How long is the sidereal day, which measures when stars return to the same position relative to Earth’s axis?
Answer
Approximately 23 hours, 56 minutes, and 4.09 seconds.
The sidereal day measures the precise time it takes for the Earth to rotate exactly 360 degrees relative to the background stars, bringing a specific star back to the same position in the sky relative to the Earth's axis of rotation. This duration is slightly shorter than the familiar 24-hour solar day because the Earth is also moving in its orbit around the Sun during that time. The exact measurement provided for this period is 23 hours, 56 minutes, and 4.09 seconds. This slight difference explains why a specific star crosses the meridian at a time that is consistently a few minutes earlier each subsequent solar day.
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