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A painting of Astronomer Johannes Kepler
Johannes Kepler: A giant in astronomy upon whose shoulders we standImage: Pictures From History/CPA Media/picture alliance

Johannes Kepler's astronomical legacy

Louisa Wright
December 27, 2021

Astronomer Johannes Kepler was born 450 years ago, on December 27. His discoveries have shaped our understanding of the planets and the way satellites orbit Earth.


You may never have heard of Johannes Kepler, and that's no problem, but his legacy lives on all around us. If Kepler were alive today, he would be celebrating his 450th birthday. He was one of those early thinkers who revolutionized science and the way we see and understand the universe — the natural world and life itself.

Kepler was an astronomer and mathematician. He stands next to giants like Copernicus, Galileo and Newton. And today's science stands on their shoulders.

A short biography

Kepler was born on December 27, 1571, near what is now Stuttgart in southern Germany.

He is best known for his discovery of three laws of planetary motion. Presented between 1609 and 1619, those laws describe how the planets orbit around our sun.

Living at a time when there was a strong divide between astronomy and physics, Kepler wanted to bring the two disciplines together. In doing so, he transformed scientific thought.

A new space telescope

Kepler's laws

It all started with Kepler's discovery that Mars orbited the sun in an ellipse — an oval shape.

That first discovery led Kepler to then realize that all the planets moved at different speeds around the sun in an elliptical orbit.

This improved on an earlier heliocentric theory, proposed by Polish mathematician and astronomer Nicolaus Copernicus, who had theorized that the planets orbited the sun in a circular movement.

Kepler's laws of planetary motion were essential for Isaac Newton's law of gravitation in the 1680s. Newton's law says that all objects — or the particles that make up objects — attract each other with a gravitational force. And that explains why planets orbit around the sun in the first place.

His laws in action

Kepler's laws are helpful when it comes to our understanding of the movement of natural objects. They help us understand stellar systems and extrasolar planets, which are outside our solar system.

Internet from space

They are also used in the design of rocket trajectories and how we have satellites orbit our planet today. Satellites can be farther or nearer to us, depending on where they are on their orbit — that's the nature of an elliptical orbit.

And that's important to know when, for instance, you are planning when and where you can take images of Earth — or when it's most efficient to send data from one place on Earth to another via satellite.

When science and religion coexisted

Kepler was born into a time when the scientific community worked within the confines of religion and the church. It wasn't always an easy match between science and religion, and many early philosophers and scientists paid the price.

In Kepler's case, religion was a positive influence. He avoided calling his discoveries about planetary motion "laws" and instead considered them "celestial harmonies" that reflected God's design for the universe.

All schooling at the time was controlled by church institutions, both Roman Catholic and Protestant. Kepler was raised in a Lutheran family. He received a scholarship through the church and that started him on his scientific journey.

He attended a seminary at the University of Tübingen from 1589. People tended to graduate from these schools to become teachers or church ministers — Kepler had initially planned to become a theologian.

That was until he studied under his mentor, Michael Mästlin.

Mästlin introduced Kepler to Copernicus' ideas, which went on to play a central role in Kepler's own discoveries.

Kepler said God had led him to study the stars. He even believed that God was symbolized by the sun — a force around which the planets revolved. It's was an idea which Newton later revised as described above.

What Kepler's story shows us, however, is how science evolves. It is in constant flux and movement, discovery and rediscovery. It reminds us how scientific thought exists in the context of its times and the lives of its thinkers.

Edited by: Zulfikar Abbany

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