On Saturday, March 17th – St. Patrick’s Day — for many locations across the United States, the length of day and night will be equal. For New York City, for instance, the Sun will be above the horizon for exactly 12 hours, rising at 7:05 a.m. and setting at 7:05 p.m. But wait! How can this be? The date we commonly associate with equal days and equal nights is the vernal equinox, which doesn’t occur until the following Tuesday, March 20th, when the Sun will stand directly over the equator at 12:15 p.m. Eastern Time. On that day, the length of daylight will be 12 hours and 8 minutes long. So, why the apparent discrepancy and what’s the explanation? Blame it on something called “atmospheric refraction.”
How Atmospheric Refraction Affects Sunrise and Sunset
First, sunrise and sunset are measured when the Sun’s uppermost rim first appears above or last disappears below the horizon. Our atmosphere also acts like a lens to bend or refract the Sun’s image above the horizon when it’s not really there. So when you see the rising or setting Sun sitting on the horizon, keep in mind that what you’re seeing is an illusion! In actuality, the Sun is still below the horizon. This effect, ends up lengthening the amount of daylight by about 6 to 8 minutes.
So thanks to atmospheric refraction we receive 12 hours of sunlight a few days before the date of the vernal equinox.