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Western Slope Skies: Fall Equinox

Daniel V. Schroeder

It’s that time of year again… Watching the leaves change color with cooling temperatures, warm apple cider, a pumpkin patch, light seems to slip away sooner.  What marks the fall season for you?

‘Fall’ is astronomically observed to begin on September 23rd this year, called the Fall (or Autumnal) Equinox.  But what is an "equinox?"  In Latin, this means "equal night."  “Equinox” is when the day and night are approximately equal in length.  This occurs twice per year, in spring and autumn.  The Fall Equinox here in the Northern Hemisphere occurs at the exact same time as the Spring Equinox in the Southern Hemisphere.  Seasons occur due to the 23.5 degree tilt of Earth’s rotational axis, like a spinner-top toy orbiting the Sun with a tilted angle.  When the Northern Hemisphere is tilted toward the Sun, we experience the summer season, beginning with the Summer Solstice. When tilted away, we experience the winter season, beginning with the Winter Solstice. The Fall and Spring Equinoxes occur between the Winter and Summer Solstices. At equinox, the Sun shines at a perpendicular angle to the equator.

Our ancestors relied upon this knowledge, using the day and night sky as both clock and calendar in order to survive.  They tracked the Sun’s path across the sky, the change in day length, the location of the sunrise and sunset.  Early humans also built the first astronomical observatories, such as Stonehenge in England and Machu Picchu in Peru.  Ancient, astronomically-aligned architecture can be seen nearby in Chaco Culture National Historic Park in New Mexico.

For us in the Northern Hemisphere, the Fall Equinox means the days continue to get shorter.  Birds, insects, and other animals begin their migrations to warmer climates.  Plants are ending their growth season, and preparations for winter start.  Take time to notice the seasonal changes this Fall Equinox by using all your senses.  Smell the crisp air and find your way through a hay stack maze.  What do you do differently as the season changes?  Enjoy and Happy Fall Equinox!

Western Slope Skies is produced by members of the Black Canyon Astronomical Society. This episode was written and recorded by Park Ranger Angela Trnka of Black Canyon of the Gunnison National Park.

Links of interest: http://earthsky.org/astronomy-essentials/everything-you-need-to-know-september-equinox

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