The length of one
day (technically, a solar day) on a planet is the time from
noon to noon. Since the planets spin on their axes at different
rates and travel about the Sun at different speeds as well,
the length of one day is different for each planet. For example,
on Earth the length of a solar day is 24 hours. But because
of its slow rotation and its rapid orbital speed, one solar
day on Mercury lasts about as long as 176 days on Earth.