Perihelian

Perihelian is a term used in astronomy to describe the point in the orbit of a celestial body, such as a planet or comet, where it is closest to the Sun. This point is also known as the perihelion. The opposite point in an orbit, where the body is farthest from the Sun, is called aphelion.

The distance of a celestial body from the Sun varies throughout its orbit because most orbits are not perfect circles but are elliptical. The shape of an orbit is described by its eccentricity. A higher eccentricity means a more elongated ellipse, resulting in a greater difference between the perihelian and aphelian distances.

The concept of perihelian is important for understanding orbital mechanics and predicting the positions of celestial objects. For instance, the Earth reaches its perihelian around January 3rd each year, and its aphelion around July 4th. While this variation in distance from the Sun occurs, it is not the primary cause of Earth's seasons; axial tilt is the dominant factor. For comets, the perihelian is often the point where they are most active and visible due to increased solar radiation.