Sidereal day
What is a sidereal day?
One full turn relative to the stars, not the Sun. On Earth that is 23 hours 56 minutes. A star rises about four minutes earlier each night because of that difference — which is the orbit showing up in the clock.
Why stars rise earlier
Face a star at 9 p.m. Tomorrow the same star is up at 8:56 p.m. Earth spun once relative to the stars in 23 h 56 m, then kept turning a little more to face the Sun. That extra is why the night sky shifts through the year.
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The number on the planets page
When the planets page says Earth’s day is 23.9 hours, it means this: one full turn. Mars 24.6 hours is the same kind of number. Sunrise to sunrise is the solar day, a few minutes longer.