The sidereal day is the time required for Earth to rotate once relative to distant stars. How long is this sidereal day approximately?

The sidereal day is the time required for Earth to rotate once relative to distant stars. How long is this sidereal day approximately?

• A. 23 hours 56 minutes
• B. 24 hours
• C. 25 hours
• D. 23 hours 30 minutes

Answer: (A)

The main idea here is how long it takes Earth to rotate once with respect to the distant stars. A sidereal day is the rotation period relative to that fixed star background, not relative to the Sun. It takes about 360 degrees of rotation to realign with the same stars, and that happens in roughly 23 hours, 56 minutes, and a few seconds. The reason it’s a little shorter than 24 hours is that Earth is also moving along its orbit around the Sun; as Earth travels, the Sun appears to drift a degree or so each day against the stars, so you need a little extra rotation to bring the Sun back to the same solar longitude. Therefore, the sidereal day is about 23 hours 56 minutes, which is why that option is the best approximation.

The main idea here is how long it takes Earth to rotate once with respect to the distant stars. A sidereal day is the rotation period relative to that fixed star background, not relative to the Sun. It takes about 360 degrees of rotation to realign with the same stars, and that happens in roughly 23 hours, 56 minutes, and a few seconds. The reason it’s a little shorter than 24 hours is that Earth is also moving along its orbit around the Sun; as Earth travels, the Sun appears to drift a degree or so each day against the stars, so you need a little extra rotation to bring the Sun back to the same solar longitude. Therefore, the sidereal day is about 23 hours 56 minutes, which is why that option is the best approximation.