Kepler's laws of planetary motion
Kepler conceives, calculates, and initially rejects his third-law relation
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Original prompt & settings (JSON) ↗Kepler left an exact date for an idea he got wrong: the day the third law came to him, the arithmetic failed and he threw it out as false.
8 March 1618: conceived and discarded the same day
Kepler later recorded that the relationship now called his third law first came into his mind on 8 March 1618. He tested it, calculated unsuccessfully, and rejected it as false.
Nothing was announced. The date marks a first conception, not a completed verification.
What the relation proposed
The idea tied planetary orbital periods to mean distances from the Sun: in modern notation, the squares of the periods stand in proportion to the cubes of the orbital semimajor axes.
Kepler was finishing Harmonice mundi at the time, hunting for a mathematical relation of exactly this kind and testing the new ratio against data derived from Tycho Brahe's observations.
15 May, and a book being prepared for print
Kepler returned to the relation on 15 May 1618 and accepted it once it agreed with his long analysis of Brahe's observations — work he described as invoking seventeen years of observation.
He wrote that when the calculation finally came out he thought at first he was dreaming, or that he had assumed his conclusion among his premises. The result forced revisions to a book already being prepared for print; Harmonice mundi appeared in 1619.
Sources
Researched 23 Aug 2026 4 sources 2 audit passes, 1 issue open
How this was checked
Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →
What the sources leave uncertain
- The dates come directly from Kepler's retrospective account in Harmonice mundi. They identify stages in a calculation rather than a witnessed public event.
- March 8 can properly be called the conception of the third law, but calling it the completed discovery without mentioning its rejection and May confirmation is misleading.