Solar system · Planet
Planet
Mercury
The swiftest planet, always close to the Sun — which is why most people have never knowingly seen it.
Look for
Catch it low in bright twilight, just after sunset or before sunrise, a few times a year when it swings farthest from the Sun.
Discovery
A tiny drift in its orbit that Newton’s gravity could not explain was solved in 1915 by Einstein’s general relativity.