The History of Time Keeping In respect to human history, time keeping is a relatively recent desire – probably 5000 to 6000 years old. It was most likely initiated in the Middle East and North Africa.
A clock is defined as a device that consists of two qualities:
•A regular, constant or repetitive process or action to mark off equal increments of time. Early examples of such processes included movement of the sun across the sky, candles marked in increments, oil lamps with marked reservoirs, sand glasses ("hourglasses"), and in the Orient, small stone or metal mazes filled with incense that would burn at a certain pace.
•A means of keeping track of the increments of time and displaying the result.
Using the Sun Around 3500 B.C. the Egyptians built “Obelisks” -- tall four-sided tapered monuments -- and placed them in strategic locations to cast shadows from the sun.
Their moving shadows formed a kind of sundial, enabling citizens to partition the day into two parts by indicating noon. They also showed the year's longest and shortest days when the shadow at noon was the shortest or longest of the year. Later, markers added around the base of the monument would indicate further time subdivisions.
Around 1500 B.C., the Egyptians took the next step forward with a more accurate “shadow clock” or “sundial.” The sundial was divided into 10 parts, with two “twilight” hours indicated. This sundial only kept accurate time (in relative terms) for a half day. So at midday, the device had to be turned 180 degrees to measure the afternoon hours.
The Egyptians improved upon the sundial with a “merkhet,” the oldest known astronomical tool. It was developed around 600 B.C. and uses a string with a weight on the end to accurately measure a straight vertical line
AstrolabeThe astrolabe is a very ancient astronomical computer for solving problems relating to time and the position of the Sun and stars in the sky.
Several types of astrolabes have been made. By far the most popular type is the planispheric astrolabe, on which the celestial sphere is projected onto the plane of the equator.
Astrolabe
“Clepsydras” or Water Clocks were among the first time-keeping devices that didn’t use the sun or the passage of celestial bodies to calculate time.
One of the oldest was found in the tomb of ancient Egyptian King Amenhotep I, buried around 1500 B.C. Around 325 B.C. the Greeks began using clepsydras (Greek for "water thief") by the regular dripping of water through a narrow opening and accumulating the water in a reservoir where a float carrying a pointer rose and marked the hours.
A slightly different water clock released water at a regulated rate into a bowl until it sank. These clocks were common across the Middle East, and were still being used in parts of Africa during the early 20th century. They could not be relied on to tell time more closely than a fairly large fraction of an hour.
Mechanical Clocks
The mechanical clock was probably invented in medieval Europe. Clever arrangements of gears and wheels were devised that turned by weights attached to them. As the weights were pulled
downward by the force of gravity, the wheels were forced to turn in a slow, regular manner. A pointer, properly attached to the wheels, marked the hours.
A technological advance came with the invention of the “spring-powered clock” around 1500-1510, credited to Peter Henlein of Nuremberg, Germany. Because these clocks could fit on a mantle or shelf they became very popular among the rich. They did
have some time-keeping problems, though, as the clock slowed down as the mainspring unwound. The development of the spring-powered clock was the
precursor to accurate time keeping.
In 1582, Italian scientist Galileo, then a teenager, had noticed the swaying chandeliers in a cathedral. It seemed to him that the movement back and forth
was always the same whether the swing was a large one or a small one. He timed the swaying with his pulse and then began experimented with swinging weights. He found that the "pendulum" was a way of marking off small intervals of time accurately.
In 1656 Dutch astronomer Christian Huygens first devised a successful pendulum clock. He used short pendulums that beat several times a second, encased the works in wood, and hung the clock on the wall. It had an error of less than one minute a day. This was a huge
improvement on earlier mechanical clocks, and subsequent refinements
reduced the margin of error to less than 10 seconds per
day.
In 1721George Graham improved the pendulum clock’s accuracy to within a second a day by compensating for changes in the pendulum's length caused by temperature variations. The mechanical
clock continued to develop until it achieved an accuracy of a hundredth-of-a-second a day and it became the accepted standard in most astronomical observatories.
The running of a Quartz clock is based on an electric property of the quartz crystal. When an electric field is applied to a quartz crystal, it changes the shape of the crystal itself. If you then squeeze it
or bend it, an electric field is generated. When placed in an electronic circuit, the interaction between the mechanical stress and the electrical field causes the crystal to vibrate, generating a constant electric signal which can then be used to measure time.
Termed NIST F-1, the new cesium atomic clock at NIST, the National Institute of Science and Technology, in Boulder, Colorado is the nation's primary frequency standard that is used to define Coordinated Universal Time (known as UTC), the official world time. NIST-7 had been the primary atomic time standard for the United States since 1993 and was among the best time standards in the world.
The 'Natural frequency' recognized currently as the measurement of time used by all scientists, defines the period of one second as exactly 9,192,631,770 oscillations or 9,192,631,770 cycles of the Cesium Atom's Resonant Frequency. The cesium-clock at NIST is so accurate that it will neither gain nor lose a second in 20 million years!
Our Solar SystemOur solar system consists of the sun, nine planets which orbit around the sun and some smaller bodies like moons and comets. The orbits of all except two planets, lie in the nearly the same plane (inclined by less than 3,4°); so that we can say: the solar system is disk-shaped. Pluto and Mercury are
the odd ones out, with Pluto's orbit inclined at 17,2° and Mercury's orbit inclined by 3,4° to earth's orbit. All planets revolve counterclockwise around the sun - as seen from the north.
