November 21, 2019

AstroEvent of the Week: A Solstice/Lunar Eclipse Tie-In.

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Astronomers and lovers of the dark rejoice; Monday we take begin to take back the night! The summer solstice occurs June 21st at 11:28UT; this is the point that the Sun is at its highest apparent northerly declination and begins its long march southward. If you’re down under, of course, it’s the beginning of winter and the reverse is true. You probably won’t notice the slow creep of darkness and ever shortening days until around September, but it’s the thought that counts. The higher northerly latitude you are, the greater the variation. And to top things off, a partial lunar eclipse occurs on Saturday, June 26th. This will be visible from the Far East at moonrise eastward across the Pacific in its entirety to North America at moonset. Only the northeastern US gets left out. This will be the first lunar eclipse over the contiguous US since February 2008, and at its maximum the Moon will be 54% eclipsed. First contact with the umbra occurs at 10:16UT and the Moon departs the umbra at 12:59 UT. This eclipse is part of saros series 120, and sets the stage for the Tahitian total solar eclipse next month. This is also a good primer for December’s total lunar eclipse, which will occur in its entirety over the US on the winter solstice!

The Astroword for this week is Gnomon. Ever wonder what that protractor-looking arm is called on a sundial? Of course you have, and now you can tell people with authority that this is known as a gnomon, complete with the silent “g”. Gnomon is Greek for “indicator” or “one who discerns”, although the phrase “she was the gnomon for all which was a failure in my life,” might be stretching it a bit. For a sundial to work function properly, the gnomon must be set parallel to the Earth’s axis, which is a fancy way to say to the north in the northern hemisphere and south…well, you get the idea. Hopefully, this knowledge won’t spark a lawsuit against the Ancient Greeks by any manufacturer of polar aligned telescopes. Now during the summer solstice is a good time to check that garden sundial against your local standard solar transit time or measure the sun fast as evinced by our friend, the equation of time… gnomon also makes a good “gn” Scrabble word, right along with “gnarled” and “Gnostic,” a sure fire way to get folks scrambling for the dictionary!

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