Learning Objectives

• Describe the roles of gravity and tangential velocity in maintaining a planetary orbit.

• Explain Kepler's three laws of planetary motion.

• Define key orbital terms including ellipse, perihelion, and aphelion.

• Relate a planet's orbital speed and period to its distance from the Sun.

Key Vocabulary

Gravity

The force that pulls objects together and keeps planets in their orbits around the Sun.

Ellipse

An oval shape with two focal points, representing the true path of a planet's orbit around the Sun.

Perihelion

The point in a planet's orbit where it is closest to the Sun, moving at its fastest speed.

Aphelion

The point in a planet's orbit where it is farthest from the Sun, moving at its slowest speed.

Orbital Period

The time it takes for a planet to complete one full orbit around the Sun, also known as its year.

What Keeps a Planet in Orbit?

• Gravity acts as the centripetal force, pulling a planet toward the Sun.

• The planet's tangential velocity keeps it moving forward along its orbital path.

• Gravity's strength depends on objects' mass and the distance between them.

• A gravity well illustrates how massive objects have stronger gravitational pulls.

Planets orbit the sun in an elliptical path.

​First Law

Planets move faster when they are closer to the Sun and slower when they are farther away.

​Second Law

Kepler's Second Law

This law provides a relationship between the distance of a planet from the Sun and the time it takes to complete one orbit. Planets that are farther from the Sun have longer orbital periods.

​Third Law

​Keplers Third Law

On your own, answer the remaining 6 Questions.