How long is a day on Mercury?
The planet Mercury rotates very slowly, taking almost 59 Earth
days to turn once (58.646 days). However, it also orbits the Sun
very rapidly, about once every 88 Earth days, which combines with
the slow rotation to create an unusual situation.
The solar day (sunrise to sunrise) on Mercury is 176
Earth days, or twice the length of its year! This further increases
the drastic temperature extremes that the surface experiences,
alternately seared for months by solar heat and then frozen for
months as it turns away from the Sun.
*Due to the tidal effects of the Sun's gravity, the equatorial
rotation of Mercury is only about 10 km/hr (far slower than Earth's
1600 km/hr). This means a "day" on Mercury is about 1400 Earth
hours (58.6 days).
(see the related question)
58.6461 Earth Days, rounded to 58.65 Earth Days
is the correct answer!
That's the rotation period, also known as the "sidereal"
The "solar day" is 3 times longer, at about 176 Earth