The notion that an equinox is precisely 24 hours long is a common misconception, stemming from the literal meaning of the word as "equal night." While it is true that day and night are nearly equal in length on these dates, the reality is a bit more complex. The equinox is less a point in time and more a condition, and the exact duration of daylight on those days varies significantly depending on location and atmospheric factors.
Defining the Astronomical Event
An equinox occurs when the plane of Earth's equator passes through the center of the Sun. This happens twice a year, around March 20 or 21 (the vernal equinox) and September 22 or 23 (the autumnal equinox). During this moment, the Sun is positioned directly above the Earth's equator, and theoretically, every place on Earth should experience exactly 12 hours of daylight and 12 hours of night. However, this "equal" split is a geometric ideal that doesn't account for the physical properties of our planet and its atmosphere.
The Role of Atmospheric Refraction
The primary reason the equinox isn't 24 hours of exact darkness and light is atmospheric refraction. Earth's atmosphere bends, or refracts, sunlight. This means that we can see the Sun for several minutes before it actually rises above the horizon and for several minutes after it sets. This optical effect effectively lengthens the day. Consequently, the moment the upper limb of the Sun appears on the horizon in the morning occurs earlier than geometric calculations would suggest, and the moment it disappears in the evening happens later.
The "Equal Night" Misconception
Because of refraction, the actual day—the period from the upper edge of sunrise to the upper edge of sunset—is longer than 12 hours during the equinox. To find the day with exactly 12 hours of daylight, you need to look a few days before the official equinox date. This discrepancy is more pronounced the farther one is from the equator, where the Sun's path is lower in the sky. The term "equinox" refers to the balance of the Sun's declination, not the precise division of day and night for a specific location.
Geographic Variations
The difference between the geometric equinox and the observed day length varies based on latitude. Near the equator, where the Sun rises almost perpendicularly, the day and night are much closer to equal. However, at higher latitudes, the Sun rises and sets at a much shallower angle, which increases the duration of twilight and further extends the period of daylight. For example, a location at 40 degrees latitude might experience a day that is 12 hours and 8 minutes long on the equinox, while a location at the poles experiences a much longer period of twilight and daylight transition.
Defining the Equinox by the Sun's Center
If we define the equinox strictly as the moment the geometric center of the Sun crosses the celestial equator, the day is not 24 hours long. The equal day and night occur when the center of the Sun is exactly on the horizon. Due to refraction lifting the image of the Sun, this specific moment happens when the geometric Sun is still about 0.83 degrees below the horizon in the morning and leaves the horizon in the evening. This creates the illusion of a slightly longer day, reinforcing the idea that the equinox is a range of balanced light rather than a strict 24-hour condition.
