A not-so-equal equinox: Why day beats night on the 1st day of spring The spring equinox marks the first day of spring today, but daylight actually lasts several minutes longer than night. Here's why the equinox doesn't appear perfectly equal. On March 20 at 10:46 a.m. Eastern Daylight Time, the vernal equinox occurs. At that moment, the sun aligns directly over the equator, casting equal light on both hemispheres. However, this moment is not the exact midpoint of day and night. For North Americans, spring officially begins on March 20, a shift from the traditional March 21. This change is due to the Earth’s elliptical orbit and the structure of the Gregorian calendar. From 1980 to 2102, the equinox will never fall later than March 20. In 2028, it will even occur on March 19 for the Western Hemisphere. The idea that days and nights are equal on the equinox is a common misconception. In reality, daylight is longer than darkness by several minutes. For example, in Pittsburgh, the equinox does not mark equal day and night but rather Saint Patrick’s Day. This discrepancy arises because sunrise and sunset times are calculated based on when the sun’s top edge appears on the horizon, not its center. The sun’s apparent diameter, about half a degree, contributes to this effect. Additionally, Earth’s atmosphere bends sunlight, creating an optical illusion. The U.S. Naval Observatory accounts for this refraction, noting that the sun’s geometric center is more than eight-tenths of a degree below the horizon at sunrise. As a result, we see the sun for a few minutes before it rises and after it sets. This atmospheric lens effect adds approximately six to seven minutes to daylight each day.#us_naval_observatory #pittsburgh #spring_equinox #earth_elliptical_orbit #joe_rao
