It's dark. You dropped your compass in the woods. Which way is North?

If it's overcast you're out of luck, but otherwise you can find Polaris, the North Star (sometimes "Pole Star"), assuming that you live in the Northern Hemisphere. This is the bright star that is closest to the line passing though Earth's axis of rotation. An amazing number of people don't know (or just forgot) how to do this, so it's worth noding. This could save your life, or impress your friends, or just get you back to camp before they said they were going to open the beer and have the trout cooked.

Wherever you are in the Northern Hemisphere, you should be able to see either Ursa Major (the "Big Dipper") or Cassiopeia, since they are roughly opposite Polaris, (so one will be high in the sky if the other one is lower or obscured by hills, trees, etc.) and they are both extremely bright and distinct. Scroll down past the makeshift diagram below, which you will need as a reference.

                        *       *                        

                              * Polaris                  

                       Ursa Major                        
                       ("big dipper")                    
                             *                *          

Method number 1: Find Cassiopeia, which looks like a big letter W (or sigma, depending on how it is oriented), with one branch of the W wider and deeper than the other. The mouths of the two V's in the W will open up in the direction of Polaris. Go about two W-lengths in that direction (sort of but not quite perpendicular to the line that would underline the W) and you are there.

Method number 2: Find Ursa Major, which looks like a big ladle; pretty much everybody recognizes the "Big Dipper" from being taught as a child. Follow the last two stars in the ladle in a straight line going up, about one dipper-length, and it should intersect Polaris almost exactly. This is easier than locating via Cassiopeia if you can do it because Polaris is remarkably close to that line.

An interesting fact: Polaris has not always been the North Star. The Earth precesses (very slowly -- the period is roughly 25,000 years) as well as rotates and revolves around the sun. This is called the "precession of the equinoxes", although it is sometimes mistakenly called the "procession of the equinoxes" (even though the latter makes perfect sense). In A.D. 14000, Vega (in the constellation Lyra) will be the North Star.

That's it. Next week: Using Orion's belt to find the Pleiades

Being along the axis of rotation of the earth, Polaris does not seem to move in the sky.

If the sky is clear find a nice comfortable spot and place a rod pointing at a star near the horizon. After about 10 minutes the star (unless it is polaris) will have drifted from the rod. If the star has moved towards the horizon you are pointing in a westerly direction, likewise if the star has risen up from the horizon you are looking in an easterly direction.

Some stars will just skim along the horizon instead of rising or setting. These are the circumpolar* stars. If they move clockwise in the sky you are pointing north and at their centre of revoloution is Polaris.

If they are moving anti-clockwise you are in the southern hemisphere and no amount of searching will find Polaris.

* A circumpolar star is a star that, from the observers point of view, allways circles the celestial pole. By definition it is a star that never dips below the horizon. If you are at a lattitude of L then any star that has a right ascenscion of ra greater than 90o - L will be circumpolar. During the summer the sun is circumpolar to observers above the arctic circle.

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