Friday, September 21, 2012

Fall Back? 2012 Autumn Equinox and Daylight Savings Time


CAPE CANAVERAL, Florida -- The 2012 Autumn Equinox is on Saturday September 22, 2012.  The Autumn Equinox, as pictured in this NASA Earth Observatory photo, is caused by the tilt of the earth's rotating axis. 
The Autumn Equinox and changing of the seasons occurs on September 22, 2012 at 10:49 a.m. Eastern Daylight Time. (The image above is a few days early.) 
Equinox means "equal night" in Latin, capturing the idea that daytime and nighttime are equal lengths everywhere on the planet. That is true of the Sun's presence above the horizon, though it does not account for twilight, when the Sun's rays extend from beyond the horizon to illuminate our gas-filled atmosphere.


It is the equal amount of day and night in the northern hemisphere for locations like Melbourne, Florida, but also in the southern hemisphere for locations like Melbourne, Australia.

The apparent change in location of the sun and moon, marks important dates for hunting, fishing, and farming.

Around 6 a.m. local time each day, the Sun, Earth, and any geosynchronous satellite form a right angle, affording a nadir (straight down) view of the terminator, the edge between the shadows of nightfall and the sunlight of dusk and dawn. The shape of this line between night and day varies with the seasons, which means different lengths of days and differing amounts of warming sunshine. (The line is actually a curve because the Earth is round, but satellite images only show it in two-dimensions.) 

On March 20 and September 20, the terminator is a straight north-south line, and the Sun is said to sit directly above the equator. On December 21, the Sun resides directly over the Tropic of Capricorn when viewed from the ground, and sunlight spreads over more of the Southern Hemisphere. On June 21, the Sun sits above the Tropic of Cancer, spreading more sunlight in the north and turning the tables on the south. The bulge of our spherical Earth blocks sunlight from the far hemisphere at the solstices; that same curvature allows the Sun’s rays to spread over more area near the top and bottom of the globe. 


Of course, it is not the Sun that is moving north or south through the seasons, but a change in the orientation and angles between the Earth and its nearest star. The axis of the Earth is tilted 23.5 degrees relative to the Sun and the ecliptic plane. The axis is tilted away from the Sun at the December solstice and toward the Sun at the June solstice, spreading more and less light on each hemisphere. At the equinoxes, the tilt is at a right angle to the Sun and the light is spread evenly.

2012 Autumn Equinox and 2012 Daylight Savings Time

Naturally, many people think that with the changing of the seasons comes the changing of their clocks at the beginning of Fall 2012.  But this is not the case.  
It is true however, that a helpful way to remember whether to set our clocks ahead or behind one hour during the daylight savings time is to "Fall Back" and "Spring Ahead."
The beginning of Fall 2012 does not mean the end of 2012 Daylight Savings Time.  2012 Daylight Savings Time occurs after the 2012 Autumn Equinox when days become shorter and shorter heading closer to the Winter Solstice.

So if you are asking, "When does daylight savings time end?" "Do we change clocks when Fall starts?"  The answer is the date Daylight Savings Time (DST) will end for Florida is on Sunday November 4, 2012.

This is one week before Veterans Day, formerly known as Armistice Day, which is on Sunday November 11, 2012 but will be observed this year on Monday November 12, 2012 . 
 

The rest of the United States will also turn their clocks back on November 4, 2012 with the exception of Hawaii and Arizona because those states do not practice Daylight Savings Time.
 

This means that we gain (not lose) an hour when we turn our clocks back an hour to 1:00 a.m. when our clocks reach 1:59 a.m. on Sunday.

Daylight Savings Time 2013 begins again on Sunday March 10, 2013 and will end Sunday November 3, 2013.
Article based in part on NASA publications.