Sundials - World's Oldest Clocks

North American Sundial Society

nass news 2019 july joplinIn downtown Joplin, MO, John Hadsall of the Globe digital edition began probing into the sundial on the face of Ron Jones's Commerce Building at 113 West 3rd St. Hadsall says, "I’ve been enchanted with this sundial ever since I noticed it during a lunch-break walk. I’ve written before about how I hoof it downtown and find all sorts of treasures under our noses, from the amazing front porches in the North Heights Neighborhood to the brick sidewalks that remain throughout that neighborhood, Murphysburg and other downtown residential areas." 

"The sundial is believed to have always been a part of the more than 100-year-old-building.  Originally built in 1904 for a machinery company, it was remodeled into a bus station in 1937."  And over the years the building was abandon, finally being purchased by Jones and his business partner Ivan McElwee.  John proceeded to see if the dial told accurate time, taking a photo of the dial and its shadow "at about 1:05 pm on Thursday [18 July 2019], I took a picture of the sundial at 111 W. Third St... It appeared to be a little behind: The shadow of the gnomon was still crawling toward dead center of the 12."

John gives several possibilities of the dial's time inaccuracy, but the truth is far more interesting.  The founders of Joplin in 1873 used magnetic north to lay out the street grid.  The recessing of the sundial in the wall aligns the dial to true north south.  A very close look at the dial construction shows that the afternoon hour lines converge on the eastern edge of the toe of the gnomon.  That is, this sundial was designed for the thick one inch gnomon where morning hour lines converge on one side of the gnomon and afternoon hour lines on the other  To read the time, one needs to drop lines from the left and right gnomon edges straight down creating a split noon.  For the morning, read the western shadow edge with reference to the west gnomon line, giving a shadow time of about 11:41 solar time.

The Joplin dial is at 37° 5.0' N, 94° 30.8' W, about 4.5° west of the central time meridian at 90° W.  That translates to 18m early. That is, when the sun is over the central time meridian, it will take another 18m before noon occurs in Joplin.  We also need to include the sun's Equation of Time, that annual variability between solar and civil time due to the earth's slightly eccentric orbit and 23.4° tilt of its axis.  Calculations of the EOT are provided by the British Sundial Society and others, showing that on July 18th the sundial was slow (early to clock time) by just a bit more than 6m, giving a total difference between sun and civil time of 24 minutes.  With above photo taken at 1:05pm central daylight time, we subtract 1h 24m to get sundial time of 11:41am. 

So now you know that this century old sundial at 111 W Third Street still keeps accurate solar time, and with a little math, you can set your watch to the minute!  You can hear an edited version of John's blog with history and his not so precise conclusions about the sundial here:

For a full version of John Hadsall's podcast "In Case You Missed It" go to: https://www.joplinglobe.com/multimedia/podcast-in-case-you-missed-it-sundial-building/audio_95587b74-ab7c-11e9-b4e8-bfc13b97fe5b.html

and you can read John's blog at: https://www.joplinglobe.com/news/lifestyles/joe-hadsall-downtown-sundial-still-working-as-intended/article_6cf74006-963d-5439-83cc-e6bdafd85dd3.html

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