The Calais Observatory is a site on which was formerly located a United States Coast Survey astronomical observatory. The observatory is located in the far eastern town of Calais, Washington County, Maine, upon an outcrop dome in a residential neighborhood. Marking the site of the observatory are a stone pillar and stone base, both used to support astronomical instruments, as well as two drill holes and a pair of stone pads chiseled out of the granite. Erected first in 1857, and utilized again in 1866 and 1895, the observations and measurements recorded at the site made nationally significant contributions to the advancement of a broad range of geophysical scientific fields. Here in December 1866, astronomical observations (star meridian passages) were taken that finalized the longitudinal measurements between America and the Greenwich Meridian in England, an important step in determining the precise shape of the earth and creating a universal geophysical framework for measuring time and space. In addition, this observatory is unique as the only remaining observatory from the Transatlantic Telegraphic Longitude Campaign of 1866 that still has in-situ the stones upon which the scientific instruments were mounted. As such, the Calais Observatory was listed in the National Register of Historic Places at the national level of significance for its role in contributing to geophysical scientific advances.