A sundial is in essence simply any form of stick - known as a style or gnomon - which casts a shadow. The position of the shadow can then be used to determine the current solar time.
A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the Sun casts a shadow from the gnomon, which is a thin rod straight edge, onto a flat surface marked with lines indicating the hours of the day. The shadow cast by the end of the gnomon is the solar time at all times. As the Sun moves across the sky, the shadow of the gnomon's edge progressively aligns with different hour-lines on the plate. Such designs rely on the gnomon that is aligned with the Earth's rotation axis. Hence, if such a sundial is to tell the correct time, the gnomon must point towards the true North and the gnomon's angle with the horizontal plane must be equal to the geographical latitude where the sundial is placed.
The installation of many dials requires knowing the local latitude, the precise vertical direction (e.g., by a level or plumb-bob), and the direction to true North.
During the day people saw that the shadow cast by a tree, a rock, or even their own body was long early in the morning and grew shorter and shorter until it almost disappeared when the Sun was in the middle of the day. They also would have noticed that the shadow grew longer again, on the other side of the tree, as night came.
The shadow stick is the earliest form of sundial. People judged the time of day by the length and position of the stick's shadow.
As the Earth turns on its axis, the Sun appears to move across the sky. The shadows the Sun casts move in a clockwise direction for objects in the northern hemisphere. If the Sun rose and set at the same time and spot on the horizon each day, sticks would have been accurate clocks. However, the Earth is always spinning like a top. It spins around an imaginary line called its axis. The axis runs through the center of the Earth from the North Pole to the South Pole. The Earth's axis is always tilted at the same angle.
Every 24 hours the Earth makes one complete turn, or rotation. The Earth rotates on its axis from west to east. The Earth's rotation causes day and night.
On the Earth's yearly trip around the Sun the North Pole is tilted toward the Sun for six months and away from the Sun for six months. This means the shadows cast by the Sun change from day to day.
Because the Earth is almost spherical, the ground at the base of a shadow stick will not be at the same angle to the Sun's rays as at the equator. Because of this, the shadow stick will not move at a uniform rate during the day.
Picture 4: Equatorial sundial at the Imperial Canal of Aragon passing through Saragossa (Spain). (Click here to enlarge)
Picture 5: Horizontal sundial in Perdiguera, Saragossa (Spain). (Click here to enlarge)
Eventually man discovered that slanting the gnomon and aiming it North made a more accurate sundial. Because its angle makes up for the tilt of the Earth, the hour marks remained the same all year long. After this discovery, people were able to construct sundials that were much better at keeping accurate time.
In the sundial of picture 4 we can see that the arrow replaces the gnomon or stick and is parallel to the axis of the world. The arrowhead points to the North Pole. There are various types of sundials. The picture 4 is the equatorial type since the gnomon projects shadows on a plane parallel to Equator.