Perihelion

Definition

Perihelion is the point in the orbit of a planet or other celestial body where it is closest to the Sun. It represents one extreme of an elliptical orbit.

5 Must Know Facts For Your Next Test

1. Perihelion occurs once per orbital period for any object orbiting the Sun.
2. The term is derived from Greek, where 'peri' means near and 'helios' means sun.
3. Kepler's Second Law, also known as the Law of Equal Areas, explains why planets move faster at perihelion than at other points in their orbits.
4. The Earth's perihelion currently occurs around January 3rd each year.
5. The distance between a planet and the Sun at perihelion can be calculated using its semi-major axis and eccentricity.

Review Questions

• What is perihelion and how does it relate to an object's orbit around the Sun?
• According to Kepler's Second Law, what happens to a planet's speed as it approaches perihelion?
• When does Earth typically reach its perihelion during the year?

Related terms

Aphelion:

The point in the orbit of a planet or other celestial body where it is farthest from the Sun.

Eccentricity:

A measure of how much an orbit deviates from being circular. An eccentricity of 0 represents a perfect circle.

Semi-Major Axis:

The longest radius of an ellipse, representing half of the longest diameter across an elliptical orbit.