Monday, May 20, 2013

How does a sundial work?

Sundials are the oldest known devices that are used to measure time. It depends on the rotation and movement of the sun. As the sun moves from east to west, the shadows formed predict the time of the day. The Egyptians were the first to use the sundials. They used a stick or pillar called the gnomon. Time was calculated depending on the length of the shadow.
The Egyptians built a t-shaped sundial comprising of a crossbar and a vertical stick. On the stick, five hours were written. In the morning, the stick was placed facing east, and in the afternoon, the stick was placed facing west. The Egyptians and Babylonians built obelisks to calculate time as the shadows cast were similar to a sundial. It also helped to calculate the longest and shortest day of the year. In time, Egyptians build portable sundials, a smaller version of the obelisks.

The Greeks used a sundial called the “pelekinon” where the gnomon or vertical rod was placed on a horizontal or half spherical face. These sundials are marked to predict time accurately throughout the year. They built a more accurate sundial based on their knowledge of geometry. The hemicycle is one of their inventions. It is a cubical block of wood or stone into which a hemisphere is cut with a stick or rod attached to one end. This created a circular arc with varied length, depending on the season. These arcs were further divided into twelve to indicate the length of each day. The famous Tower of Winds in Athens comprised of eight sun dials that faced cardinal points of the compass.
The first sundial in Rome was captured from the Samnites. Vitruvius gives the city its own sundial, including portable ones. “Rigou” as sundials were called in China were in existence from the Zhou Dynasty. The most popular sundial was the portable sundial during the Song Dynasty.
The sundials specify time by casting shadows or light on the base plate or face plate. This plate is flat but it can be shaped spherical, circular, conical or just about any shape. The face has markings on it to indicate the time. Some sundials help calculate the dates and the dial may contain multiple markings. In some cases, the dial may have settings to change the date so that time may be calculated accordingly. Time can be calculated most accurately at noon.
The working of a sundial is not so simple due to the tilted axis of the earth. If this is not accounted for, the sundial will have a different time each week. By aligning the gnomon with the earth’s axis, the difference can be compensated. Different sundials have different principles for calculating time. In a horizontal sundial, the gnomon or needle is tilted in line with the earth’s axis while the base is kept horizontally. In an equatorial sundial, the base plate where the shadow falls is kept at an angle that is parallel to the equator while the gnomon is perpendicular to the plate. The most common type is the vertical sundial where the base plate is vertical and the gnomon is aligned to the earth’s axis.
A sundial’s position in latitude in one hemisphere should be the opposite in the other hemisphere. For instance, a direct south vertical sundial in the northern hemisphere becomes a direct north vertical sundial in the south hemisphere. Positioning the sundial accurately requires finding true north or south and positioning them parallel to the axis in terms of longitude. The rotation of the sun is also calculated when building and placing a sundial.

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