The apparent orbit of the sun around the earth - that is in fact the orbit of the earth around the sun - is called ecliptic. It is devided into 12 equal parts named the zodiac circle. The angle of 23 degrees between the ecliptic and the equator brings about the seasons as the variing length of day and night. On June 21 the summer solstice is in the northern hemisphere and with it the longest day. In the southern hemisphere it is winter solstice on June 21 with the shortest day. The sun stands vertically above 23 degrees northern latitude (Tropic of the Cancer). On March 21 the sun stands vertically above the equator, so day and night is identical.