Earth's Seasonal Change: Not-So-Equal Equinox

The seasons on Earth will officially change Wednesday, heralding their shifting nature with an astronomical feat: the autumnal equinox.


On Sept. 22, at 11:09 p.m. EDT (8:09 p.m. PDT), the fall season will begin in the Northern Hemisphere while the Earth's Southern Hemisphere residents ring in their spring. This date – one of two each year – is called an equinox, from the Latin for "equal night," alluding to the fact that day and night are then of equal length worldwide but this is not necessarily so.

The not-so-equal equinox


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 Earth lacked 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. Thus, when we see the sun as a reddish-orange ball just sitting on the horizon, we're looking at an optical illusion. [Top 10 Extreme Planet Facts]


It is actually completely below the horizon. So from our point of view, the day on an equinox appears longer than it actually is. This illusion means that the appearance of equal day and night, from a skywatcher's view, will come several days later.


In addition to refraction hastening sunrise and delaying sunset, there is another factor that makes daylight longer than night at an equinox: Sunrise and sunset are defined as the times when the first or last speck of the sun's upper limb is visible above the horizon – not the center of the disk.


This is why, when you check your newspaper's almanac or weather page on Wednesday of this week to look up the times of local sunrise and sunset, you'll notice that the duration of daylight from sunrise to sunset still lasts a bit more than 12 hours – not exactly 12 as the term "equinox" suggests.