Kepler’s First Law — Elliptical Orbits: Why don’t planets move in circles?

Before Kepler, everyone believed planets move in circles. Why? Because a circle is “perfect” and the heavens must be perfect — or so they thought. But the observational data said otherwise.

Johannes Kepler spent 20 years analysing Tycho Brahe’s observatory data before discovering something simple yet revolutionary: orbits are ellipses.

Kepler’s First Law

Every planet travels in an elliptical path around the Sun, with the Sun located at one of the two foci of that ellipse — not at the centre.

Perihelion and Aphelion

Because the Sun sits at a focus — not the centre — the planet’s distance from the Sun changes throughout its orbit:

• Perihelion: the closest point to the Sun. Earth reaches perihelion in early January.
• Aphelion: the farthest point from the Sun. Earth reaches aphelion in early July.

Interestingly, Earth is closest to the Sun in January — yet the Northern Hemisphere is cold. Why? Because the seasons are caused by the angle of sunlight, not the distance from the Sun.

How Elliptical Are They?

Most solar system planets have nearly circular orbits — very “fat” ellipses. Earth’s perihelion and aphelion differ by only about 3%. Pluto and many comets, however, have highly elongated orbits.

References

• Halliday, Resnick & Krane — Physics, Ch. 13
• Feynman Lectures on Physics — Vol. 1, Ch. 7
• NASA Jet Propulsion Laboratory — Solar System Exploration