The definition of the equinox as being a time of equal day and night is a convenient oversimplification.
For one thing, it treats night as simply the time the sun is beneath the horizon and completely ignores twilight. If the sun were nothing more than a point of light in the sky, and if thelacked an atmosphere, then at the time of an equinox the sun would indeed spend one half of its path above the horizon and one half below. But in reality, atmospheric refraction raises the sun's disk by more than its own apparent diameter while it is rising or setting.
In St. Louis, Missouri, for instance, sunrise is at 6:48 a.m. local time with sunset coming at 6:57 p.m. local time. So, the amount of daylight is not 12 hours, but rather 12 hours and 9 minutes. It won't be until Sept. 25 that the day and night are truly equal . And at the North Pole, the sun currently is tracing out a 360-degree circle around the entire sky, appearing to skim just above the edge of the horizon. At the moment of this year's autumnal equinox, it should theoretically disappear completely from view. And yet its disk will still be hovering just above the horizon. It's not until 75 hours and 29 minutes later that the last speck of the sun's upper limb will finally drop completely out of sight.
This strong refraction effect also causes the sun's disk to appear oval when it is near the horizon. The amount of refraction increases so rapidly as the sun approaches the horizon, that its lower limb is lifted more than the upper, distorting the sun's disk noticeably.