Aug 6, 2026 · 22 min · 7 segments
Johannes Kepler had the relative sizes of the orbits of our planets in the solar system mapped by the 1630s. The shape of the solar system had been known relatively well, but it was a shape without…
The first real attempt at measuring astronomical distance came in 1671.
Italian-French mathematician and astronomer Giovanni Cassini, working from Paris, and his colleague Jean Richer, who was stationed across the Atlantic in French Guiana, simultaneously tracked the position of Mars against the background stars.
The two landed on one hundred forty million kilometers, or eighty-seven million miles.
Today's accepted value is roughly one hundred fifty million kilometers, or roughly ninety-three million miles.
They had gotten within about 7% of the truth, the first time in human history anyone had a real evidence-based figure for the scale of the solar system.
But there was an even earlier clue that nobody had yet followed up on.
A little over three decades before Cassini's measurement, a young Englishman named Jeremiah Horrocks had been rechecking Kepler's calculations for fun and realized Kepler had made a mistake.
It wouldn't miss.
Horrocks wrote to his friend he'd never met in person, English astronomer and mathematician William Crabtree, urging him to watch the transit as well.
Neither man trusted the math completely, though.
On the day of the actual transit in December 1639, he was called away on urgent business and nearly missed the entire thing.
He got back to his observatory with only minutes of daytime left.
Crabtree, thirty miles away, was so overwhelmed at the sight that he simply stood and stared, too stunned to take proper notes.
Between the short window and the sheer disbelief of it, neither man walked away with usable numbers.
But he filed a report anyway, noting that a transit like this could someday help measure the distance to the sun.
He died the following year at twenty-two years old due to unknown causes and before he ever published the idea in full.
It would take another Englishman, Edmond Halley, to turn Horrocks' forgotten discovery into an actual plan.
The first real attempt at measuring astronomical distance came in 1671.
Italian-French mathematician and astronomer Giovanni Cassini, working from Paris, and his colleague Jean Richer, who was stationed across the Atlantic in French Guiana, simultaneously tracked the position of Mars against the background stars.
The two landed on one hundred forty million kilometers, or eighty-seven million miles.
Today's accepted value is roughly one hundred fifty million kilometers, or roughly ninety-three million miles.
They had gotten within about 7% of the truth, the first time in human history anyone had a real evidence-based figure for the scale of the solar system.
But there was an even earlier clue that nobody had yet followed up on.
A little over three decades before Cassini's measurement, a young Englishman named Jeremiah Horrocks had been rechecking Kepler's calculations for fun and realized Kepler had made a mistake.
It wouldn't miss.
Horrocks wrote to his friend he'd never met in person, English astronomer and mathematician William Crabtree, urging him to watch the transit as well.
Neither man trusted the math completely, though.
On the day of the actual transit in December 1639, he was called away on urgent business and nearly missed the entire thing.
He got back to his observatory with only minutes of daytime left.
Crabtree, thirty miles away, was so overwhelmed at the sight that he simply stood and stared, too stunned to take proper notes.
Between the short window and the sheer disbelief of it, neither man walked away with usable numbers.
But he filed a report anyway, noting that a transit like this could someday help measure the distance to the sun.
He died the following year at twenty-two years old due to unknown causes and before he ever published the idea in full.
It would take another Englishman, Edmond Halley, to turn Horrocks' forgotten discovery into an actual plan.
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