Aphelion

Definition

Aphelion is the point in the orbit of a planet, asteroid, or comet at which it is farthest from the Sun. This term contrasts with perihelion, the point in the orbit where the object is closest to the Sun.

5 Must Know Facts For Your Next Test

1. Aphelion occurs because planetary orbits are elliptical, as described by Kepler's First Law.
2. At aphelion, a planet's orbital speed is at its minimum due to being furthest from the Sun’s gravitational pull.
3. The Earth reaches its aphelion around early July each year.
4. Kepler's Second Law states that a line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time; thus, planets move slower at aphelion compared to perihelion.
5. The difference in distance between aphelion and perihelion affects seasonal variations for planets with significant axial tilt.

Review Questions

• What happens to a planet's orbital speed when it reaches aphelion?
• How does Kepler's Second Law explain planetary motion at aphelion?
• When does Earth reach its aphelion?

Related terms

Perihelion:

The point in an orbit where a planet or other celestial body is closest to the Sun.

Elliptical Orbit:

An oval-shaped trajectory followed by planets and other celestial bodies around a star.

Kepler's First Law:

\text{Also known as the Law of Ellipses, this law states that planets move in ellipses with the Sun at one focus.}