Q:  What is a solar day?

            The time it takes for the Sun to move from its highest point
            in the sky one day to the time it is at its highest point
            in the sky again.

            

   Q:  What is a sidereal day?

            The time it takes for a star to move from its highest point
            in the sky one day to the time it is at its highest point
            in the sky again.


   Q:  Why are they different?

            Because the Earth is revolving around the Sun -- and so 
            the Sun appears to change its position slightly each
            day relative to the distant stars.