How is a solar day different from a sidereal day?

The primary difference between a solar day and a sidereal day lies in the reference point used to measure the Earth's rotation.

• Sidereal day: This is the time required for the Earth to complete one full rotation relative to the distant, fixed stars. It is measured by the interval between two successive transits of a star over the same meridian and lasts approximately 23 hours, 56 minutes, and 4 seconds of mean solar time.12
• Solar day: This is the time between two successive transits of the Sun over the same meridian. Because the Earth moves along its orbit around the Sun during its rotation, the Sun appears to shift eastward relative to the stars. Consequently, the solar day is about four minutes longer than the sidereal day, averaging 24 hours.1

While the sidereal day is essential for astronomical observations, the solar day serves as the fundamental unit of time for civil life and standard timekeeping.12