ABSTRACT
The mean sun-Earth distance is approximately 150,000,000 km, varying at any given time by an average of 1.7% due to the elliptical eccentricity of the earth's orbit. Earth's orbit is a nearly circular ellipse with an eccentricity of 0.017, a value that varies over a cycle of thousands of years due to gravitational attractions of other celestial bodies. The Earth orbits the sun on the ecliptic plane, which passes through both the center of the Earth and the center of the sun. While the tilt of the Earth's rotational axis is essentially fixed, its apparent tilt relative to the ray of the sun changes constantly as the Earth moves along its orbit. This changing relationship results in seasons and varying day lengths over the course of a year. Solar declination is the angle of the solar ray relative to the Earth's equatorial plane on a given day.
