What phenomenon causes the difference between the apparent solar day and the sidereal day?

Answer

The Sun's daily eastward progression along the ecliptic

Because the Sun advances nearly one degree eastward daily along the ecliptic, the Earth must rotate slightly further each day to bring the Sun back to the same apparent position, making the solar day about four minutes longer than the sidereal day.

What phenomenon causes the difference between the apparent solar day and the sidereal day?
sunastronomypathCelestial sphereecliptic