Keplerā€™s First Law, also called the Law of Ellipses, states that every planet moves in an elliptical orbit, with the Sun located at one focus of the ellipse.

What Is Keplerā€™s First Law?

Before Johannes Kepler, astronomers believed that planetary motion must be perfectly circular. Circles were thought to be the most ā€œidealā€ shape.

Kepler showed that this assumption was wrong. By carefully analyzing astronomical data, he discovered that planetary orbits are actually ellipses, not circles.

This insight fundamentally changed our understanding of planetary motion.

What Is an Ellipse?

An ellipse is a closed curve that looks like a stretched or flattened circle.

  • A circle has one center point.
  • An ellipse has two special points, called foci (plural of focus).

In our solar system:

  • The Sun sits at one focus of the ellipse.
  • The other focus is empty space.

This means the planetā€™s distance from the Sun changes as it moves along its orbit.

Keplerā€™s First Law: Nearly Circular Elliptical Orbit A low-eccentricity elliptical orbit showing a realistic planetary path with the Sun at one focus. Focus (Sun) Empty Focus Planet
Although planets follow elliptical paths, most appear almost circular, with the Sun slightly offset from the cente.

Key Features of an Elliptical Orbit

  • Semi-major Axis ($a$):
    The average distance between the planet and the Sun.

  • Eccentricity ($e$):
    A number that describes how stretched out the orbit is.

    • If $e = 0$, the orbit is a perfect circle.
    • If $e$ is close to $1$, the orbit is very elongated.

Most planetary orbits in our solar system have small eccentricities, meaning they are nearly circular but not perfectly so.

Why This Law Matters

Keplerā€™s First Law explains why planets:

  • Move faster when they are closer to the Sun
  • Spend more time farther away from the Sun
  • Do not follow perfectly circular paths

This law laid the foundation for later discoveries, including Newtonā€™s Law of Universal Gravitation.

Interactive Match: Orbital Vocabulary

Match the key terms related to elliptical orbits and Keplerā€™s First Law.

Click a term and then its matching definition. Match all pairs to complete!

šŸ’” Quick Concept Check:

If an orbit has an eccentricity of 0.02, what does the shape look like?

Click to Reveal Answer
Since 0.02 is very close to zero, the orbit is almost perfectly circular, though it is technically still an ellipse.

Build on your understanding of Keplerā€™s First Law with these related skills:

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