text: 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.\nSorry for my English\nIt's difficult to explain it in a better way without a picture