He is it who made the sun -the central body of the solar system- emit light*, and the moon** -the satellite of the earth- derive its light from it. and reflect it to earth. He determined the divisions of the ecliptic*** for the moon to occupy them on successive days and present its phases according to the extent of its illumination**. This helps you to know the number of the years or reckon the time occupied by the sun in its apparent passage through the signs of the Zodiac and the period of the earth's revolution round the sun forming a natural unit of time, so that you be able to compute time and determine it by calculation. Never did Allah institute this and bring it about but for a serious end that conforms with the truth and agrees with reality. Thus does Allah expound His revelations and render His discourse readily understood by those who reflect.