Selected to be about 31 miles (50 km) west of the then southernmost point in Philadelphia, the observatory was used to determine the precise latitude of its location.

[3] Mason and Dixon's survey was the final step in the resolution of a border dispute between Pennsylvania and Maryland that lasted over 80 years.

In 1760 the Crown intervened, defining the border as the line of latitude 15 miles (24.1 km) south of the southernmost house in Philadelphia.

The proprietors of the colonies, the Penns and Calverts, then commissioned Mason and Dixon to survey the newly established boundary.

[4] Mason and Dixon used the finest instruments of their day in the survey, including a type of telescope, the zenith sector built by John Bird, used for measuring latitude and an "equal altitude and transit" instrument for sighting survey lines, as well as a less accurate quadrant for faster rough estimates of latitude, and a chronometer built by John Harrison, used for measuring longitude.

13 Lodged at Esquire Worths Arrived at Mr. Harlands and set up the sector in his Garden (inclosed in a tent), and in the Evening brought the Instrument into the Meridian, and took the following observations[8]The new site of the observatory was near marks that had been made by surveyors from New Jersey in 1730 and 1736.

They moved the observatory to this point where they measured the latitude again and remeasured back to the Star Gazers' Stone.

They were reputed to be heavy drinkers, and local lore says that a nearby tributary of the Brandywine, Punch Run Creek, was named to commemorate their drinking.

[3] Brothers George and Michael Harlan were Quakers who were born in Durham, England and immigrated to Pennsylvania about 1687 from County Down, Ireland.

Sometime between 1758 and 1760 Joel returned to Newlin Township after living in other locations in Chester County, took ownership of the farm and built an addition to the house.