Polaris shows the geographic north direction because it lies almost exactly along the direction defined by the Earth's axis of rotation. Thus, it is almost directly overhead when you are at the North pole, near the horizon when you are at the Equator, and cannot be observed when you are in the southern hemisphere.

As a result of the above, Polaris' angle of elevation above the horizon can determine your latitude, but only in the northern hemisphere. To determine your longitude you need something beyond just Polaris. For that you need to determine what is the angle above the horizon of other stars (any bright star you can recognize will do). In combination with a star map, knowledge of the exact time, and the date, you can determine your longitude.

The reason you need to know the date is that although the direction to any star is almost entirely unaffected by the location of the Earth in its orbit around the sun, that location December 21, the overhead direction at the Equator, at midnight, will be 180 degrees away from the overhead direction at the same location at midnight on June 21. This is due to the fact that the Earth will be on the opposite side of the sun, so midnight will have you pointed in the opposite direction in winter with respect to summer.

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