Here's why:

* Sidereal period refers to the time it takes a celestial object to complete one orbit around another object, relative to the stars.

* Io orbits Jupiter much faster than the other Galilean moons (Europa, Ganymede, and Callisto).

* Io's sidereal period is approximately 1.77 Earth days.

Let me know if you have other questions about the Galilean moons!