Science › Space, time & gravity
Mercury's orbit puzzle solved
1915· Discovery / experiment
ProvenTested many times; part of working physics.
Mercury's oval orbit slowly swings around the Sun slightly faster than Newton's gravity allows; general relativity accounted for the gap exactly.
See it on the Science map →How it works
Each orbit, the point where Mercury comes closest to the Sun shifts a little. Most of this shift comes from the pull of other planets. But in 1859 Le Verrier found a small leftover, refined by 1882 to about 43 arcseconds per century. Astronomers proposed a hidden planet, "Vulcan," which was never found. In November 1915 Einstein calculated that curved spacetime near the Sun produces exactly 43 extra arcseconds.
Why it matters
It was general relativity's first success, explaining an old puzzle without any adjustable fudge factor. Einstein said the result gave him heart palpitations.
How we know
The 43 arcseconds per century is measured from centuries of observations and modern radar ranging, and matches general relativity's prediction to better than one percent.
Still a mystery
Settled. Similar orbital shifts are now measured for stars orbiting the black hole at our galaxy's center, testing the theory under extreme gravity.
Connections (2)
▸General relativity: gravity is curved spacetime explained Mercury's orbit puzzle solvedProven
The relationship
Curved spacetime near the Sun slightly changes Mercury's orbit, causing the closest-approach point to rotate a little extra each orbit. The calculated extra amount, 43 arcseconds per century, matched the long-known discrepancy with Newton's gravity.
How the link was found
Einstein calculated it in November 1915, just before finalizing his field equations.
Why it's proven
Modern radar ranging to Mercury confirms the shift to better than one percent.
Open questions and mysteries
Settled. Similar tests now probe stars orbiting the black hole at the galaxy's center.
▸Mercury's orbit puzzle solved exposed a flaw in Newton's law of gravityProven
The relationship
Mercury's elliptical orbit slowly rotates around the Sun. Newton's gravity, including tugs from other planets, accounts for most of that drift but leaves about 43 arcseconds per century unexplained. That small leftover showed Newton's law is not exact; Einstein's general relativity later accounted for it precisely.
How the link was found
Urbain Le Verrier identified the anomaly in 1859, Simon Newcomb refined it in 1882, and Einstein explained it in November 1915.
Why it's proven
Modern radar ranging and spacecraft tracking confirm both the anomaly and general relativity's prediction to high precision.
Open questions and mysteries
Settled, but instructive: a tiny discrepancy overturned a theory. Physicists now watch other anomalies, such as galaxy rotation, the same way.