The sun is far about the Earth in the average ( 149.6 ) million kilometers. And earth turns round its axis each ( 4 second ? 56 minute? 23 A hour ) and what is known by the solar day . And the earth’s axis is an imaginative line extends between the north pole and the south pole and tilts by the corner of its amount ( 23.45 is his staircase ) About the vertical direction on the earth level round the sun throughout ( 91.315 day ) Then when earth takes a complete round the sun in a white path quickly estimates by ( 29.79 Km ) a second in a temporal average estimates by ( 365.26day ) Causes an operation of inclination to the south pole and the north pole during this course ? then we find in ( 21 December ) A non noticeable tendency to the south pole towards the sun during the earth circulation happens and it gradually becomes the inclination in ( 21 June ) To the north pole ? and continues the movement so in all of the years and the mile that happens to the south pole in ( 21 December ) it well-known by the wintry reversal . As for the inclination that happens to the north pole in ( 21 June ) it well-known by the summer reversal. As a result to that is the beginning point in ( 21 December ) and what it is called a point ( the zero ) . As for the end point then is in ( 21 June ) That estimates by ( 18 2.63day ) And the sun sunrise changes during this period According to the change of the movement of the circulation of the earth and what it is called the four seasons ? then the winter starts in ( 21 December ) A point ” The zero ” ( the beginning ) and in ( 21 March ) the spring starts . trkeg.com/trk/modules/3/