# Perihelion – Definition & Detailed Explanation – Astronomical Objects Glossary

## What is Perihelion?

Perihelion is a term used in astronomy to describe the point in an object’s orbit around the Sun when it is closest to the star. This phenomenon is not exclusive to Earth but can occur with any celestial body that orbits another. The opposite of perihelion is aphelion, which is the point in an object’s orbit when it is farthest from the Sun.

## How is Perihelion Calculated?

The calculation of perihelion involves determining the distance between the object and the Sun at the closest point of its orbit. This distance is known as the perihelion distance. To calculate perihelion, astronomers use Kepler’s laws of planetary motion, which describe the motion of objects in orbit around a central body.

Kepler’s laws state that planets move in elliptical orbits with the Sun at one of the foci of the ellipse. The perihelion distance can be calculated using the semi-major axis of the orbit and the eccentricity of the orbit. The eccentricity of an orbit is a measure of how elongated the ellipse is, with a value of 0 representing a perfect circle and a value of 1 representing a straight line.

## What Causes Perihelion?

The phenomenon of perihelion is caused by the gravitational pull of the Sun on the object in orbit. As the object moves along its elliptical path, the gravitational force of the Sun causes it to speed up as it approaches perihelion and slow down as it moves away towards aphelion.

The gravitational force of the Sun is strongest at perihelion, which is why the object is closest to the star at this point in its orbit. This gravitational force is what keeps the object in orbit around the Sun and prevents it from flying off into space.

## When Does Perihelion Occur?

The timing of perihelion varies depending on the object in orbit. For Earth, perihelion occurs around January 3rd each year. This is when the Earth is closest to the Sun in its elliptical orbit. The exact date of perihelion can vary slightly from year to year due to the gravitational influences of other planets in the solar system.

For other celestial bodies, such as comets and asteroids, perihelion can occur at different times depending on their orbits. Some objects have very elongated orbits that bring them close to the Sun only once every few hundred or thousand years, while others have more regular orbits that bring them close to the Sun on a more frequent basis.

## What is the Significance of Perihelion in Astronomy?

Perihelion has significant implications for astronomers studying the motion of celestial bodies in the solar system. By observing the timing and distance of perihelion for different objects, astronomers can gain insights into the gravitational interactions between these objects and the Sun.

Perihelion also plays a role in the study of comets and asteroids, as these objects can be influenced by the gravitational forces of the planets as they pass through the inner solar system. By tracking the perihelion distances of comets and asteroids, astronomers can predict their future paths and potential impacts on Earth.

Overall, perihelion is a key concept in astronomy that helps scientists understand the dynamics of the solar system and the motion of objects in orbit around the Sun. By studying perihelion, astronomers can unlock the mysteries of the universe and gain a deeper understanding of the forces that shape our cosmic neighborhood.