There are four very awesome days in the solar calendar that we use to measure a year. Well, five if you count Teddy Roosevelt’s birthday. But cool as he was, his birthday (October 27th, if you were wondering) doesn’t have anything to with the awesomeness that creates those other four days. See, on four days out of the year the Earth’s position relative to the sun, coupled with its 23.5 degree tilt, creates what we call the solstices and the equinoxes. 

The solstices are days of extremes: the day of the summer solstice marks the longest amount of daylight (in the northern hemisphere), and the winter solstice the shortest (again, in the northern hemisphere; it’s reversed in Australia). The equinoxes, on the other hand, are as their name suggests: days when the amount of daytime and nighttime are almost completely equal. They happen in the spring and fall, and the fall one is coming up here real soon.

The spring equinox is exactly between the winter and summer solstices: from the shortest day on, the days get longer, until they reach the longest day. Then they start to get shorter, and the exact middle of that process is the autumnal equinox. Almost exactly 12 hours of daylight and 12 hours of night, because neither hemisphere of the Earth is facing towards the sun or away from it. 

The Earth’s tilt is 90 degrees off from the sun instead of straight towards it as it is on the solstices. Cultures around the world have traditionally celebrated the solstices and equinoxes, and for us they’re most important as the days that mark the official changes of seasons. On our autumnal equinox, Sept. 22, summer will officially end and fall will officially begin.