2021, June 20: Summer Solstice

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June 20, 2021:  The sun reaches the summer solstice mark in the sky, to signal the beginning of astronomical summer in the northern hemisphere.

Photo Caption – Sunrise from the International Space Station. (NASA Photo)

by Jeffrey L. Hunt

Our ancestors watched the sky and the world around them to note the beginning of seasons.  For those who lived at mid-northern latitudes, knowing the growing season was important.  If they were cultivators, they needed to anticipate the last frost of the spring season to plant.

The Old Farmer’s Almanac lists the typical intervals between the last spring frost and autumn’s first frost for several places in the US. It lists Atlantic, Iowa (136 days) and Duluth, Minnesota (119 days) as having shorter growing seasons than many locations that are not at lower latitudes or higher elevations.  Knowing the approximate time to plant so that the crop matured before the first fall frost was important.

In his book, Living the Sky, Ray Williamson shows how Native Americans in the American Southwest used the sun’s rising location on the horizon as a calendar for agriculture and ceremony.  Across the globe, our ancestors used similar means of sunlight and shadow, not to tell time of day, but to indicate time of year.

During most of the year, the sun’s rising and setting points move rapidly, either northward or southward along the horizon.  Near the solstices, they moves very little, almost imperceptibly.  Without a calendar or scientific equipment, one could begin counting days starting around April 15, noting where the sun appeared on the horizon.  The sun’s rising point moves northward, reaches the solstice, and starts southward along the horizon. When the sunrise point reaches the place again where the count started, that gives the total number of days for the measurement.  Divide the value by two and that yields the number of days it took for the sun to move from the starting point to the solstice.  Next year, when the sun reaches the starting point start counting the number of days until the solstice. Similar counting activities could occur for the winter solstice, the first or last frost, or other events.

The solstice occurs at 10:32 p.m. CDT on June 20! Happy summer !

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Categories: Astronomy, Sky Watching

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