Polaris and your latitude

From Seoul, at latitude 37.6° north, Polaris stands about 37.6° above the northern horizon.

The star that hardly moves

In the last unit the whole sky turned round the celestial poles. Look north from Korea and there is a moderately bright star almost exactly at the northern pole: Polaris, the North Star. It is not the brightest star in the sky. Its catalogue magnitude is 2.02, so dozens of stars outshine it. What makes it special is its place. In the Yale Bright Star Catalogue, which gives positions for the year 2000, its declination is 89.26°, about 0.74° from the pole. Precession has since brought it a little closer, to about 0.63° in 2026, a bit more than the width of the full Moon. As the sky turns, Polaris traces a tiny circle and stays almost still while everything else wheels round it.

Polaris has not always been the pole star, and it will not stay one. Earth's axis wobbles slowly like a spinning top, a motion called precession that takes about 26,000 years for one full circle (NASA). Working the precession formulas forward from the catalogue position, the pole comes closest to Polaris around 2100, at about 0.46°, and then drifts away. NASA's StarChild pages note that in about 13,000 years the bright star Vega will be the pole star.

Why the pole's height equals your latitude

Picture yourself at the North Pole. Earth's axis points straight up, so the celestial pole is at your zenith and Polaris is overhead at altitude 90°, which is also your latitude. Now walk south to the equator, latitude 0°. You have walked a quarter of the way round Earth, and your idea of "straight up" has tipped by 90° with you. The axis now lies flat along the northern horizon, so Polaris sits on the horizon at altitude 0°. Anywhere between, the tilt is in proportion: every degree of latitude you move south lowers the pole by one degree. That is why the altitude of the celestial pole equals your latitude.

Sailors used this for centuries. Measure the height of Polaris above the sea horizon with a quadrant or sextant, add a small correction for the offset of about 0.6°, and you have your latitude without any map. Try it in the model: Seoul's latitude is 37.57° and the card shows Polaris at about 37.9° on the night of 18 October 2026. The extra few tenths are that offset; the model uses the catalogue's year-2000 positions, so its offset is the slightly larger 0.74°.

Stars that never set

Near the pole some stars circle without ever touching the horizon. They are circumpolar. A star stays up all night, all year, if its distance from the pole is less than the pole's height. At Seoul the pole is 37.6° up, so any star with a declination above 90° − 37.6° = 52.4° never sets. Dubhe, at declination 61.8°, is circumpolar from Seoul and never dips below about 9° altitude. Alkaid, the end of the Dipper's handle at 49.3°, is not: for a short while each day it slips about 3° below the northern horizon. Go farther north and more stars become circumpolar; at the North Pole every visible star is, and at the equator none is.

The southern sky

The same rule works in reverse for stars far to the south: a star whose declination is more than 52.4° south of the celestial equator never rises at Seoul. Canopus, the second brightest star at night, has a declination of −52.7°, just past that limit. EarthSky notes that Canopus never rises for places north of about 37° north. From Seogwipo on Jeju, at about 33.3° north, it climbs about 4° above the southern horizon on winter evenings, and the Seogwipo astronomy centre holds viewing nights for it each February. Korean tradition calls it Noinseong, the star of the old man, linked with long life. The Southern Cross is farther south still: its brightest star, Acrux, is at declination −63.1°, so it only rises south of about 27° north.

The south celestial pole has no bright marker. The nearest naked-eye star, Sigma Octantis, is magnitude 5.47 in the catalogue, too faint to see from most towns. Southern navigators instead use the long axis of the Southern Cross, which points roughly towards the pole. Pick Sydney in the model and look south.