text: what is the 4 min difference between solar & sidereal day related to? as you know  sidereal day is 4 min less than solar day,why? solar day is the time to have the sun in the same meridian. sidereal day is the time to have a star in the same meridian. while earth is rotating around its axis it also is orbiting around the sun, so to see the sun in the same direction after a rotation the earth need a further time because meanwhile it is in another position in its orbit. sorry for my english it's difficult to explain it in a better way without a picture