The 2013 Equinoxes
 
There are only two times of the year when the Earth's axis is tilted neither toward nor away from the sun, resulting in a "nearly" equal amount of daylight and darkness at all latitudes. These events are referred to as Equinoxes and occurred on March 20th at 1114 pm MDT (Vernal Equinox - the first day of Spring) and will again on September 22nd at 444 pm MDT (Autumnal Equinox - the first day of fall).  The word equinox is derived from two Latin words - aequus (equal) and nox (night). At the equator, the sun is directly overhead at noon on these two equinoxes.  The "nearly" equal hours of day and night is due to refraction of sunlight. or a bending of the light's rays that causes the sun to appear above the horizon when the actual position of the sun is below the horizon.  Additionally, the days become a little longer at the higher latitudes (those at a distance from the equator) because it takes the sun longer to rise and set.  Therefore, on the equinox and for several days before and after the equinox, the length of day will range from about 12 hours and six and one-half minutes at the equator, to 12 hours and 8 minutes at 30 degrees latitude, to 12 hours and 16 minutes at 60 degrees latitude.

The 2013 Solstices

The winter solstice marks the shortest day and longest night of the year.  In the Northern Hemisphere, it occurs when the sun is directly over the Tropic of Capricorn, which is located at 23.5° south of the equator and runs through Australia, Chile, southern Brazil, and northern South Africa. The summer solstice marks the longest day and night of the year. It is directly over the Tropic of Cancer, located at 23.5° north of the equator.  This year, the Northern Hemisphere summer solstice occurred at 1104 pm MDT on June 20th. The winter solstice will occur at 1011 am MST on December 21st. For a complete listing of the dates of the winter and summer solstice's and spring and fall equinox's through 2020, check out this site from the U.S. Naval Observatory.

The Seasons


We all know that the Earth makes a complete revolution around the sun once every 365 days, following an orbit that is elliptical in shape.  This means that the distance between the Earth and Sun, which is 93 million miles on average, varies throughout the year.  During the first week in January, the Earth is about 1.6 million miles closer to the sun. This is referred to as the perihelion.  The aphelion, or the point at which the Earth is about 1.6 million miles farther away from the sun, occurs during the first week in July.  This fact may sound counter to what we know about seasons in the Northern Hemisphere, but actually the difference is not significant in terms of climate and is NOT the reason why we have seasons.  Seasons are caused by the fact that the Earth is tilted on its axis by 23.5°.   This can be seen graphically in the picture below from NASA. To see and use a simulator from the University of Nebraska at Lincoln that illustrates the earth's position from the sun throughout the year, click here.

 


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