Difference between revisions of "Sundial"
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[[File:Gardensundial.jpeg|left|An Analemmatic Sundial|]]
Revision as of 21:54, 5 December 2010
The Sundial is a device that tells time by using a flat surface and a long stick called a gnomon to turn the sun's reflection into a shadow that corresponds to a particular marking. The sundial was once used by many civilizations, including the likes of the Ancient Egyptians, the Babylonians, the Ancient Greeks, the Roman Empire, the Chinese, the Mayans, and the European countries during the Renaissance.
- 5000-1500 BC: The first device to tell time was made; the simple model consisted of a pillar sprouting from the ground, similar to the later models of the sundial. Shadow clocks were also developed by Egyptian and Babylonian astronomers.
- 800 BC: The earliest known sundial preserved came from this time period. The model had a base with six different time divisions inscribed, and a raised crosspiece that casted a shadow pointing to a division.
- 560 BC: The Ancient Greeks develop the basic principles of the sundial, derived originally from Babylonian ideas. Since the Greeks founded geometry as well as the concept of conic sections, sundial construction came natural to them.
- 400 BC: The Ancient Greeks begin using the water clock, which measures time by the outflow of water from a particular location..
- 300 BC: Babylonian priest Berossus creates a sundial in the form of a half sphere cut into a large block. A small bead was placed at the center of the half sphere, and its shadow would move in a circular arc throughout the day. The arc was divided into 12 parts, each representing a different hour.
- 263 BC: Around this time the sundial assumed the role as a constant unit of time. The first sundial was also imported to Rome from Sicily.
- 100 BC: Astronomer Theodosius of Bithynia invents a universal sundial that can be used anywhere on Earth, regardless of its position relative to the sun.
- 30-25 BC: Roman author Vitruvius outlines 13 different styles of the sundial.
- 10 BC: The Romans build a very large sundial and call it the Solarium Augusti.
- 150 AD: The Greeks use trigonometry to plot hours with a slanted gnomon.
- 1200: Equal hours of the day are introduced relative to the sundial.
- 1500-1800: Due in part to the spark of the Renaissance, European sundials become extremely popular as the main time-telling device.
- 1700s AD: Clocks and watches begin to replace sundials since they do not require sunlight, but they are frequently inaccurate and must depend on sundials for exact times.
- 1724: The Prince of Dials is placed in Old Delhi, it was the largest sundial in the world at the time. The sundial was so big that the shadow moved at a rate of five feet per minute.
- 1777: French General Lafayette gives General and eventual President George Washington a silver Explorer Sundial as a gift of respect.
- Early 1800s: Mechanical clocks become accurate and inexpensive enough to begin replacing sundials.
How the Sundial Works
The basic model of the sundial consists of a sloping face or flat surface that faces the sun directly. A gnomon (pronounced 'no men') or a long stick/pole is attached to the base of this dial, usually near the middle, and the shadow of the gnomon falls on the dial as the sun hits. Also on the dial were markings that designate the hour of the day; the the time was determined by which of these markings the the shadow falls on. These lines had to be positioned based on latitude and longitude, and they usually ran parallel from top to bottom, sometimes also showing the different months of the year as well as the summer and winter solstices. The markings were usually formatted as a long line from the base of the dial to the edge, where a number from 1-12 would be written, often in Roman numerals. Many sundials also included religious or philosophical proverbs written across the base of the dial. Sundials were built and marked based on their position relative to the earth's axis; the angle of the gnomon had to be parallel to this axis. Initially they were not portable past a small range because the position relative to the axis would be skewed if a sundial was moved a certain distance without being adjusted. The dial plate was usually made of light and refractory material, which was unalterable by heat or moisture in order to withstand outside weather conditions. Greek sundials tended to use bowl-shaped dials, with gnomons to tell time as well as seasonal information with the size of the shadow being indicative of the time of year. Placing the gnomon across a curved surface would indicate the summer and winter solstices and the equinoxes in fall and spring.
Since the Earth moves in an elliptical orbit, and since the equator is tilted about 23.5 degrees off the orbital path, a discrepancy in time can occur. This difference is usually about 15 minutes from a watch, depending on the time of the year.