Earth's Movement around the SunThe planets revolve around the sun not in neat circles, but in ellipses, with the sun at one focus of their ellipse. For Earth this ellipse is nearly a circle - earth's distance from the sun varies by less than 2%. According to Kepler's second law of planetary motion, the line from the sun to the earth sweeps out equal areas in equal times.
This means that if area A is equal to area B; then tA = tB ( see figure on the right). Since the distance covered during tA is longer than the distance covered during tB, it follows that Earth must move faster during tA than during tB.Therefore when the earth is closer to the sun it travels faster and if it is further away from the sun it travels slower. Earth is closest to the sun in January - about 147.5 million km and furthest from the sun in June at about 152.6 million km.
Earth's Rotation around its AxisEarth rotates around its own axis through 360° once every 24 hours. This is the cause of day and night. If we divide 360° by 24, we get 15°. In other words, it takes the Earth 1 hour to turn through 15°.The Earth's polar axis tilts at an angle of 23,5° with its orbital plane around the sun. This inclination is responsible for the climate changes and the seasons experienced on Earth.
The Declination
Earth's equator lies in the same plane as the equator of the sun, but since the Earth goes around the sun with its head tilted to one side, the plane of Earth's orbit makes an angle of 23,5° with the celestial equator. This is called the declination of Earth.
Earth’s coordinate systemAny surface is two dimensional and we can define any point on a surface by two coordinates. To define a position on the earth's surface we use two coordinates called latitude and longitude.
LatitudeThe earth rotates around its own axis and the two end points of the axis are called the north and south pole. The imaginary line that divides the earth in half between the two poles is called the equator. The latitude of the equator is 0°. The latitude of any other point on the surface of the earth is the angle subtended at the centre of the earth between that point and the equator.
For instance, the latitude of Pretoria is 25° 45' S; that means that the angle q = 25° 45' south of the equator.
LongitudeA north-south line is called a meridian or longitude. Meridians are measurements of width and give the angle between the Greenwich meridian and any other place. The longitude is the angular distance (east-west) from the standard meridian. If we say that the longitude of Pretoria is 28° 15' E, we mean that Pretoria lies on a meridian 28° 15' east of the Greenwich meridian. If it is noon at one place on a specific meridian it will also be noon at all other places on the same meridian.
A nautical mile is defined as 1 minute of longitude and since the earth is not a perfect sphere the nautical mile is longer at the equator than at the north or south pole.An international nautical mile = 1,852 km.
A major advance that made early navigation much more accurate was the invention of the chip log (c.1500-1600). Essentially a crude speedometer, a light line was knotted at regular
intervals and weighted to drag in the water. It was tossed overboard over the stern as the pilot counted the knots that were let out during a specific period of time. The knots were spaced at a distance apart of 47 feet 3 inches and the number of these knots which ran out while a 28-second sand glass emptied itself gave the speed of the ship in nautical miles per hour. The proportion of 47 feet 3 inches to 6,080 feet is the same as 28 seconds to one hour. Interestingly, the chip log has long been replaced by equipment that is more advanced but we still refer to miles per hour on the water as knots.
History of the Sundial"In today's complex digital world, the sundial has become a forgotten timepiece. While not as convenient as a wristwatch, a sundial links timekeeping to its ancient celestial origins, and a well-designed one can accurately tell time to the minute." (Mayall, 1994)About this tutorial
The sundial dates back to the Egyptian Period, around 1500 B.C. It was also used in ancient Greece and Rome. In central Europe it was the most commonly used method to determine the time, even after the mechanical clock was developed in the 14th century. The sundial was actually used to check and adjust the time on mechanical clocks until late into the 19th century.
The oldest dial known c. 1500 B.C. The sundial is considered to be the first scientific instrument.
Sundials come in all shapes and sizes, from tiny pocket dials to huge, meter high dials in observatories or sundial parks. Although their main purpose is to tell the time, they are often used as focus points in gardens, as art in the form of sculptures and even as jewelry.
Types of SundialsSundials are divided into two main groups:Altitude dials determine the time from the sun's
altitude (the sun's height above the horizon). These dials either have to be correctly oriented to the compass directions or they have to be aligned to the sun.
Azimuth dials determine the time from the hour angle of the sun (the sun's angle on its daily arc). These dials have to be correctly oriented and therefore a magnetic compass is often incorporated in the instrument.
Sundial TypesMost planar (flat) sundials are categorized by the orientation of the face - the direction faced by the surface that receives the shadow.
Horizontal This is the type found commonly on pedestals in gardens. The dial plate is horizontal. The gnomon (which casts the shadow) makes an angle equal to the latitude of the location for which it was designed
Vertical Vertical sundials may be direct south dials if they face due south (in which case the gnomon will be at an angle equal to the co-latitude of the place, and the hour lines, if delineated for local time at the place, will be symmetrical about the vertical noon line).
If they do not face directly south, they are described as
declining dials.
Equatorial dials have the dial plate fixed in the plane of the equator. The gnomon is perpendicular to the dial plate. The hour lines are spaced equally at 15 degree intervals. The armillary sphere is a development of this idea, and consists of a series of rings in the planes of the equator and the meridian, and a rod parallel to the earth's axis and passing through the center of the rings.
Analemmatic dials are not very common. They are unusual because the gnomon is vertical, and the hours are marked not by lines but by points falling on the circumference of an ellipse. The gnomon has to be moved depending on the time of year, so that the shadow falls on the correct point.
Portable dials come in many varieties, such as the shepherd's dial, the tablet dial, the ring dial and others. They are not strictly a separate type of dial, but can be of the types listed above.
