A point in the sky that corresponds with Earth's own North Pole. Because this point always
lies directly above the Earth's pole, the bright star that lies
close to it, Polaris, always lies due north from an observer's point of view.
From the surface of the Earth, it appear that all celestial objects, from the Sun to the stars,
rotate around this point once each day. This illusion is due to the fact that it is the observer who is
carried round the Earth's own poles by the planet's rotation.
The declination of the Pole Star (its angular distance from the Celestial Equator) is fixed, by definition, at +90°.
However, its altitude (the angle it makes with the horizon) is not fixed, but varies according to the latitude of the
observer. This is very useful for navigation in the northern hemisphere, since the altitude of the Northern Celestial
Pole is always equal to the observer's own latitude: by measuring the Pole's angle to the horizon, it's possible to
exactly calculate your own distance from the Earth's equator.
For northern navigators, this measurement is made particularly straightforward by the fact that a bright
and easily distinguishable star lies very close to the Celestial Pole (navigators south of the equator have no such
luxury). This star is Polaris, the brightest of the stars in Ursa Minor, which is easily located by the two
Pointer stars in Ursa Major. Polaris is presently some 44' (about three quarters of a degree) from the
Northern Celestial Pole itself.
Polaris hasn't always been the Pole Star. In fact, it has only been close to the Pole for the last thousand
years or so, and over the next millennium it will gradually move further away. This is because the Earth's motion
is constantly affected by the pull of other bodies in the Solar System, especially the Moon and the Sun,
which causes a 'wobble' in its orbit. This, in turn, causes the Pole to move relative to the stars.
The effect of this wobble (properly called precession) is that both Celestial Poles follow a broad
circle through the sky. For most of the time, there is no Pole Star at all, but occasionally the Pole will
pass near a conspicuous star - we are lucky to live in a time when Polaris fulfils this role. For the
ancient Egyptians, the Pole Star was not Polaris, but Thuban in Draco, while observers in the far future will see
yet other stars at the Northern Celestial Pole.