The line between day and night on a planet is called the terminator. Mercury is
not tidally locked to the Sun; instead, it spins on its axis once every
58.6 Earth days (its sidereal rotation period) while orbiting the Sun once every
88 Earth days, both in the prograde sense. Mercury's radius is
2.44×106 m. How fast does Mercury's terminator move across its surface at the equator?