Historical Meridian Stones on Nantucket & Elsewhere The pair of marble obelisks located in downtown Nantucket and known as "Meridian Stones" is probably the most enigmatic historical monument on Nantucket. The inscriptions engraved on them state: "Northern (Southern) Extremity of the Town's Meridian Line" (Figure 1). The year of their creation, 1840, is indicated on the Northern stone. There is little doubt that the person who erected them was William Mitchell (1791-1868), the most prominent astronomer and surveyor on Nantucket at that time and the father of the first American woman astronomer, Maria Mitchell (1818-1889). But why did he erect them? In 2002, we started an investigation aimed at answering this question. This article is the final report on our study. Some preliminary results were presented at two meetings of the American Astronomical Society (Amory et al. 2002; Boyce and Davis 2003).