The earth's equator makes an angle of 23.5° with its orbital plane around the sun (the angle which the ecliptic makes with the celestial equator) the northern hemisphere receives more sunlight in June, less in December the southern hemisphere receives less sunlight in June, more in December(Fig. 2-9). four seasons Fig. 2-10 The cycle of four seasons occur because Earth's axis is inclined 23.5° from the perpendicular. As Earth circles the sun, its axis remains pointing in the same direction in space. In June, the Northern hemisphere receives more sunlight than the Southern hemisphere, so it is summer at the Northern hemisphere and winter in the Southern hemisphere. In December, the reverse situation occurs. Vernal equinox: The sun crosses the celestial equator moving N (Fig. 2-11). Summer solstice: The sun at farthest N Autumnal equinox: The sun crosses the celestial equator moving S. Winter solstice: The sun at farthest S. Fig. 2-11 Following the ecliptic (dashed line), the Sun crosses the celestial equator at equinoxes. The solstices mark the Su's most northerly and most southerly points.