Define Sidereal day. * a) It is defined as one complete rotation of the earth relative to the fixed stars. It is measured as 23h 56m 04s mean solar time. b) It is defined as one and half rotation of the earth relative to the fixed stars. It is measured as 23h 56m 04s mean solar time. c) It is defined as one complete rotation of the earth relative to the fixed stars. It is measured as 24h mean solar time. d) None of above