With Thanksgiving and the Christmas season just around the corner, Santa and the elves have been laboring mightily at the North Pole assembling everything from X-boxes to rocking horses. Above their heads the stars twinkle in a sky that’s dark 24 hours a day. That’s right. Neither sunshine nor the faintest blush of twilight mar what must seem like eternal night. As I write, the temperature at the pole is -27 F with an expected high today between 10 and 20 below under mostly cloudy skies.
At the North Pole, the sun sets for good on the first day of fall, and the last sign of its presence occurs in early November, when a sharp-eyed elf might see the faintest hint of dusk in the southwestern direction. True and utter night settles in for good on November 13. Not until the end of January will dawn return by slow degree until the sun finally rises again on March 20. The season’s lightning bill alone must weigh heavily on Santa’s back.
Before packing up all those binoculars and telescopes intended for good little girls and boys, the elves undoubtedly check them out under the stars for quality assurance purposes. Any clear night would do as would any clear day. Imagine the freedom to go out and look at the sky anytime you wanted without having to worry about daylight. OK, there are a few downsides, but I’d love a taste of seamless polar darkness.
So what would you behold on looking up? You’ll recall yesterday’s discussion about the Big Dipper and how low it sinks in the sky for mid-northern latitudes this time of year. To see it in November, you either have to live in the northern U.S. and peer along the northern horizon or wait until just before dawn, when the Earth’s rotation finally carries the constellation back up in the eastern sky.
From the North Pole, the Dipper not only never sets, it pivots about the North Star near the very top of the sky. Earth’s imaginary north polar axis extends into space in the direction of the North Star (Polaris). As the planet spins, all the stars describe arc-like paths across the sky, a reflection of our spinning motion. All except Polaris. It alone remains in one spot in the sky, because Earth’s axis points directly and unflinchingly at it.
For observers north of the equator, Polaris is as high in the sky as your latitude. That translates to 47 degrees high for Duluth, Minn., 42 degrees for Chicago, 24 degrees for Key West and directly on the horizon for an observer standing on her tiptoes in Quito, Ecuador. If you stepped outside to share a hot cocoa with Santa at the Pole this week, you’d see the North Star 90 degrees high or directly overhead at the zenith point. Any star groups near Polaris like the Big Dipper and Cassiopeia would circle around it and never set.
Stars that never set are called circumpolar. They’re close enough to the North Star that they don’t get cut off by the horizon as Earth’s rotation spins them about. To be precise, any star that’s within the latitude of the observer from Polaris never set. For Duluth, that means stars within 47 degrees of the North Star never set, for Key West, where Polaris is much lower in the sky, only stars within its latitude of 24 degrees are circumpolar. At the equator, where Polaris sits on the horizon, there are no circumpolar stars.
Guess what’s in store for observers at the North Pole? ALL stars are circumpolar. The sky never changes at the Pole. Nothing rises, nothing sets. Only the moving planets and sun make appearances and change positions, otherwise the same group of constellations are ALWAYS visible. There’s no such thing as the seasonal star groups we experience in the mid-latitudes: Virgo in spring, the Summer Triangle in summer and Orion in the winter. No matter what time of year you go out – whether for real or in your imagination – Polaris will always be overhead, the Big Dipper always visible and the Belt of Orion forever below the horizon. In a sense, all star seasons are visible simultaneously. What you see is what you get.
Can you see any of the southern stars from the North Pole? In a word, no, but let’s first define what we mean by southern stars. Just as Earth’s equator divides northern and southern hemispheres, the celestial equator defines the boundary between northern and southern stars. Picture the celestial equator as an imaginary extension of Earth’s equator out into the sky. From mid-northern latitudes the celestial equator cuts midway across the southern sky, which means most of us see a mix of both northern and southern stars when we look up at night.
An observer’s horizon is that great circle located 90 degrees from the zenith. From the North Pole, the horizon is 90 degrees from the North Star and coincides with the celestial equator. Santa and his staff would be forever denied a look at brilliant Sirius and constellations like Scorpius and Sagittarius, because they’re south of the celestial equator and always below the horizon. Only the top half of Orion clears the horizon – two of the three famous Belt stars are invisible. The uppermost star in Orion’s Belt might be seen on a favorable night via atmospheric refraction (bending of light), since it lies directly on the celestial equator.
So yes, it might be nice to have 24 hours of night days on end, but never seeing cool stuff like the Orion Nebula (located below the Belt of Orion) would soon have me yawping for points south. Surely Santa must smile at the sight of Sirius ascending in the sky as he speeds south on his sleigh every December.