Welcome to Fall! It's the Equinox, but why aren't we seeing equal day and night?
Equinox vs Equilux: When do we see equal day and night this fall?
Despite the name, our days of equal night and day do not fall on the equinoxes. So, when do they occur and why?
On September 22, 2025, at exactly 2:19 p.m. EDT, the Sun will be directly above the equator, marking the date of the Equinox. This will be the start of astronomical Fall for us here in the northern hemisphere, while the southern hemisphere will see the start of astronomical Spring.
During the day, itself, there is little that occurs to note the occasion. If you were using a solargraph to track the path of the Sun each day between the solstices, you could note that it was halfway between its highest and lowest point in the sky (although you'd have to wait until the winter solstice to develop the picture to see it).
Or, if you happened to be under the subsolar point at the time — the location on Earth directly 'under' the Sun — you could look down and see one of the smallest shadows you'll ever cast.
The location of the subsolar point (yellow dot) on Earth during the 2025 Fall Equinox, on the equator, in the Pacific Ocean, to the west of northern South America. (NASA/DSCOVR/EPIC, Scott Sutherland)
Although the name equinox literally means 'equal night', and implies that there is equal day and night on that day, that isn't the case.
No matter where you are on Earth on the 22nd, you will see between 6 and 16 'extra' minutes of daylight on that day. Here in Canada, we will see 9-10 minutes of 'extra' daylight, depending on how far north you are.
The actual day of equal day and night, aka Equilux, for Canada happens on September 25 for communities throughout the southern half of the country. For the northern part of the provinces and in the territories, it will be on Sep 24.
So, what's going on here?
Both the northern and southern hemispheres are equally lit by the Sun on the equinox. (NOAA)
On September 22, both the northern and southern hemispheres will be lit equally by the Sun. So, why doesn't equal day and night come with that?
There are three main reasons for this. The first is due to how we define 'sunrise' and 'sunset'. The second is the difference in the Sun's position in our sky based on how far away from the equator we are. The third is a physical effect caused by Earth's atmosphere.
Each day, sunrise is recorded as the exact moment that the edge of the Sun's disk crests the eastern horizon. Every night, sunset is the time when the Sun's disk has completely disappeared beyond the western horizon.
(Stellarium/Scott Sutherland)
If we wanted equal day and night on the equinoxes, we would have to change those definitions so that they were recorded at the exact moment when the Sun's centerpoint touched the horizon.
However, even then, that would only give equal day and night on the equinoxes to anyone living on the equator. Since the angle of the Sun in the sky affects the timing of sunrise and sunset, anyone living at higher latitudes would still have to wait an extra day or two after the fall equinox.
Plus, we'd still need to account for the atmosphere, as it also shifts the timing of sunrise and sunset, due to diffraction.
When the Sun is near the horizon, rays of sunlight are diffracted by the atmosphere just enough that the Sun's position in the sky appears slightly shifted — we will see it just before it actually peaks above the eastern horizon at dawn, and we will still see it for a short time after it has slipped beyond the western horizon at dusk. This is enough to add a few minutes of daylight to each day. Exactly how much daylight is added depends on the weather, specifically the atmospheric pressure, temperature, and humidity.