On October 2, 2009, an important event took place on the corner of Main and Fair Street – the unveiling of a memorial plaque explaining the purpose and the significance of two marble obelisks located in downtown Nantucket, on Main and Fair streets and known as “Meridian Stones.” The plaque is on display on the wall of the Pacific Bank, next to the Northern Meridian Stone.
The purpose of the two stone monuments has long been one of Nantucket’s great historical mysteries. It was known that the stones had been erected by the Town in 1840, on a proposal of William Mitchell (1791-1868), the most prominent astronomer and surveyor on the island at that time and the father of the first American woman professional astronomer, Maria Mitchell (1818-1889). However, the exact reason these stones were cut and placed was unknown until recently.
In 2002, a group of enthusiasts led by the Maria Mitchell Association (MMA), set out to solve the enigma. The project took several years. Two MMA summer students participated in the project and one of them, Lucy Amory, the history major from Vassar College, finally found in archives decisive materials that clarified the purpose of the stones. They were erected by Mitchell, using precise astronomical methods, to represent the true geographic meridian (north-south line) and use it for monitoring the variations of the angle between the true North and the magnetic North (the so called “magnetic declination”). The direction to the magnetic North was the working reference direction for the surveyors of that epoch who used a compass to measure bearings. An accurate correction for the magnetic declination was necessary to present the results of the survey on a geographic map, where the reference direction is the true (not magnetic) North.
The investigation showed further that the Mitchell Meridian Line was one of the first of this kind in the country. Mitchell preceded by three decades the mandatory installation of similar constructs elsewhere in Massachusetts and other states. Only in 1870 the Massachusetts General Court passed a resolve that demanded the installation of “true meridian lines” in every county of the state.
The plaque will immortalize the memory of two most distinguished residents of our island – William and Maria Mitchell who brought to Nantucket one of the important flavors of its fame – to be an “Astronomical Island.”