Thursday, March 21, 2013

Spring Equinox and the Four Seasons

Spring began  at 6:02 A.M. (CDT) on March 20, 2013 in the Northern Hemisphere. The technical name is the "Vernal Equinox" and in the fall it is called the "Autumnal Equinox".  The seasons are reversed in the Southern Hemisphere so this was the start of Fall or autumn for them.

The word equinox is derived from the Latin words meaning “equal night.” Days and nights are approximately equal everywhere and the Sun rises and sets due east and west (making driving on East-West roads difficult at sunrise and sunset).  Due to the way we measure sunrise and sunset, i.e. at what point the sun's disk has risen and set, there are a few extra minutes at the Equinoxes so it is not exactly 12 hours of daylight.

As we head toward the first day of Summer (the Summer Solstice) the sun in Lincoln rises further and further north and sets further and further north on the horizon.  By June the sun is rising in the northeast east and setting in the northwest sky in Lincoln. For those lucky enough to travel to the Arctic during the summer, the sun in June is visible for all 24 hours there and will be directly north around midnight (but very low in the sky).  This area is often referred to as the "Land of the Midnight Sun"

Lincoln will have around 15 hours of daylight by June which is in contrast to our minimum of around only 9 hours of daylight in December (see exact totals below here).. 

Lincoln, NE:  Start of each Season with Sunrise and Sunsets for 2013:
Spring Equinox: 6:02 am CDST, March 20:  Sunrise 7:30 am CDST; Sunset 7:39pm CDST
Summer Solstice: 12:04 am, CDST, June 21:  Sunrise 5:55am CDST; Sunset 9:02pm CDST
Autumnal Equinox:  3:44 pm CDST, September 22:  Sunrise 7:14 am CDST; Sunset 7:23pm CDST
Winter Solstice: 11:11am CST, December 21:  Sunrise 7:48 am CST; Sunset 5:02pm CST

Lincoln's longest daylight length = June 21 =  15 hours and 7 minutes
Lincoln's shortest daylight length = December 21 =  9 hours and 14 minutes

The actual starting times and dates of the start of each of the seasons changes from year to year.
Here is a link to the exact dates and times of the start of the 4 seasons, present out through  future years (NOTE the times are in GMT which is 5 hours later than our time zone during CDST and 6 hours later than us during CST):
Dates and times for the start to the seasons. 

Here is a link to an excellent Internet site to create a table for the sunrise and sunset times for any location for a complete year: Sunrise/Sunset Tables for any Location

 The following diagram (although not to scale since the sun is actually much larger than the earth) shows the concept of the Northern Hemisphere being pointed toward the sun during our Summer and away from the sun during our winter.  This is the cause of the season and results in the variation in daylight during the year and the intensity of solar radiation received at the earth's surface.

Equinox solstice cycle

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