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.

Saturn's Auroras Filmed by Hubble

SPACE.com -- Saturn's Auroras Filmed by Hubble

A new Hubble movie shows a rare view of Saturn with its many rings sitting edge-on, providing a dazzling glimpse of the planet's poles and the auroras that dance above them.

It takes Saturn almost thirty years to orbit the sun, with the opportunity to image both of its poles occurring only twice in that period, when the planet reaches its equinox (the point in a planet's orbit when the sun's rays fall perpendicular to the planet's equator).

Saturn hit this position last year, providing Hubble with the unique chance to keep a sustained view of the planet with both its poles in view. The movie they created from the data, collected over several days during January and March 2009, has aided astronomers studying both Saturn's northern and southern auroras.

Given the rarity of such an event, this new footage will likely be the last and best equinox movie that Hubble captures of our planetary neighbor.

As Saturn was approaching its equinox, both poles were equally illuminated by the sun's rays.

NASA Cassini: JPL Video explains Saturn's Equinox

NASA Cassini mission observing Saturn bringing back pictures and scientific data from the ringed planet.

Click the picture to run the JPL video

The pictures below are extracted from the video.



The pictures show the point, during Saturn's orbit of the Sun, when Saturn tilts from South facing to North facing.

At this point, the Equinox, Saturn's rings are edge on to the Sun' rays and because of this, the rings begin to cool.



This picture shows the temperature differential between Saturn's rings normal temperature (120 kelvin) and their temperature at the transient Equinox (40 kelvin).

Cassini Equinox Mission: Scientist for a Day

Cassini Equinox Mission: Scientist for a Day







Shared via AddThis