While many stars appear to rise and set like our Sun, some exhibit a unique behavior, remaining perpetually visible from certain locations on Earth. These celestial objects offer a continuous display, never dipping below the horizon throughout the night or across the year. Their constant presence provides a fascinating insight into our planet’s motion and perspective from its surface.
What Are Circumpolar Stars?
Circumpolar stars are celestial bodies that, when viewed from a specific point on Earth, never set below the local horizon. Instead, they appear to continuously circle a celestial pole, tracing a complete path above the observer’s visible sky. This means they are visible every night of the year, even though their orientation shifts with the Earth’s rotation. Unlike stars that rise in the east and set in the west, circumpolar stars remain consistently in view.
Why Stars Appear Circumpolar
The phenomenon of circumpolar stars is a direct consequence of Earth’s rotation and an observer’s geographical location. Our planet spins on an axis, and if this imaginary line is extended into space, it points to two fixed points in the sky: the North Celestial Pole and the South Celestial Pole. Stars appear to rotate around these celestial poles as Earth completes its daily spin.
The height of the celestial pole in an observer’s sky is directly equal to their latitude on Earth. For instance, if you are at 40 degrees North latitude, the North Celestial Pole will appear 40 degrees above your northern horizon. A star becomes circumpolar if its angular distance from the visible celestial pole is less than the observer’s latitude. This means that the closer an observer is to either the Earth’s North or South Pole, the larger the number of stars that appear circumpolar to them. At the geographic poles, all visible stars are circumpolar, while at the equator, no stars are.
How to Find Circumpolar Stars
Locating circumpolar stars involves first identifying the celestial pole relevant to your hemisphere. For observers in the Northern Hemisphere, the North Celestial Pole is marked by Polaris, commonly known as the North Star. Polaris is particularly helpful because it remains nearly stationary in the sky, positioned less than one degree away from the true celestial pole. This makes it a reliable guide for navigation, as its altitude above the horizon directly corresponds to your latitude.
To find Polaris, one common method involves using the “pointer stars” of the Big Dipper (part of Ursa Major), which direct the eye toward Polaris. Prominent Northern Hemisphere circumpolar constellations include Ursa Major (the Great Bear), Ursa Minor (the Little Bear, which contains Polaris), Cassiopeia, Cepheus, and Draco. These constellations are always visible in the northern sky, though their specific orientation changes throughout the night and year.
In the Southern Hemisphere, the situation is slightly different, as there is no bright star equivalent to Polaris marking the South Celestial Pole. Sigma Octantis is the closest star to the South Celestial Pole, but it is quite dim and barely visible to the unaided eye. Instead, observers in the Southern Hemisphere often use the Southern Cross (Crux) constellation, specifically its two brightest stars, Alpha and Gamma Crucis, to pinpoint the approximate location of the South Celestial Pole.