how sundials work
The earliest sundials were in essence a stick (gnomon) which casts a shadow. The position of the shadow can then be used to determine the current solar time. link
The earliest sundials were in essence a stick (gnomon) which casts a shadow. The position of the shadow can then be used to determine the current solar time. link
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 or straight edge, onto a flat surface marked with lines indicating the hours of the day. The shadow cast by 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 base plate. Such designs rely on a 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 (star) and the gnomon's angle with the horizontal plane must be equal to the geographical latitude where the sundial is placed.
The installation of any sundial requires knowing the local latitude, the precise vertical direction (e.g by a level or plumb), and the direction to true North (star).
The time of day is read by the length and position of the gnomon's shadow.
As the Earth turns on its axis, the Sun moves 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 Sun's shadows length cast changes 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.
The installation of any sundial requires knowing the local latitude, the precise vertical direction (e.g by a level or plumb), and the direction to true North (star).
The time of day is read by the length and position of the gnomon's shadow.
As the Earth turns on its axis, the Sun moves 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 Sun's shadows length cast changes 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.
Eventually man discovered that slanting the stick 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. link