the Sun at one focus.

The point of the Earth's orbit that is the closest to the Sun is called the perihelion while the aphelion is the point that is farthest from the Sun (Fig. 2.6). a is half of the major axis and b half of the minor axis. The shape of the ellipse is then characterised by its eccentricity (ecc), defined by:
The parametres of the Earth's orbit vary with time (see section 5.4.1) but at present ecc = 0.0167, meaning that the Earth's orbit is very close to a circle (which of course corresponds to an eccentricity of zero).
The distance from the Sun to the Earth (r) can be computed as a function of v, the true anomaly, according to the formula for an ellipse:
The amount of incoming solar electromagnetic radiation per unit area at the top of the atmosphere is a function of r. We can define r_{m} as the mean distance between the Earth and the Sun by:
This means that a circle with a radius r_{m} would have the same area as the ellipse corresponding to the Earth's orbit. The total energy emitted by the Sun is equal to the total energy received on the surface of a sphere of radius r, centred on the Sun, and to that received on a sphere of radius r_{m}. S_{r} the amount of solar radiation per unit area measured on the outer surface of Earth's atmosphere in a plane perpendicular to the rays at a distance r from the Sun can then be computed as a function of the Solar Constant S_{0}: