Why the Autumnal Equinox is so special
Follow along with the attached image gallery.
As we change seasons from Summer to Autumn Friday, we're about to celebrate a special event that only happens twice a year, a point in time when the sun's rays are evenly distributed across the northern and southern hemisphere.
As we all know the earth is basically a rotating giant ball of dirt and water, but this ball's axis of rotation is not perfectly pointed at the sun, its tilted (Figure 1). This is key in causing seasons, without such a tilt the planets climate would be mostly dominated by latitude. The further away from the equator the colder you'd be and that's that.
Also so most people know our giant rotating ball of dirt and water, circles (orbits) an even larger ball of flaming gas, the sun. The most common misconception about the changing of the seasons is the the earth changes its tilt towards and away from the sun, that is FALSE. The tilt of the earth is fairly constant (23.5°) and its orientation does not change. It is the position of the Earth at different times of the year (figure 2) along its orbit that causes the earths tilt to favor the northern and southern hemispheres, with more (summer) or less (winter) sunlight.
What makes the equinox so special? It only happens twice a year and is when the earths tilt favors neither the northern or southern hemisphere (figure 3). The Autumnal Equinox this Friday, Sept. 23, at 5:05 a.m., EDT, is that moment when there is equal light distributed upon the earth from the sun. After that more light ( and more important energy) from the sun will be focused on the southern hemisphere. The difference at first is almost immeasurable but the sun's angle will continue to get lower in the sky and days will get shorter through the Winter Solstice in December (figure 4).
Bottom line, the sun is lower in the sky, days are shorter, less direct sunlight and this will cause temperatures to cool all thanks to the Earth being a little tipsy on its axis.
To learn more about the science of the seasons check this link from http://www.physicalgeography.net/fundamentals/6h.html