The Star Gazers’ Stone in the Natural Land Trust’s ChesLen Preserve in Coatesville, Pa is an unassuming quartzite rock measuring no more than 18 inches in height, but it played an outsized role in the political fates of our nation.

For many years, the Stone was difficult to access, but—thanks to a new parking area constructed by Natural Lands Trust and Newlin Township—it is now easy to visit and appreciate its significance to early colonial history.

In 1763, 240 years before the widespread use of GPS, surveyors and astronomers Charles Mason and Jeremiah Dixon came to the New World to end a bloody, 80-year boundary dispute between Pennsylvania and Maryland. Their survey, which established the official line between the two colonies, stands among the greatest scientific achievements of the time.

While the duo began their expedition in Philadelphia, they quickly realized that they would need to travel along a straight line due south of the capital city, requiring a difficult river crossing. Instead, they selected a location due west of Philadelphia—the forks of the Brandywine Creek in Embreeville, Chester County.

They negotiated with a local farmer to use his property to set up an observatory from which to make their calculations. A reference point, now known as Star Gazers’ Stone, was placed to mark the astronomical meridian line north of their observatory. Using a device with a six-foot-long brass telescope that allowed them to establish their position relative to the stars, they spent the winter nights charting the sky.

In the spring of 1764, they ventured due south from the farm with a team of axemen clearing a wide swath through the dense forests. Using chains and levels, they surveyed in straight, 12-mile segments, then made detailed astronomical calculations to adjust to the exact latitude.

All told, the survey took five years—through farm fields and woodlands, frigid winters and oppressive summers, colonial villages and sometimes hostile Native American territories—before Mason and Dixon were satisfied. They’d established the 233-mile-long boundary line between Pennsylvania and Maryland, and the 83-mile-long north-south boundary between Maryland and present-day Delaware.

Star Gazers’ Stone is listed on the National Register of Historic Places and was designated a National Historic Civil Engineering Landmark, one of 125 sites in the country.

7/29/2013