The seasons on Earth willofficially change Wednesday, heralding their shifting nature with an astronomicalfeat: the autumnal equinox.
On Sept. 22, at 11:09 p.m. EDT (8:09p.m. PDT), the fallseason will begin in the Northern Hemisphere while the Earth's Southern Hemisphereresidents ring in their spring. This date ? one of two each year ? ?is called anequinox, from the Latin for "equal night," alluding to the fact thatday and night are then of equal length worldwide.?
But this is not necessarily so.
The not-so-equal equinox
The definitionof the equinox as being a time of equal day and night is a convenientoversimplification.?
For one thing, it treats night assimply the time the sun is beneath the horizon, and completely ignorestwilight. If the sun were nothing more than a point of light in the sky and if Earthlacked an atmosphere, then at the time of an equinox the sun would indeed spendone half of its path above the horizon and one half below.
But in reality, atmospheric refraction raises the sun?s diskby 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?relooking at an optical illusion. [Top10 Extreme Planet Facts]
It is actually completely belowthe horizon. So from our point of view, the day on an equinox appears longerthan it actually is. This illusion means that the appearance of equal day andnight, from a skywatcher?s view, will come several days later.
In addition to refraction hasteningsunrise and delaying sunset, there is another factor that makes daylight longerthan night at an equinox: Sunrise and sunset are defined as the times when thefirst 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 yournewspaper?s almanac or weather page on Wednesday of this week to look up thetimes of local sunrise and sunset, you?ll notice that the durationof daylight from sunrise to sunset ?still lasts a bit more than 12 hours ? notexactly 12 as the term "equinox" suggests.?
In New York City, for instance,sunrise is at 6:43 a.m. and sunset comes at 6:54 p.m. So the amount of daylightis not 12 hours, but rather 12 hours and 11 minutes.
Not until Sept. 26, will the daysand nights truly equal (sunrise is at 6:47 a.m., sunset coming 12 hours later).
And at the North Pole, the suncurrently is tracing out a 360-degree circle around the entire sky, appearingto skim just above the edge of the horizon.
At the moment of this year?sautumnal equinox, it should theoretically disappear completely from view, andyet its disk will still be hovering just above the horizon. But it will take another52 hours and 10 minutes later until the last speck of the sun?s upper limbfinally drops completely out of sight.
This strong refraction effect alsocauses the sun?s disk to appear oval when it is near the horizon. The amount ofrefraction increases so rapidly as the sun approaches the horizon, that itslower limb is lifted more than the upper, distorting the sun?s disk noticeably.
Equinoxes on other planets?
The word equinox is also used foreither of the two points in the sky where the sun is located on the first dayof spring and autumn. These points are the intersections of the ecliptic withthe celestial equator, but they're not necessarily confined to Earth.
To determine when another planetexperiences equinoxes, we need to know its axial tilt. The Earth?s axis istilted at a 23.44-degree angle.?
The planet whose axial tilt is mostsimilar to ours is Mars, whose axis is tilted at a 25.19-degree angle. TheMartian autumnal equinox this year comes on Nov. 12, while the spring equinoxoccurs next year on Sept. 13.
Mercury has no significant axialtilt, so the sun (which appears about 2 1/2 times larger than here on Earth)always shines directly down on Mercury's equator.
In contrast, Uranus has an axialtilt of 97.77 degrees, giving it seasonal changes completely unlike those ofthe other major planets.? Uranus rotates more like a tilted rolling ball.
At the time of a Uranian solstice,one pole continually faces the sun while the other pole faces away.? Each polegets around 42 years of continuous sunlight, followed by 42 years ofdarkness.
Near the time of the equinoxes, thesun faces the equator of Uranus giving a period of day-night cycles similar tothose seen on most of the other planets.
Uranus reached its most recentequinox in 2007. The next will come in 2049.?
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Joe Rao serves as an instructor andguest lecturer at New York's Hayden Planetarium. He writes about astronomy forThe New York Times and other publications, and he is also an on-camerameteorologist for News 12 Westchester, New York.