One day on Mercury equals 176 earth-days. This drawing follows the position of a single point, in yellow, as Mercury spins (day to night) and as Mercury travels around the Sun (year). The yellow point faces the Sun only once every 176 earth-days or two Mercury years.
Image from: NASA
Mercury's Orbital Resonance
It takes Mercury about 59 Earth days to spin once on its
axis (the rotation period), and about 88 Earth days to complete
one orbit about the Sun. However, the length of the day on
Mercury (sunrise to sunrise) is 176 Earth days. How can this
happen? The figure shows the path of Mercury about the Sun
with a mark indicating the same spot on the surface of the
planet at different times in the orbit. A point initially
pointing toward the Sun will point in the same direction
after one rotation (59 days or 2/3 of the orbital period),
but that point will no longer be directed toward the Sun.
It takes three rotations of the planet during two orbits
of the planet about the Sun, or 88 x 2=176 days, for the mark
to get back to the same position.
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