Celestial Pole

Definition

The celestial poles are the two points where the extension of the Earth's rotation axis meets the celestial sphere. As the Earth turns, the whole sky seems to rotate around them once every sidereal day, 23 h 56 min 4 s. Long-exposure photographs pointed at a pole show the stars drawing concentric arcs around it.

The poles define the equatorial coordinate system. The celestial equator lies 90° from both poles, and declination is the angle measured from the equator: +90° at the north celestial pole, −90° at the south one.

The altitude of the pole is your latitude

For an observer at latitude φ, the celestial pole of their hemisphere stands at an altitude equal to φ above the horizon, due north in the northern hemisphere and due south in the southern one. At the North Pole the north celestial pole is at the zenith; at the equator both poles lie on the horizon.

This was the simplest way to measure latitude at sea for centuries: measure the altitude of Polaris, apply a small correction for its distance from the pole, and read off the latitude.

Pole stars

The north celestial pole currently has a bright marker, Polaris (Alpha Ursae Minoris), magnitude 2.0, about 0.64° from the pole. Precession is still bringing it closer: it will be nearest, about 0.45°, around the year 2100, then slowly move away.

The south celestial pole has no bright star. The nearest visible one, Sigma Octantis, magnitude 5.4, is about 1° from the pole and needs a dark sky to be seen.

• Finding the north pole: follow the two stars at the end of the bowl of the Big Dipper, Merak and Dubhe, about five times their separation, to Polaris.
• Finding the south pole: extend the long axis of the Southern Cross about four and a half times its length, or go to the point halfway between the Southern Cross and Achernar.

Circumpolar stars

A star close enough to the visible pole never sets: it circles the pole above the horizon all night, all year. At latitude φ (north), a star is circumpolar if its declination is greater than 90° − φ, and never rises if it is less than −(90° − φ).

At Paris (48.9° N), every star north of declination +41.1° is circumpolar: the whole of the Big Dipper, Cassiopeia and Capella. Every star south of −41.1° stays invisible, including the Southern Cross and Alpha Centauri.

The poles move: precession

The Earth's axis precesses like a spinning top, under the torque of the Sun and the Moon on the equatorial bulge. Each celestial pole describes a circle of radius about 23.4° around the ecliptic pole in about 25,800 years. The north ecliptic pole is in Draco, at right ascension 18h and declination +66.6°.

Superimposed on precession, nutation makes the pole wobble by about 9″ with an 18.6-year period. Meeus applies both corrections, precession and nutation, whenever star positions are carried from a catalogue epoch such as J2000.0 to the date of observation.

Explore it on AstroAlgos

• Precession — the slow motion of the poles
• Night sky — the stars turning around the pole
• Declination — angular distance from the equator
• Horizontal coordinates — altitude of the pole