Science history: Kepler’s optics and orbits

In a time of witch trials, Johannes Kepler was a champion of reason. Jeff Glorfeld reports.

An engraving depicting Kepler discussing his findings with Holy Roman Emperor Rudolf II.

An engraving depicting Kepler discussing his findings with Holy Roman Emperor Rudolf II.

Nastasic/Getty Images

In March 2009, the US space agency NASA launched the Kepler mission, a space telescope designed to survey a portion of the Milky Way in search of planets outside the solar system.

Using data from the mission, the agency reports, scientists have so far identified more than 4500 candidate exoplanets and confirmed more than 1000 of these as “bona fide”.

“A handful of planets are thought to be rocky like Earth (but a bit bigger), and orbit in the habitable zone of their stars, where liquid water – an essential ingredient of life as we know it – might exist,” NASA’s website reports.

The mission was named for Johannes Kepler, a mathematician, astronomer and astrologer, born on December 27, 1571, in the town of Weil der Stadt, now part of the Stuttgart region in Germany.

Just a few items from NASA’s biographical List of Kepler's Firsts reveals the breadth of his scientific interests.

He was the first to correctly explain planetary motion, thereby becoming the founder of celestial mechanics and the first "natural laws" in the modern sense of principles being universal, verifiable, and precise.

In his book Astronomia Pars Optica, for which he earned the title of father of modern optics, he was the first to investigate the formation of pictures with a pinhole camera; first to explain the process of vision by refraction within the eye; first to formulate eyeglass designing for nearsightedness and farsightedness; and first to explain the use of both eyes for depth perception.

In another book, Dioptrice, he was the first to describe real, virtual, upright and inverted images and magnification, ans well as explain the principles of how a telescope works. He also discovered and described the properties of total internal reflection.

A third book, Stereometria Doliorum, formed the basis of integral calculus.

Kepler is also credited with explaining that the tides are caused by the moon, and that the sun rotates on its own axis.

Kepler’s scientific accomplishments seem all the more extraordinary when considering the times in which he lived.

For example, in 1620 he was forced to put aside his research and defend his mother against a charge of witchcraft, in a case that would last six years, “the last 14 months of which saw her attached with an iron chain to the floor of a prison cell”, according to a 2015 Guardian review of a book about the matter, The Astronomer and the Witch, by Cambridge professor Ulinka Rublack.

Kepler himself died just four years later, on November 15, 1630.

“Religious leaders were reluctant to relinquish their ideas about the heavens,” writes Dan Lewis, a curator at California’s Huntington Library, on the website. “Talk by astronomers of a sky filled with objects moving in non-circular orbits and other phenomena that went against an Earth-centric model threatened their beliefs.”

Actually, Kepler's motivation was not to upset the church, says astrophysicist Paul Sutter, in another article. “Rather, his defence of the Copernican model was not on physical or mathematical grounds. Kepler's argument was religious. Kepler said that since the son of God was at the centre of the Christian faith, the sun ought to be at the centre of the universe.”

Jeff Glorfeld is a former senior editor of The Age newspaper in Australia, and is now a freelance journalist based in California, US.
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