Ships entering Portland's harbor could not be seen directly from the wharfs, which made it hard for merchants to prepare for cargoes. In 1807, Captain Lemuel Moody organized the construction of an observatory on Munjoy Hill.
The tower is 86 feet (26 m) tall, has seven stories, and is octagonal to lessen wind pressure. Using a telescope and signal flags, people could talk to ships hours before they reached the docks. It is the only known surviving tower of its type in the United States.
It was designated a National Historic Landmark in 2006 and is now managed by Greater Portland Landmarks as a museum.