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Circumpolar stars are those that remain above the horizon at all times during the year for a given observer’s location. This characteristic is primarily due to their proximity to one of the celestial poles, which means they complete a circular path around the pole without dipping below the horizon.
In the Northern Hemisphere, for example, stars like Polaris or the Big Dipper are classified as circumpolar because as the Earth rotates, these stars move in a circular motion around the North Star and are visible throughout the entire year, regardless of the season.
This is in contrast to stars that are seasonal (which are only visible during specific times of the year) or those that rise and set, like most stars in the night sky, which do so due to the Earth's rotation. Circumpolar stars also aren’t limited to any specific hemisphere; for instance, circumpolar stars can be observed in both the Northern and Southern Hemispheres, but the specific stars that fit this description vary based on one's latitude. Therefore, the defining characteristic of circumpolar stars is their continuous visibility throughout the year.