The earth spins in space rather like a top.it rotates on its axis.At the same time it revolves round the sun.There is always a part of the earth facing the sun.it is then day for that part of the earth and night for the other part.We measure our days and nights by these rotations of the earth.One rotation of the earth takes twenty-four hours which is one day.One revolution of the earth round the sun takes 365 days.These revolutions are our years