Yesterday, March 20, was the first day of spring in the Northern Hemisphere, and the first day of autumn in the Southern Hemisphere. The Sun is just about directly above the Tropic of Cancer at noon now - the Sun rises with the constellation of Cancer the Crab.

At night, in the Northern Hemisphere, the spring time constellations are taking the stage as the winter constellations are exiting, stage right. Leo the Lion, Virgo the Virgin, and Bootes the Herdsman are all dominant in the eastern sky before midnight. At the same time, Orion, Canis Major, Auriga, and Taurus are sinking toward the western horizon In a few months, the prominent summer constellation including Lyra, Aquilla and the Northern Cross (Cygnus the Swan) will make their claim on the eastern horizon.

In the mid latitudes of the Southern Hemisphere, you can still see Canis Major and Orion. If you recall from our question in January, Orion pretty much straddles the Celestial Equator - it's visible in both hemispheres. However, you might notice that Orion looks a bit different. It's upside down from the perspective of a viewer in Argentina or Australia - the sword lies above the familiar 3-stars in a row belt. Not only are Orion and Canis Major easy to spot, but the constellations of the zodiac such as Taurus, Gemini, Leo, and Virgo are all visible too. Leo, Bootes and Virgo can be seen now in the Southern Hemisphere, in the early evening, rising in the northeast. At the same time, the bright constellations of Centaur and the Southern Cross (Crux) ride high in the southeastern sky.

Of course there are things observers in the Southern Hemisphere can see that we can't here north of the Equator and vise-versa. For instance, the Southern Hemisphere has the Large and Small Magellanic Clouds, two satellite galaxies of our Milky Way galaxy, which are visible on most clear, moonless nights throughout the year. These "clouds" are circumpolar meaning that, although they rotate across the sky, for people living south of about 40 degrees south latitude, they're close enough to the South Celestial Pole so that they never quite dip below the horizon. Rather, they appear to circle the pole throughout the year, The Southern Cross and Centaurus are also both circumpolar constellations. Therefore, in the mid latitudes of the Northern Hemisphere, we can't see them.

If you live at 40 degrees north latitude, and if you have an unobstructed view of the southern horizon, it's theoretically possible to see a star that would be overhead for someone at 50 degrees south latitude (40 + 50 = 90). From our view, such a star would be kissing the horizon. If we lived on a planet with no atmosphere, it would be possible to see a star lying on the horizon, but our atmosphere is too dense to permit viewing of stars closer than about 5 degrees to the horizon - too much starlight is scattered and absorbed by the atmosphere. Anyway, the point is, just because we're in the Northern Hemisphere, it doesn't mean we can't see stars in the Southern Hemisphere. We just can't see the ones that are more than about 80 degrees or so of our latitude.

Of course, the Northern Hemisphere has circumpolar constellations too. However, our circumpolar constellations (at about 40 degrees north latitude) are feeble compared to the ones in the Southern Hemisphere. In the Northern Hemisphere, there are 5 circumpolar constellations; Ursa Major (the Big Dipper), Ursa Minor (the Little Dipper), Cephus (the King), Cassiopeia (the Queen), and (Draco the Dragon). Not a single one of the stars in any of these constellations is considered to be a very bright star (first order magnitude or brighter). Sure, the Big Dipper is easy to identify, and 5 of the 7 stars of the dipper are relatively bright second order magnitude stars, but all-in-all, the northern sky is rather devoid of light. By contrast, in the Southern Hemisphere, there are 11 constellations which are circumpolar and 6 first order magnitude stars!

So what is it that the Northern Hemisphere has that the Southern Hemisphere doesn't. The answer is a "pole star." Polaris or the North Star is nearly directly above the North Pole (it's actually about 1 degree away from the celestial pole). You might think that with all of the stars in the sky, it shouldn't be that unusual for a given star to rest above the pole, but really, it's an extremely unlikely occurrence. It's even more unlikely that our pole star would be relatively bright - second order magnitude. If you divided the night sky into squares that are one degree latitude by one degree longitude in size, there would be 41,253 square degrees in our night sky. There are approximately 2,000 stars that we can see on the clearest night, and perhaps 6,000 different stars are visible to us throughout the year, but only 50 of these are as bright or brighter than Polaris. The chances of a star like Polaris occupying a place over the pole are about slim indeed - about 1 in 1,000. Nevertheless, Polaris defies the odds and has become our guiding light.

Inhabitants of the Southern Hemisphere can't see Polaris, even if they just live one degree south of the Equator, and they don't have a pole star they can call their own. There's nothing even close to the South Pole. A couple of faint stars are about 10 degrees from the South Pole, but the nearest star of equivalent brightness to Polaris is nearly 15 degrees from the pole.

Finding Polaris is a snap. You can simply use the "pointer stars" in the bowl of the Big Dipper to point the way. However, trying to locate the spot in the night sky over the South Pole is a chore. For someone in South Africa, for instance, it's first necessary to find south. The Southern Cross can be used for this purpose. The northern most and southern most stars in this 4 star constellation points toward the South Pole. To zero in on the actual location of the spot above the South Pole, a line can be drawn from the first order magnitude star, Canopus, to the line extending south from the Southern Cross. Canopus can be found to the lower right or southeast of the Southern Cross and is about the same distance to the pole as is the cross. These two lines intersect at an approximately 90 degree angle (right angle) and fairly close to the South Pole.

Knowing the direction of the either pole nowadays is basically an academic exercise, but before it was understood that clocks and sextants could be used for establishing your latitude and longitude, mariners and explorers often relied on old Polaris. The height of the North Star above the horizon is equal to your latitude. Of course, compasses point to the north, but they point toward the geomagnetic pole and not toward the true geographic pole. In some cases, such as in the central US, there's very little offset between these two, but depending upon where you are in the Northern Hemisphere, the offset can be considerable. For example, from England, there's approximately a 45 degree difference between true north and geomagnetic north - geomagnetic north is in a northwest direction from the British Isles.

The North Star isn't just a run-of-the-mill star in terms of its size either. It's huge, and it shines with the light of about 1,600 Suns. The only reason it isn't brighter is because it's so far away - 600 light years. Polaris and the Sun are now about as close to each other as they'll ever get. Alas, all good things must come to an end, and in a few centuries, Polaris will drift away from it's current heralded post to a location carrying much less esteem, somewhere to the south of where it is now. If it's any consolation, Polaris will return to the pole again but not for another 20,000 thousand years.

For more about the North Star see the book Secrets of the Night Sky by Bob Berman. Also see, last year's question about the Big Dipper (May, 2001).