Happy Equinox!!! I celebrated by baking some sourdough and making a wonderful breakfast (equinox was officially 6:30am PST)
It is widely believed that on the Equinoxes there is equal day and night all around the world. After all, "Equinox" literally means "equal night", giving the impression that the night and day on the equinox are exactly the same length; 12 hours each. But this isn't entirely accurate.
Even if the name suggests it and it is widely accepted, it is not entirely true that day and night are exactly equal on the equinox – only nearly. Earth spins around its own axis approximately every 24 hours (a sidereal day is 23 hours, 56 minutes, 4 seconds). The time it takes Earth to orbit once around the Sun is around 365 days, 5 hours, 48 minutes and 45 seconds (365.242189 days). And, like two spinning tops connected at the tips, it also wobbles around on its axis, making a complete rotation every 26,000 years (axial precession).
Earth is actually tilted at an angle of around 23.4 degrees toward the celestial pole, which is a certain point in the sky. As Earth makes its yearly orbit, one hemisphere faces the sun more than the other, the side that has summer.
On any other day of the year, the Earth's axis tilts a little away from or towards the Sun. However, the equinoxes marks the exact moment, twice a year, when the Earth's axis is not tilted toward or away from the Sun at all. However, the axial tilt of around 23.4 degrees, remains the same. Even if day and night aren’t exactly equal on the day of the equinox, there are days when day and night are both very close to 12 hours.
However, this date depends on the latitude, and can vary by as much as several weeks from place to place. On the equator the days and nights are similar all year round. The further north you go, the closer the day and night are at the equinox. For example, at 5º North, the closest to a 12 hour day this year will be October 17th, but at 45ºN and above it is September 25th.
Now you may be saying, "Wait, I thought today, September 22nd, was the Equinox!" You're right, but the day will always appear a little longer than 12 hours on the Equinoxes, due to the reasons below.
On the equinoxes, the geometric center of the sun is above the horizon for 12 hours, and you might think that the length of the day (hours of daylight) would be 12 hours too.
However, ‘sunrise’ is defined as the moment the upper edge of the sun's disk becomes visible above the horizon... not when the center of the sun is visible. In the same sense, ‘sunset’ refers to the moment the Sun's upper edge, not the center, disappears below the horizon. The time it takes for the sun to fully rise and set, which is several minutes, is added to the day and subtracted from the night, and therefore the equinox day lasts a little longer than 12 hours. Another reason why the day is longer than 12 hours on an equinox is that the Earth's atmosphere refracts sunlight. This refraction, or bending of the light, causes the Sun’s upper edge to be visible from Earth several minutes before the edge actually reaches the horizon. The same thing happens at sunset, when you can see the sun for several minutes after it has actually dipped under the horizon. This causes every day on Earth – including the days of the equinoxes – to be at least 6 minutes longer than it would have been without this refraction.
The extent of refraction depends on atmospheric pressure and temperature. The calculations in the Sunrise and Sunset Calculator assume the standard atmospheric pressure of 101.325 kPa and temperature of 15° C or 59° F. Good thing we have experts for that.