Ah, autumn: falling leaves, warm mugs of tea, and astronomy! Though the autumn equinox is primarily associated in our minds with the advent of fall, it’s actually determined by a very specific astronomical event, not the whims of weather. The equinox occurs when the “subsolar point”—the spot on the Earth where the sun is directly overhead—lands exactly on our planet’s equator. This happens just twice a year, once in March (spring equinox) and once in September (fall equinox). In 2014 it occurred (for the second time) at 10:29 pm Eastern time on September 22; in 2015 it occurred at 4:21 a.m. (EDT) on September 23. At that moment, Earth was positioned such that its axis was tilting neither toward nor away from the sun.
It’s the 23-degree tilt of the Earth that gives us the seasons; now, as the Earth continues to move in its elliptical path around the sun, the Northern Hemisphere will begin to point further and further away from the sun, while the southern half of the planet comes more and more into the light. Days in the north get shorter and shorter until they reach their minimum on the winter solstice (which falls on December 21, at 6:03 pm Eastern time).
You might think that Earth’s position in its orbit around the sun must play a role in the timing of the equinox—but it actually doesn’t! Perihelion, the point at which our planet is closest to the sun, occurs in early January; it doesn’t coincide with the equinoxes or the solstices. In fact, at perihelion, Earth tends to be colder than it is at aphelion, the orbital point furthest from the sun. This is primarily due to the fact that the Northern Hemisphere contains more land than the Southern Hemisphere. Land heats up more easily than water, but at perihelion the land-rich half of our planet is tilted away from the sun. When the Northern Hemisphere is tilted towards the sun, the easily warmed land masses raise the average temperature of the globe.
Also, Northern Hemisphere dwellers can be grateful that their spring and summer occur when the Earth is at aphelion; planets move slower at the points in their orbit further from the sun, so the seasons are not all of equal length. The time between the spring equinox (from the Northern perspective) and the summer solstice, and the summer solstice and the autumn equinox, are 92.8 and 93.6 days, respectively; the time between the autumn equinox and the winter solstice is just 89.8 days, and between the winter solstice and the spring equinox just 89 days. So Northerners enjoy a slightly shorter fall and winter—and longer spring and summer—than Southern Hemisphere residents.
Although “equinox” is Latin for “equal night,” neither the spring nor fall equinoxes coincide with days and nights that are exactly 12 hours long. Why not? Well, first off, day length on a single date naturally varies across various latitudes—places on the equator will always be closer to having nights and days of equal length. But there are other factors that cause equal days and nights to actually arrive slightly later than the autumn equinox (even at the equator). Sunlight is bent by Earth’s atmosphere, so the sun always appears to rise a bit sooner than it actually does. And while sunrise and sunset are determined when the sun’s edge crosses the horizon, the equinox is marked by the position of the center of the sun. These tiny offsets end up making the date of the equinox not quite line up with the meaning of its name.
Updated 9/23/2015 to reflect the time and date of the 2015 autumnal equinox.