The Calais Observatory is a pair of granite fixtures in Meridian Park in Calais, Maine. The two stones were used for mounting scientific equipment used in various astronomical observations, principally for the accurate calculation by the United States Coast Survey of Calais’s longitude with respect to meridians in Cambridge, Massachusetts, and Greenwich, England. The site was listed on the National Register of Historic Places in 2012, and is part of the National Oceanic and Atmospheric Administration’s Heritage Trail program. At the time of its longitude determination in 1866 with respect to the Greenwich Meridian, it was arguably one of the most precisely located places in the United States. The Calais Observatory site is located on a granite knob in Meridian Park, on the grounds of the former Calais Academy, at the corner of North and Lincoln Streets. The surviving elements of the observatory are two granite fixtures, flat pads chiseled out of the bedrock, and several drill holes. The site was known to be sheltered by a nominally permanent structure in both 1857 and 1866 but had by 1895 lost whatever shelter it had.