Different Types of Sundials
During the European Dark Ages Muslims used trigonometry to make flat circular sundials. These sundials were the first to propose hours of equal length, and they often marked the hours that Muslims prayed. This model also helped to record events on a schedule.
Known as the 'garden sundial', the horizontal model was found commonly on pedestals in gardens. The dial plate was horizontal, and the gnomon formed an angle equal to the latitude of the location. The plane was aligned horizontally to match the gnomon angle. This model was advantageous because it was easy to read, and because the sun would light the plane throughout the entire year.
The vertical model was usually found on the walls of buildings because they were easy to read from far away. A common practice was to position vertical sundials on all four sides of a rectangular tower, so that time could be told throughout the day in four divisions. Vertical models were split up into two subdivisions. If the sundial faced south it was called a direct south dial, and the gnomon would have an angle aligned with the Earth's axis of rotation. Dials that faced north, east or west were called vertical direct, and had limited hours. If the dial faced east it could only tell time in the morning, as the sun would not shine down in its direction in the afternoon; thus the model with four sundials on a rectangular tower was put in use. Dials that faced north could only tell time before 6 AM and after 6 PM, and consequently it was the least used sundial. Vertical dials that faced east or west were called polar dials.
In this model the dial plate was in the plane relative to the equator, and the gnomon was perpendicular to the dial plate. This model also included an armillary sphere, which consisted of rings in the planes of the equator. The shadow was cast from below in the winter and from above in the summer. A drawback of this design was that no clear shadow was produced on the dial near the equinoxes in spring and fall, since the sun would move on a circle that was very similar to that of the equatorial plane.
These models resembled a sphere, with the dial being a semicircle. They were popular in France to create and maintain train schedules prior to World War I.
This model was not common because the gnomon was vertical, and the hours were marked by points instead of lines. The gnomon had to be moved depending on the time of year. This dial was most commonly found laid out on lawns where a person could act as the gnomon. Advantages included easiness in set up and in reading the time, although it was typically less accurate.
Universal Equinoctial Ring Dial
Inspired by the astrolabe, this model also included the armillary sphere. The most portable of the sundial models, it contained a thin slit that allowed sun rays to fall on the hour lines of the ring after the user tilted the dial towards the true north.
Altitude Based Sundial
This model measured the height of the sun in the sky, instead of the shadow of the suns rays. The suns altitude is the same at times equally far from noon (ex: 8 AM and 4 PM) so the user had to make a judgment call to decide whether or not it was morning or afternoon.
Death of the Sundial
The Sundial's death cannot be pinpointed to a single day; it is still utilized today in certain situations. The primary cause of its decline was the rise of clocks and mechanical watches, which first arose in the 1700's. Initially they were inaccurate and relied on the sundial for accurate times, but as their design was improved they eventually became much more practical than the sundial. By the 18th century clocks and watches could function 24 hours a day, whether or not the sun was out, thus propelling them as more universal than sundials. Mechanical watches and clocks were easier to make, easier to use, and more compact, thus sundials lost popularity.
Today sundials are used mainly as decoration pieces in gardens; mini models of the sundial are often found for purchase at gift shops and nurseries. In 1998, Bill Nye the "science guy" noticed the general outline of a sundial while examining pictures of a panoramic camera that was used on the Mars Surveyor Lander. He talked to NASA and got approval to attach a simple model of a sundial to two Mars rovers, one that was similar to a device they would have included anyway. The Mars sundials had no hour marks since the rover would constantly be changing positions. The primary purpose of the Mars sundial was to distinguish between colors; in the past everything on Mars appeared pink since the Martian sky is pink, but with the sundial the team would be able to tell if this was an illusion caused by pink light bouncing off the sky.
Another modern form of the sundial is the sundial bridge, which opened at Turtle Bay across the Sacramento River in Redding, California. The bridge is 700 feet long, 23 feet wide, and 215 feet high, and weighs 1,600 tons. The hour markers are located on the ground behind the gnomon tower, which stands at the end of the bridge.