The North Celestial Pole is the point in the sky about which all the stars seen from the Northern Hemisphere rotate. The North Star, also called Polaris, is located almost exactly at this point in the sky. If you go out at night and find the north star you will notice that it does not move during the course of the night, while all the other stars do move, they rotate from east to west around the north star.
You might wonder why this is important for building sundials. The Sun is also a star, so the Sun also rotates around the North Celestial Pole (Because we are so close to the Sun, the tilt of the Earth actually varies the exact axis of rotation of the Sun slightly away from the North Celestial Pole.) It is this rotation that will allow us to use the Sun to tell time.
We need to know where the North Celestial Pole is to use our sundials, but during the day, it is too bright to see the north star. How else can we know where to find this special place in the northern sky?
No matter where you live in the Northern Hemisphere there is an easy way to find the north star. As you might have guessed it is located due north, but how high in the sky is it? The north star is up from the horizon exactly an angle equal to your latitude. So if you live at 50 degrees latitude, the north star will be due north, up 50 degrees.
(An average person has about 10 degrees, from little finger to folded thumb, in a closed fist when held an arm's length away.)
The arrow you see pointing toward the North Celestial Pole in the diagram above is close to the axis about which the Sun rotates. All we need to do to make a well designed horizontal sundial tell time is to point the gnomon of the sundial along this axis. Then, as the Sun rotates around the gnomon it will cast a shadow onto our sundial surface where we will mark the appropriate hourlines. Both the angle of the gnomon and the position of the hourlines depend on the latitude where the sundial will be used.
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