Home space resource Kepler’s First Law of Planetary Motion

Kepler’s First Law of Planetary Motion

In the early 1600s, the German astronomer Johannes Kepler used decades of extremely precise observations of the planets collected by the Danish astronomer Tycho Brahe. At the time, astronomers believed that planets moved in perfect circles, an idea that had dominated astronomy for nearly two thousand years. However, despite repeated attempts, circular orbits could not accurately explain the observed motion of Mars across the sky. By carefully analysing Brahe’s measurements, Kepler discovered that the planets do not move in circles at all. Instead, they move in elliptical orbits, with the Sun located at one of the two foci of the ellipse. This became known as Kepler’s First Law of Planetary Motion. This discovery was revolutionary because it replaced the long-held belief that celestial motion must be perfectly circular. Kepler’s work provided a much more accurate description of planetary motion and laid the foundation for Isaac Newton’s later development of the law of universal gravitation.

Year 9-10, Years 11-13

This activity is part of our live SpacewardBound sessions delivered in schools across New Zealand.

Earth & Space Science

Astronomy

20 mins

Interactive

Why is this good to know

Kepler's First Law was one of the most important discoveries in the history of astronomy. It showed that planets move in elliptical orbits rather than perfect circles, providing a much more accurate description of the Solar System. This discovery helped lay the foundation for modern astronomy and physics, leading to Isaac Newton's explanation of gravity and our understanding of how planets, moons, comets, and spacecraft move through space.

Learning outcomes

Analyse how orbital distance and velocity vary throughout an elliptical orbit.

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Explain Kepler's First Law in terms of elliptical orbits and orbital eccentricity.

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Use models to represent and explain phenomena in the natural world.

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Evaluate how scientific knowledge develops through observation and evidence.

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Develop an understanding of the Solar System as a dynamic system governed by physical laws.

Prior knowledge

Students should have a basic understanding of the Solar System, including that planets orbit the Sun. They should be familiar with using diagrams and models to represent scientific ideas and understand that scientific explanations are developed from observations and evidence. No prior knowledge of ellipses or orbital mechanics is required.

“The orbit of every planet is an ellipse, with the Sun at one focus.”

In this simulation, the grey curve shows the planet’s elliptical orbit, while the blue circle shows what the orbit would look like if it were perfectly circular. The yellow marker represents the Sun at one focus of the ellipse, the red marker represents the second focus, and the white marker represents the geometric centre of the ellipse. By varying the eccentricity and comparing orbits with the reference circle, you can see how real planetary orbits deviate from perfect circles and explore the key insight that led Kepler to one of the most important discoveries in the history of astronomy.

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This activity is part of our live SpacewardBound sessions delivered directly in schools. We run it as a full interactive experience using our mobile planetarium and hands-on activities.

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