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Stargazer's Stone

USA / Pennsylvania / Thorndale /
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"The Stargazers' Stone marked an important astronomical observation point used by Charles Mason and Jeremiah Dixon in 1764 in surveying the Mason-Dixon line, which lies 15 miles south of the stone."


"The southernmost part of Philadelphia was determined by the survey commissioners to be the north wall of a house on the south side of Cedar Street (the address is now 30 South Street) near Second Street. Mason and Dixon had a temporary observatory erected 55 yards northwest of the house, and after detailed celestial observations and calculations, they determined the latitude of the house wall to be 39o56’29.1”N.

Since going straight south would take them through the Delaware River, they then surveyed and measured an arbitrary distance (31 miles) west to a farm owned by John Harland, in Embreeville, Pennsylvania, at the “Forks of the Brandywine.” They negotiated with Harland to set up an observatory, and set a reference stone, now known as the Stargazers' Stone, at the same latitude. They spent the winter at Harland’s farm making astronomical observations on clear nights and enjoying local taverns on cloudy nights. The Harland house still stands at the intersection of Embreeville and Stargazer Roads, and the Stargazers' Stone is in a stone enclosure just up Stargazer Road on the right. Its latitude is 39 56’18 9” N which they calculated to be 356.8 yards to south of the parallel determined in Philadelphia.

At Harland’s they observed and timed predicted transits of Jupiter’s moons, as well as a lunar eclipse on March 17th 1764. The average (sun) time of these events at the Stargazers’ Stone was 5 hours 12 minutes and 54 seconds earlier than published predicted times for the Paris observatory (longitude 2o20’14”E). So they were able to estimate their longitude as (5:12:54)/(24:00:00) x 360o = 78 13’30 west of Paris, and thus 78 13’30” - 2 20’14” = 75 53’6” west of Greenwich. They published these findings in the Royal Society’s Philosophical Transactions in 1769. "
Text from - "road less trcled" - www.flickr.com/photos/road_less_trvled/2989977012/
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Coordinates:   39°56'21"N   75°43'57"W
This article was last modified 14 years ago