Saturday's Fall Equinox: Earth's Season Change Explained

On Saturday, Sept. 22, the sun crosses the celestial equator heading south. This is the autumnal equinox in the northern hemisphere and the vernal equinox in the southern hemisphere. CREDIT: Geoff Gaherty/Starry Night Software/SPACE.com View full size image

When is the first day of the northern autumn in 2012? A carefully worded answer is that on Saturday, Sept. 22), fall begins astronomically in the Northern Hemisphere, while spring begins in the Southern Hemisphere. The exact time of the event will be 10:49 a.m. EDT (1449 GMT).

This season-changing equinox, like a similar event on March 20 that heralded northern spring, is gets its name 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 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. 

Not so equal

If the sun were nothing more than a point of light in the sky, and if the 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, interference by Earth's atmosphere (which refracts the sun's light) raises the sun’s disk by more than its own apparent diameter as it rises or sets. [Earth's Equinoxes & Solstices (Infographic)]

So when we see the sun as a reddish-orange ball just sitting on the horizon, we’re actually looking at an optical illusion. It is still completely below the horizon.

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

And this is why if you check your newspaper’s almanac or weather page for Saturday this weekend for the times of local sunrise and sunset, you’ll notice that the duration of daylight, or the amount of time from sunrise to sunset, still lasts a bit more than 12 hours, and not exactly 12 as the term “equinox” suggests. [How An Equinox Looks From Space (Video)]

In New York City, for instance, sunrise is at 6:43 a.m. EDT and sunset comes at 6:54 p.m.  EDT So the amount of daylight is not 12 hours, but rather 12 hours and 11 minutes.  Not until Wednesday, Sept. 26, will the days and nights truly be equal (sunrise is at 6:47 a.m. EDT, sunset coming 12 hours later).

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.

Not until 52 hours and 10 minutes later will the last speck of the sun’s upper limb 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.

Not as dark as it seems

Certain astronomical myths die hard.

 One of these is that that the entire arctic region experiences six months of daylight and six months of darkness. Often, “night” is simply considered to be when the sun is beneath the horizon, as if twilight didn’t exist.

This fallacy is repeated in innumerable geography textbooks, as well as travel articles and guides. But twilight illuminates the sky to some extent whenever the sun’s upper rim is less than 18 degrees below the horizon. This marks the limit of astronomical twilight, when the night sky is indeed totally dark from horizon to horizon.

There are two other types of twilight.

Civil (bright) twilight exists when the sun is less than 6 degrees beneath the horizon. It is loosely defined as when most outdoor daytime activities can be continued. Some daily newspapers provide a time when you should turn on your car’s headlights. That time usually corresponds to the end of civil twilight.

So even at the North Pole, while the sun disappears from view for six months beginning on Sept. 25, to state that “total darkness” immediately sets in is hardly the case! In fact, civil twilight does not end there until Oct. 8. 

When the sun drops down to 12 degrees below the horizon it marks the end of nautical twilight, when a sea horizon becomes difficult to discern. In fact, at the end of nautical twilight most people will regard night as having begun. At the North Pole we have to wait until Oct. 25 for nautical twilight to end.

Finally, astronomical twilight — when the sky indeed becomes completely dark — ends on Nov. 13. It then remains perpetually dark until Jan. 29 when the twilight cycles begin anew. So at the North Pole the duration of 24-hour darkness lasts almost 11 weeks, not six months. 

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.