In the early seventeenth century, the astronomer Johannes Kepler, working from decades of extraordinarily precise naked-eye observations collected by Tycho Brahe, worked out that planets don't orbit the sun in perfect circles at constant speed, as had been assumed for nearly two thousand years. They move in ellipses, and their speed changes constantly along the way — faster near the sun, slower far from it, in a precise, calculable pattern. He had no idea why. That explanation wouldn't arrive for another seventy years.
Equal areas in equal times
Kepler's second law states that a line drawn from the sun to a planet sweeps out equal areas in equal intervals of time, no matter where the planet is in its orbit. Near the sun, where that line is short, the planet has to move a greater distance along its path to sweep the same area in the same time — so it moves faster. Far from the sun, with a long line, a much smaller arc of travel covers the same area — so it moves slower. Kepler derived this purely from Brahe's positional data, fitting a pattern to observation with no underlying physical mechanism to explain why it should be true at all.
The "why" arrives seven decades later
It took until 1687, and Isaac Newton's law of universal gravitation combined with his laws of motion, for anyone to explain why planets should move this way. Newton showed that Kepler's laws — including the equal-areas rule — follow directly and necessarily from a single, simple assumption: that gravity pulls two masses together with a force that falls off with the square of the distance between them. Kepler had found the precise pattern by painstaking data-fitting; Newton showed the pattern was the inevitable consequence of one universal force.
What we're still unsure about
Newton's explanation held for over two centuries as a complete account, until Mercury's orbit revealed a tiny, persistent discrepancy — its closest point to the sun shifts slightly more each orbit than Newtonian gravity alone predicts. That gap remained genuinely unexplained until Einstein's general relativity in 1915 accounted for it as a curvature-of-spacetime effect, too small to matter for most orbits but measurable for a planet as close to the sun as Mercury — a reminder that even a theory that matches observation almost perfectly for centuries can still be hiding a real, physically meaningful gap nobody's found yet.
This sits inside Gravitation & Kepler's Laws, one of eight topics in Mechanics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.