BATON ROUGE, LA (WAFB) - Wednesday, March 20 at 6:02 AM (CDT) – we officially kick-off spring! Wednesday is the Spring (Vernal) Equinox, which means 12 hours of daylight and 12 hours of dark, right? Well, not exactly.
If it's called the equinox: why do the sunrise/sunset tables show a few minutes more daylight on the Equinox than exactly 12 hours? Which means "nighttime" is less than 12 hours. The presumption that Equinox means exactly "12-and-12" is based on astronomical geometry (the relationship between the Earth and Sun), and it would be essentially correct if there were no atmosphere.
First off, this "12-and-12" rule ignores the indirect sunlight we get before sunrise and after sunset -- the period officially referred to as "twilight." And twilight is a result of sunlight "scattered" by the atmosphere even when the solar disk is below the horizon. (The moon, for example, with no atmosphere, has no twilight periods.) In fact, there are three types of twilight -- Civil Twilight, Nautical Twilight and Astronomical Twilight -- each effectively defined by the amount of indirect lighting available. (We'll leave it to you to dig up the details of these three definitions.)
But even if we exclude "twilight," the daylight period during the Equinoxes (Remember, there are two each year: Spring/Vernal and Fall/Autumnal) is longer than 12 hours. There are two basic reasons.
First, since the Sun is a "disk" and not a simply a point in the sky, the official "12-and-12" would occur when the solar disk was evenly split through the middle while "sitting" on each horizon -- in the morning and the evening. But the official definition of sunrise/sunset is the time when the upper edge of the solar disk just touches the horizon in the morning as it "rises" and when the upper sits level with the flat horizon as it "sets" in the evening. The time it takes for the Sun to "move" from the solar mid-point to the upper edge (relative to the horizon) adds a couple of minutes to the official "daylight" period at the start and the end of the day.
Second, the Earth's atmosphere refracts (bends) the Sun's direct-beam light (the light from the solar disk). Because of the refraction, we actually see the top edge of the solar disk before that edge has actually risen above the horizon (from a purely geometric perspective). So sunrise "appears" before it would actually occur if there were no atmosphere. Likewise in the evening, atmospheric refraction keeps the solar disk above the horizon a little longer, delaying the time when the solar disk would disappear from view without the atmosphere's refractive effect.