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              Astronomical Information

                   Spring Semester 2015

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              Astronomical Zoo
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The Seasons


The Sun, Time & Seasons

White light photosphere image of Sun
      from SBO heliostat Spring Equinox: The Sun appears to cross directly over the equator, moving in a south-to-north direction, on March 20th at 4:45 PM MDT. On that day the Sun reaches an altitude of 50 deg in Boulder's southern sky, the same elevation as the celestial equator. Discounting the effects of atmospheric refraction, on this day there are equal hours of daytime and nighttime (12 hours each) - hence the name "equinox". The event is also referred to as the "Vernal Equinox", and the location of the Sun in the sky where it occurs is called (for historical reasons) the "First Point of Aries" - even though it actually is located in the constellation of Pisces.

Summer Solstice: The Sun reaches its highest declination (23.5 degrees north of the equator) at 10:38 AM June 21th Mountain Daylight Time. This day marks the longest period of time when the Sun is above the astronomical horizon for people in the northern hemisphere (approximately 15 hours 2 minutes for Boulder-ites). The following noon, the Sun reaches an altitude of 73.5 degrees above the southern horizon, only 16.5 degrees from directly overhead.) The word "solstice" means "to stand still", because the Sun halts its apparent northward movement for this day, after which it reverses direction and appears to move southward in our skies once again.

Fall Equinox: The Sun crosses the equator once again, traveling from north to south, on September 23rd at 2:20 AM MDT. The event is also called the Autumnal Equinox. Again, we experience equal daytime and nighttime hours, and at Boulder the Sun reaches 50 degrees (90 degrees minus our 40 degree latitude) above the horizon at noon that day.

Winter Solstice: Finally, at 9:48 PM Mountain Standard Time on December 21st the Sun dips to its lowest declination of 23.5 degrees south of the equator. That day at noon the Sun will only rise +26.5 degrees above the southern horizon here at Boulder. There will only be about 9 hours 20 minutes of daylight. The event marks the official beginning of Winter.

And then, in complete disregard for the Sun and its position in the sky:



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