StarTrails

As the Earth turns on its axis, the position of the sun, moon, and stars changes in the sky. This causes all of these heavenly bodies to rise and set. The rising and setting of the sun and the moon provide the most dramatic manifestation of this, but it is by observing the motion of the stars that one gets the clearest picture of what is happening.

This image was produced by taking a night-long exposure of the stars on a moonless night. Most of the stars trace arcs in the sky, as the Earth rotates on its axis. However, the axis of the Earth's rotation does not move from night to night (it takes centuries for it to move appreciably with respect to the stars), and the stars that lie near the axis of rotation of the earth do not change their position in the sky during the night.

This realization was key to allowing humans to navigate the oceans, where there are no landmarks to tell one's position. The sun can be used to gauge east and west during the day, and if one can keep track of the seasons, its elevation can tell someone their latitude. At night, the moon is not always up (during a new moon, it lies above the day side of the earth), but the location of the Polaris (also called the pole star or the north star) clearly marks the direction of north, and its elevation above the horizon reveals the latitude.

Once Europeans invented clocks, the time of day could be compared to the position of the sun and the stars, allowing sailors to determine their longitude as well. This enabled the trade that led to the growth of Europe.