The following astronomy primer is designed to introduce the most basic concepts of orbital mechanics to the reader, so as to provide the names that are applied by astronomers to various orbital elements that are used during the main sections of this work.
The following illustration shows the elliptical orbit of the Earth around the Sun. It is not drawn to scale and is a simplified illustration.
Perihelion is the name that has been given to the point in the orbit where the Earth is at it closest to the Sun at which its angular velocity is at its maximum.
Aphelion is the name that has been given to the point in the orbit where the Earth is at its maximum distance from the Sun and its angular velocity is at its minimum.