A circumpolar star or constellation is one that is visible all the year round from any particular location on the Earth’s surface. For example, if you were stood at the Earth’s north pole you would be able to see all stars above (to the north) of the celestial equator, but none of the stars or constellations below. Because the celestial equator is 90° away (and equidistant) from the north celestial pole, an observer at the north pole would see the celestial equator on, and extending all around, the horizon. In other words, whatever the time of year, the night sky as seen from the north pole would contain all stars and constellations above the celestial equator, those below (to the south) of the celestial equator being permanently hidden from view.
Taking this a stage further, the south celestial pole will never be visible to an observer located in a country north of the equator, such as England. The reason for this is simply that the Earth itself gets in the way. This means that, for anyone located to the north of the Earth’s equator, the stars close to the south celestial pole never rise above the horizon. However, for northern hemisphere observers, the stars and constellations close to Polaris will never set and, assuming the sky is dark and clear, will always be visible.
The well-known constellations Ursa Major (the Great Bear) and Cassiopeia, are good examples. Lying fairly close to Polaris, this two constellations sweep round the north celestial pole and, even when at their lowest points in the sky, are still located well above the horizon as seen from mid-northern latitudes. In other words, we say that Ursa Major and Cassiopeia are circumpolar.
The bright stars Castor and Pollux however, which lie further south in the sky in the constellation Gemini (the Twins), rise and set as normal and are not circumpolar. This is because their angular distance from the north celestial pole is so large that, when at their lowest point in the sky, they disappear below the horizon.
Ursa Major and Cassiopeia (along with a few other constellations such as Draco (the Dragon) and Ursa Minor (the Little Bear) are circumpolar as seen from countries at mid-northern latitudes such as England. However, if we travel further south, this situation will change. For example, when seen from Cuba in the Caribbean or Bombay in India the angular distance of Polaris above the northern horizon would be only around 20° which would mean that any stars or constellations lying further away than 20° from Polaris would not be circumpolar. The constellation Ursa Minor would remain circumpolar, although many of the other stars which are circumpolar as seen from England would, at some point, disappear below the horizon. This includes Cassiopeia and Ursa Major which, for part of their daily journey around the sky, would set and be hidden from view. However, the upside of this (at least from the point of view of observers in Cuba or Bombay) is that many southern hemisphere stars that would be hidden to observers in England would come into view. These include, for example, the brilliant star Canopus in Carina (the Keel) which is well placed for backyard astronomers checking out the night skies of India and the Caribbean.
Just as stargazers in the northern hemisphere have their own circumpolar stars, observers south of the equator, such as those in Australia and New Zealand, have theirs. From locations in these countries, the circumpolar skies are graced by constellations such as the magnificent Crux (the Southern Cross) and, amongst others, the somewhat-fainter Tucana (the Toucan) and Octans (the Octant).