The summer solstice occurs at the moment the earth's tilt toward from the sun is at a maximum. Therefore, on the day of the summer solstice, the sun appears at its highest elevation with a noontime position that changes very little for several days before and after the summer solstice. In fact, the word solstice comes from Latin solstitium or sol (the sun) + -stit-, -stes (standing).  The summer solstice occurs when the sun is directly over the Tropic of Cancer, which is located at 23.5° latitude North, and runs through Mexico, the Bahamas, Egypt, Saudi Arabia, India, and southern China.  The sun will be directly over the Tropic of Cancer at  529 am MDT on June 21, 2010.  For every place north of the Tropic of Cancer, the sun is at its highest point in the sky and this is the longest day of the year.

 

A pinhole camera (no lens and a single small aperature) can be effectively used to document the change in elevation of the sun during the year.  The image below is a solargraph made with a pinhole camera, in which the path of the sun as it crosses the sky is captured for an extended period.  It was made in Tijeras, and depicts the period from the summer solstice (highest streaks) to the winter solstice (lowest streaks) in 2009.  

 
 Becky Ramotowski

The Relationship between Length of Day and Temperature

Some of you may be wondering why the longest day of the year is not normally the warmest day of the year. There is a lag between the longest day of the year and the warmest average temperatures not only across New Mexico, but for most of the mid and high latitude locations. In the graph at the end of this section, the length of daylight is plotted in red while the average daily temperature is plotted in blue, and the date of the solstice is depicted by yellow shading. The sun angle is high before and after the summer solstice with a maximum number of daylight minutes. As the sun begins to move lower in the sky the length of daylight decreases.

In Albuquerque, the maximum daily temperature occurs nearly 3 weeks later in mid July. This lag in temperature occurs because even though the minutes of daylight are decreasing, the earth's surface and atmosphere continues to receive more energy than just what it receives from the sun. Average temperatures continue to climb until the sun drops lower in the sky. While the effect is evident in the daily temperature plot, it is more readily apparent by looking at changes in the monthly average temperature (below and to the right). In Albuquerque, July averages 3.7F degrees higher than June, with August also warmer than June by 1.3F, even though the length of days in August is considerably less than the length of days in June.
Another interesting fact depicted in the graph below is that while the summer solstice is the longest day of the year, with 14 hours and 31 minutes of daylight in Albuquerque, it is just several seconds longer than days on either side of the solstice. In fact, the U.S. Observatory lists the length of daylight in Albuquerque as 14 hours and 30 minutes or more from June 13th through June 28th (2010).
 

The Solstices, Equinoxes and 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.

graphic depicting earth's orbit around the sun

The summer solstice occurs when the sun is directly over the Tropic of Cancer, which is located at 23.5° North, and runs through Mexico, the Bahamas, Egypt, Saudi Arabia, India, and southern China.  Because of the Earth's tilt, the sun will be directly over the Tropic of Cancer at 529 am MDT on June 21, 2010.  For every place north of the Tropic of Cancer, the sun is at its highest point in the sky and this is the longest day of the year. The winter solstice occurs when the sun is directly over the Tropic of Capricorn, which is located at 23.5° south of the equator. The sun will be directly over the Tropic of Capricorn at 438 pm on December 21, 2010.

There are two times of the year when the Earth's axis is tilted neither toward nor away from the sun, resulting in an equal amount of daylight and darkness at all latitudes.  These events are referred to as equinoxes and occur near March 21 (Vernal Equinox) and near September 21 (Autumnal Equinox).  At the equator, the sun is directly overhead at noon on the two equinoxes. The Vernal Equinox occurred at 1132 am MDT on March 20, 2010.  The Autumnal Equinox will occur at 909 pm MDT on September 22, 2010.


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