Fall officially begins in the Northern Hemisphere this week with the September equinox — supposedly the one day of the year when day and night are equal. There’s just one problem: they’re not. Thanks largely to Earth’s atmosphere, daylight will still last several minutes longer than nighttime, with the true “equal night” — known as the equilux — arriving a few days later.

The September equinox occurs at 8:05 p.m. EDT on Tuesday, Sept. 22, marking the beginning of astronomical fall in the Northern Hemisphere and astronomical spring in the Southern Hemisphere.

Because the equinox occurs at one exact moment worldwide, that translates to 7:05 p.m. CDT, 6:05 p.m. MDT, 5:05 p.m. PDT, 4:05 p.m. AKDT and 2:05 p.m. HST on Tuesday, Sept. 22.

At that moment, the sun crosses the celestial equator heading south. Neither hemisphere is tilted significantly toward the sun, with Earth effectively presenting itself side-on to our star. It supposedly means that day and night are equal — but, in practice, they are not.

Why Aren’t Day And Night Equal?

The word “equinox” comes from the Latin aequus , meaning “equal,” and nox , meaning “night.” That makes it tempting to assume that everywhere on Earth experiences exactly 12 hours of daylight and 12 hours of darkness on the equinox. However, that’s not quite what happens. Earth’s atmosphere refracts — or bends — sunlight. According to Stuart Clark in The Guardian , this lets us see the sun when it is technically still below the horizon before sunrise and again after it has dipped below the horizon at sunset. It’s a little like looking at an object through a glass of water: its apparent position is slightly displaced. In this case, atmospheric refraction effectively extends daylight by several minutes.

There’s another factor. Sunrise and sunset are defined using the sun’s upper edge rather than its center, adding still more daylight.

Equilux: When Day And Night Are Actually Equal

That comes a few days later in the Northern Hemisphere, on a date known as the “equilux” — Latin for “equal light.” The precise date depends on latitude. Across much of the U.S., the equilux occurs a few days after the September equinox, according to Timeanddate.com , around Sept. 25 or 26. After that, nights become longer than days across most of the Northern Hemisphere, continuing to lengthen until the December solstice — the shortest day and longest night of the year.

The Equinox Aurora Effect

The September equinox may also have consequences far above Earth. Geomagnetic activity tends to peak around the March and September equinoxes, making these periods statistically favorable for displays of the northern and southern lights. One explanation is the Russell-McPherron effect, which describes how the geometry of Earth’s magnetic field relative to the interplanetary magnetic field carried by the solar wind becomes particularly favorable around the equinoxes. That orientation can enhance magnetic reconnection — effectively allowing more energy from the solar wind to enter Earth’s magnetosphere — and increase the chances of geomagnetic storms and intense auroras. It doesn’t mean an aurora will definitely occur on the equinox itself — that depends on solar activity — but September is traditionally one of the better months for aurora watchers.