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 $2.44 \times 10^6\ \text{m}$. How fast does Mercury's terminator move across its surface at the equator?