Perihelion

Definition

Perihelion is the point in the orbit of a planet, asteroid, or comet where it is closest to the Sun. At this point, the celestial body travels at its maximum orbital velocity due to the gravitational pull of the Sun.

5 Must Know Facts For Your Next Test

1. The Earth reaches perihelion around January 3rd each year.
2. Perihelion and aphelion describe points in an elliptical orbit; perihelion being the closest and aphelion being the farthest from the Sun.
3. Kepler's Second Law (the Law of Equal Areas) states that a planet moves faster at perihelion than at any other point in its orbit.
4. The term 'perihelion' comes from Greek words 'peri,' meaning near, and 'Helios,' meaning Sun.
5. The difference between Earth's perihelion and aphelion distances from the Sun is about 5 million kilometers.

Review Questions

• What is perihelion and why does a celestial body travel fastest at this point?
• When does Earth typically reach its perihelion during the year?
• How does Kepler's Second Law relate to a planet's speed at perihelion?

Related terms

Aphelion: The point in an orbit where a celestial body is farthest from the Sun.

Ellipse: An oval-shaped path that planets follow around the Sun, described by two foci.

Orbital Velocity: The speed needed for an object to stay in orbit around another object, varying along different points of an elliptical orbit.