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What Is a Circumpolar Moon?

Circumpolar is an astronomical term that refers to an object that is always visible in the sky and is never seen to set. The term is most often used in reference to stars and constellations. Polaris, also called the North Star, is an example of a circumpolar star for those in the northern hemisphere because it remains fixed in the sky and at no point does it move below the horizon.

Historic Occurrences
- Historic occurrences of a circumpolar moon would have occurred at different latitudes than they do today, thanks to a slight change in the tilt of the Earth's axis. In 41,000 years the tilt of the Earth's axis has fluctuated between 22 and 25 degrees. There is as much as half a degree difference between now and when early humans were first present in Europe. At that time a circumpolar moon could be observed at a location where it can no longer be seen today. For example, a circumpolar moon could have been observed in the Shetland Islands during neolithic times, but can no longer be seen there today.
Megalithic Structures
- There is evidence to suggest that Stonehenge was an early astronomical observatory.
  
  It is believed that many megalithic structures, the most famous of which is Stonehenge, were built by early humans as a way of keeping track of astronomical changes and occurrences. That may not be readily apparent to today's observers since the change in the tilt of the Earth's axis means that we do not see the night sky in quite the same way that our ancestors did. A complex of structures in the Orkneys, which includes the megalithic structures of Brodgar, Stennes and Maes-Howe, would have offered ancient observers a glimpse of a circumpolar moon where one can no longer be seen in the present day.
Observing a Circumpolar Moon
- Lunar standstill seasons are when circumpolar moons can be observed. In the northern hemisphere an observer would have to be positioned at a latitude of 63 degrees north to observe a circumpolar moon. In the southern hemisphere an observer would have to be at latitude of 63 degrees south observe a true circumpolar moon. Observers that are near this latitude can still observe what is termed a near circumpolar moon. Today observers at the Orkney complex, which is located at about 59 degrees north, can during a major lunar standstill see a moon that rolls along the horizon without rising or setting as opposed to a stationary true circumpolar moon.
Lunar Standstills
- Lunar standstills occur in cycles that occur over an 18.6- to 19-year period. They are considered the opposite of a lunar eclipse based upon the position of the moon. These events are divided into minor lunar standstills and major lunar standstills. The last major lunar standstills occurred in 2005 and 2006. The next minor lunar standstills will occur in 2015. The major lunar standstills offer the best chance of observing a circumpolar or a near circumpolar moon.

Astronomy