Wednesday, June 21, 2017

Happy Summer Solstice 2017!

Wheat ripening on the Summer Solstice

If you were to arise at dawn every day of the year and record the point on the horizon where the sun rises you would be able to track the progression from the Summer Solstice (in which the sun rises furthest North) to the Winter Solstice (in which the sun rises furthest South) and back again.  Tracking the position of the rising sun was one of the earliest astronomical observations.  Many ancient monuments were constructed to act as solar observatories, recording the longest and shortest days of the year.  These observations were used to plan planting dates for various agricultural crops.

Today the sun reached its northernmost point on the horizon.  Officially, at 12:24 AM EST Spring ended and Summer began in the Northern Hemisphere.  This moment of change is known as the Summer Solstice.  At the same time, the Southern Hemisphere was experiencing its Winter Solstice as their Fall ended and Winter began. The word Solstice comes from the Latin words sol (sun) and sistere (to stand still).  Today the sun has reached its highest position in the northern Sky, giving us our longest day of the year.  With the sunrise at 5:58 AM and sunset at 9:24 PM, mid-Michigan will experience approximately 15 hours 26 minutes of daylight today.  By comparison, Christchurch, New Zealand (which is approximately the same latitude south of the equator as Mid-Michigan is north of the equator), will have only 8 hours 56 minutes of sunlight today.
Field Corn (Zea mays) requires long summer days to grow and ripen

Why does the length of daylight vary?  The Earth rotates around its axis approximately once every 24 hours.  However this axis is tilted at 23.5 degrees from the vertical.  The points on the globe that the axis revolves around are referred to as the North and South Poles.  The axis is always pointed toward the same location in the sky.  The North Pole points toward the "North Star" - Polaris.

At any given time, fifty percent of the earth is in sunlight (Day) and the other fifty percent is in darkness (Night).  However, because the Earth is tilted on its axis sunlight does not always strike the Earth at the same angle.  This means that during different seasons different parts of the Earth will receive varying amounts of sunlight and darkness.
As the earth revolves around the sun, sometimes the North Pole is closer to the sun, sometimes the South Pole is closer to the sun.  When the North Pole is at its closest, the sun lights a larger portion of the Northern Hemisphere than it does the Southern Hemisphere.  When this happens, we experience Summer in Mid-Michigan and the Southern Hemisphere experiences Winter.  During our Northern Summer, not only does the sun light more of the Northern Hemisphere, but the sun also lights every Northern Hemisphere location for a higher percentage of the day than a comparable Southern Hemisphere location. 

When the North Pole is at its furthest from the sun, we experience Winter and the Southern Hemisphere experiences Summer.  During our Northern Winter, the sun is striking a a larger portion of the Southern Hemisphere than it is the Northern Hemisphere, and consequently the sun lights every Southern Hemisphere location for a higher percentage of the day than a comparable Northern Hemisphere location.  On two days of the year, the Spring and Fall Equinoxes, the sun lights the Northern and Southern Hemispheres equally and the length of day for will be the same for both.

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