Falling into Autumn | What’s an Equinox Scientifically?

The Autumnal Equinox. To most, it’s an indicator of the approach of cooler weather, shorter days, and pumpkin-based beverages. We seldom think of the first day of autumn in context with the heavens and space, but it’s an important marker of our planet’s annual journey around our star. The equinox is the point at which the sun and our celestial equator (the projection of Earth’s terminator – dividing line between daytime and nighttime) passes directly through Earth’s north and south poles. If you’re a fan of right angles in cosmic geometry, this is your moment. The equator and terminator are perpendicular to each other only for a single day, twice a year.

The date of the autumnal equinox varies from year to year. While it typically falls on the 22nd or 23rd of September, it occasionally falls on the 21st or 24th thanks to a disconnect between our Gregorian and solar calendars. While the Gregorian calendar defines a year as 365 days, the Earth actually takes 365.25 days to orbit the sun, meaning the equinox will occur six hours later than it did the year before. Leap years reset the equinox’s continual march forward on our calendars, which is why the equinox remains in the same little corner of September every year.

Along with our planet’s two annual solstices, the equinoxes signify a change in the seasons. Yes, yes — I know you knew that already, but the reason why the equinox marks a change in season might be slightly more nuanced than you remember. Our planet is tilted at about 23.5°; this tilt, known as obliquity, changes our exposure to the sun’s radiation depending on location and time year. During the northern hemisphere’s summer, Earth’s north pole is pointed towards the sun. During the northern hemisphere’s winter, it’s pointed away. It’s a common misconception that the hemisphere that’s tilted towards the sun is warmer because it’s closer to the sun, and being that much closer to our greatest source of heat accounts for the difference in temperature. This enduring “explanation” has a fatal flaw that few of its proponents take into account: our planet’s orbit is actually several million miles closer to the sun in January than it is during the summer! If the proximity of the Earth to the sun determined our seasons, we’d all be sweltering in January.

The actual reason that the hemisphere pointed towards the sun experiences summer is due to the angle of the sunlight that’s reaching it. Sunlight hitting the hemisphere that’s tilted towards the sun receives the most concentrated amount of light, while sunlight hitting the hemisphere tilted away is more diffuse. The same total amount of light is hitting both hemispheres, but in the winter that light is much more spread out. In addition to this effect, higher sun angles result in longer days, so more total energy is heating your hemisphere in the summer than in the winter.

Sunlight reaches both hemispheres evenly during the equinoxes, making them a temperate midway point between the two extremes of winter and summer. It’s easy to see why they’ve historically been moments of human celebration; being neither too hot nor too cold, the time around an equinox is ideal for congregating out of doors. The Neopagan harvest festival of Mabon, celebrating the gathering of crops and the fertility of Earth’s soil, takes place during the autumnal equinox. China celebrates the Moon Festival, which happens during the harvest moon, or the full moon that occurs closest to the equinox (this year’s harvest moon is September 24th). Don’t know any Pagans? Not in a position to pack off to China on such short notice? No problem. There are endless ways you can still celebrate and enjoy this cosmic event. Get your friends together for a picnic dinner, give thanks for the edible bounty in your life, and have a howl or two at the harvest moon!