Tonight at 8:05 p.m. EDT, summer ends. Not gradually, not "sometime this week" — at one precise instant, the center of the Sun will cross the plane of Earth's equator, and astronomical summer will hand the baton to astronomical fall. (If you're reading this south of the equator: congratulations, your spring starts at the same moment.)
Nothing about the weather changes tonight. What changes is geometry — the geometry of our entire planet relative to the Sun. And since this blog is, at its core, about sunlight, the equinox is basically our Super Bowl.
It's all about a 23.5° tilt
Earth's axis is tilted about 23.5° relative to its orbit. That tilt is the whole reason we have seasons — not our distance from the Sun, as most people guess.
In June, the Northern Hemisphere leans toward the Sun: longer days, a higher noon Sun, summer. In December it leans away: shorter days, winter. And twice a year — in March and September — the tilt points neither toward nor away. The Sun's rays strike the equator dead-on.
Here's where we are right now:
Tonight's alignment
At the equinox, two elegant things happen at once:
- The Sun is directly over the equator. At local noon on the equator today, shadows all but vanish.
- The line between day and night runs pole to pole. Every latitude on Earth gets roughly 12 hours of daylight and 12 of darkness — hence the name: equinox, "equal night."
Compare the solstice geometry with tonight's:
A purist's footnote: day and night aren't exactly equal today. Earth's atmosphere bends sunlight, and sunrise and sunset are measured from the Sun's upper edge rather than its center, so we actually get a few extra minutes of daylight — about eight at mid-latitudes. Close enough that the ancients, with no instruments at all, could still find this day.
Two more equinox-only party tricks: today the Sun rises due east and sets due west, a trick it only pulls off twice a year. And from here, the Sun's most direct rays march steadily south of the equator until March.
What it means for your daylight
Here's the part that matters for us sunlight-trackers: around the equinox, day length changes faster than at any other time of year — roughly two to three minutes a day at mid-latitudes. The noon Sun sits a little lower every single day. Astronomical fall runs from tonight until the winter solstice on December 21, and the light only gets scarcer from here.
That makes right now the highest-leverage moment of the year for a morning light habit. The minutes are still there to be caught — but the window is closing, a little more each day. (Meteorologists, by the way, already called it: meteorological fall started September 1. Astronomers just take a little longer to agree the party's over.)
If you track your daylight with Daylight Goals, watch your numbers over the next few weeks. There's something motivating about seeing the season change in your own data — and something grounding about knowing exactly why it's happening: a 23.5° tilt, a 93-million-mile orbit, and one precise instant tonight at 8:05 p.m.
Happy equinox.