Observing the Sky: Motion of the Earth - Seasons

In the Northern Hemisphere, the day the North Pole leans most toward the Sun is called the Summer Solstice and it is usually June 21. Three months later, near September 23, the Earth reaches a place in its orbit where neither the North or South Pole leans more toward the sun. On this day, called the Autumnal Equinox (Fall Equinox) , the hemispheres receive equal amounts of sunlight. Equinox means "equal night." This is when the Northern Hemisphere passes into fall and leans farther and farther away from the Sun each day. Meanwhile, the Southern Hemisphere leans more toward the Sun each day and passes into spring.

The Earth doesn't pause in its trip around the Sun, so three months later the North Pole is leaning as far as it can away from the Sun. This day is called the Winter Solstice and usually falls on December 21. On the Vernal Equinox (Spring Equinox), around March 21, the hemispheres again receive equal amounts of light with neither pole leaning more toward the Sun. These dates mark the first day of each season astronomically, but not meteorologically (in terms of weather).