How Long Is A Day On Mercury?
QUICK ANSWER
One day on Mercury, measured from sunrise to sunrise, lasts about 176 Earth days. That's longer than Mercury's entire year, which is only 88 Earth days. Mercury also rotates once on its axis every 59 Earth days, so depending on how you measure a day, you get very different answers.
Mercury's day is one of the strangest in the solar system. Depending on what you mean by 'a day,' the answer can be 59 Earth days, 176 Earth days, or somewhere in between. And weirder still: on Mercury, a single day lasts longer than a year.
What counts as a day on Mercury?
There are two ways to measure a day on any planet. A sidereal day is one full rotation on the planet's axis, measured against the distant stars. A solar day is the time from one sunrise to the next, which depends on both rotation and the planet's orbit around the Sun. On Earth, those are almost identical (about 23 hours 56 minutes vs. 24 hours). On Mercury, they're radically different because Mercury orbits the Sun so quickly relative to its rotation.
How long is a Mercury sidereal day?
Mercury rotates once every 58.6 Earth days, according to NASA. That's the sidereal day, the time it takes for any given point on Mercury to rotate back into the same orientation relative to the stars. It's a slow rotation, which is part of why Mercury has such extreme temperature swings: each side of the planet faces the Sun for weeks at a time before turning away.
How long is a Mercury solar day?
About 176 Earth days. As Mercury slowly rotates, it's also racing around the Sun, completing a full orbit every 88 Earth days. The combination means that by the time the Sun rises again at any given spot, Mercury has rotated and orbited just the right amount that two full Mercury years have passed. So one solar day on Mercury equals two Mercury years. From the surface, the Sun would appear to move very slowly across the sky, and stay 'up' for around 88 Earth days at a time.
Why is Mercury's day so weird?
Mercury is locked into a 3:2 spin-orbit resonance with the Sun, meaning it rotates exactly three times for every two orbits. This unusual arrangement is the result of the Sun's gravity acting on Mercury's slightly elongated shape over billions of years. Most planets either rotate freely or end up tidally locked (one side permanently facing their star). Mercury landed in a strange middle ground, where its rotation and orbit are precisely synchronized but not in a way that fixes one face to the Sun.
Mercury's day depends on what you're measuring. A rotation takes about 59 Earth days; a sunrise-to-sunrise solar day takes about 176. Mercury's year is shorter than its solar day, which is true nowhere else in the solar system. It's one of the most genuinely strange timekeeping situations in our cosmic neighborhood.
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