circumpolar star

The northern circumpolar stars revolving around the north celestial pole. Note that Polaris, the bright star near the center, is almost stationary. Polaris is circumpolar and can be seen at all times of the year. (The graphic shows how the apparent positions of the stars move over a 24-hour period, but in practice they are not visible when the Sun is also in the sky.)

The North Star and circumpolar stars in a photograph with a long shutter speed. Note that the stars near the celestial pole make less of a trail with the long exposure.

A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets (that is, never disappears below the horizon), due to its proximity to one of the celestial poles. Circumpolar stars are therefore visible (from said location) for the entire night on every night of the year (and would be continuously visible throughout the day too, were they not overwhelmed by the Sun's glare).

Explanation

As Earth spins daily on its axis, the stars appear to rotate in circular paths around one of the celestial poles (the north celestial pole for observers in the northern hemisphere, or the south celestial pole for observers in the southern hemisphere). Stars far from a celestial pole appear to rotate in large circles; stars located very close to a celestial pole rotate in small circles and hence hardly seem to engage in any diurnal motion at all. Depending on the observer's latitude on Earth, some stars – the circumpolar ones – are close enough to the celestial pole to remain continuously above the horizon, while other stars dip below the horizon for some portion of their daily circular path (and others remain permanently below the horizon).

The circumpolar stars appear to lie within a circle that is centered at the celestial pole and tangential to the horizon. At the Earth's North Pole, the north celestial pole is directly overhead, and all stars that are visible at all (that is, all stars in the northern celestial hemisphere) are circumpolar. As one travels south, the north celestial pole moves towards the northern horizon. More and more stars that are at a distance from it begin to disappear below the horizon for some portion of their daily "orbit", and the circle containing the remaining circumpolar stars becomes increasingly small. At the Earth's equator this circle vanishes to a single point – the celestial pole itself – which lies on the horizon, and there are therefore effectively no circumpolar stars at all.

As one travels south of the equator the opposite happens. The south celestial pole appears increasingly high in the sky, and all the stars lying within an increasingly large circle centred on that pole become circumpolar about it. This continues until one reaches the Earth's South Pole where, once again, all visible stars are circumpolar.

The north celestial pole is located very close to the North Star (Polaris), so, from the northern hemisphere all circumpolar stars appear to rotate around Polaris. Polaris itself remains almost stationary, always at the north (i.e., the azimuth is 0°), and always at the same altitude (angle from the horizon), equal to the latitude of the point of observation on Earth.

Definition of circumpolar stars

The circumstances making a star circumpolar is solely dependant on the observer's hemisphere and their latitude.^[1] As the altitude of either the north celestial pole (NCP) or south celestial pole (SCP) is the same as the observer's latitude ^[1], any star whose position from the pole is less than the latitude, will be circumpolar and will never set below the horizon. I.e. If the observer latitude is 45°N and is facing north, then any star will become circumpolar if it lies less than 45° from the north celestial pole. If the observer's latitude is -35°S and is facing south, then these stars are circumpolar within 35° of the south celestial pole. Stars on the celestial equator will not be circumpolar when seen from any latitude in either hemisphere of the Earth.

This is easily calculate if some star will be circumpolar (or not) at the observer's latitude by just knowing the star's declination (δ).

For northern stars, this is calculated by (90° - δ).^[1]

• For example, if Dubhe / α Ursa Majoris declination is +61° 45′, then this star will be circumpolar anywhere north of +28° 15′ latitude.

For southern stars, this is calculated by (90° + δ).^[1]

• For example, if Alpha Centauri declination is -60° 50′, then this the star will be circumpolar anywhere south of -29° 10′ latitude.

Conversely, the calculated results above also determines whether a star will rise above the local horizon. Using the results above;

• Dubhe will be visible towards the northern horizon anywhere north of -28° 15′ latitude by (δ - 90°).^[1] Similarly,

• Alpha Centauri will be visible towards the southern horizon anywhere south of +29° 10′ latitude. (δ + 90°)^[1]

Some stars within the far northern constellations, such as Cassiopeia, Cepheus, Ursa Major, and Ursa Minor, roughly north of the Tropic of Cancer (+23½°), will be circumpolar stars that never rise or set.^[1]

For British observers, for example, the first magnitude stars Capella (declination +45° 59) and Deneb (+45° 16') do not set from anywhere in the country. Vega (+38° 47') is technically circumpolar north of latitude +51° 13' (just south of London); taking atmospheric refraction into account it will probably only be seen to set at sea level from Cornwall and the Scilly Isles.

Some Stars within the far southern constellations, such as Crux, Musca, and Hydrus, roughly south of the Tropic of Capricorn (-23½°), will also be circumpolar stars.^[1]

Stars (and constellations) that are circumpolar in one hemisphere are always invisible in the high latitudes of the opposite hemisphere, and these never rise above the horizon. For example, the southern circumpolar star Acrux is invisible from most of the Continental United States, likewise, the seven stars of the northern circumpolar Big Dipper asterism are invisible from most of the Patagonia region of South America.

See also

• Pole Star
• Voyages of Christopher Columbus
• North Star
• South Star
• Celestial sphere
• Celestial pole

References

External links

• Bartleby.com: Circumpolar Star
• North Circumpolar Star Trails
• South Circumpolar Star Trails