Can the Sun rise twice in one morning on Mercury?

NASA describes a morning Sun that appears to rise briefly, set, and then rise again from some locations on Mercury. At sunset, observers elsewhere could see the sequence happen in reverse.

The strange sunrise is not a second day beginning. It is an apparent reversal within the same long Mercury morning.

Why it happens

Mercury turns slowly: one rotation takes about 59 Earth days. It also travels around the Sun on a strongly elliptical orbit, speeding up as it approaches its closest point to the Sun. Near that part of the orbit, the changing orbital motion temporarily overtakes the ordinary apparent motion produced by the planet’s rotation.

That changes the Sun’s apparent direction in Mercury’s sky. The result, from the right longitude, is the brief rise-set-rise sequence NASA describes.

What gets lost in retelling

“The Sun rises twice on Mercury” sounds like a planet-wide daily event. NASA is narrower: it happens from some parts of the surface. A complete Mercury solar day is already extraordinary—176 Earth days—but that is a different fact from the temporary reversal around sunrise or sunset.

What the source establishes

NASA’s Mercury facts page directly supports the rise-set-rise observation, limits it to some locations, connects it to Mercury’s fastest orbital motion, and gives the 59-day rotation and 176-day solar-day figures.

Next card: what the square-window story leaves out →