lesson 15—the riddle of time btlew part two enter

Lesson 15—The Riddle of Time

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Part TwoPart Two

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I. Development of Clocks

II. Biological Clock

III. Calendars

Background Background InformationInformation


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I. Development of Clocks—I. Development of Clocks—Sun ClocksSun Clocks


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Not until somewhat recently (that is, in terms of human history) did people find a need for knowing the time of day. As best we know, 5,000 to 6,000 years ago great civilizations in the Middle East and North Africa began to make clocks to augment their calendars. With their attendant bureaucracies, formal religions, and other burgeoning societal activities, these cultures apparently found a need to organize their time more efficiently.


To be continued on the next page.


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The Sumerian culture was lost without passing on its knowledge, but the Egyptians were apparently the next to formally divide their day into parts something like our hours. Obelisks (slender, tapering, four-sided monuments) were built as early as 3500 BC. Their moving shadows formed a kind of sundial, enabling people to partition the day into morning and afternoon. Obelisks also showed the year’s longest and shortest days when the shadow at noon was the shortest or longest of the year. Later, additional markers around the base of the monument would indicate further subdivisions of time.




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Another Egyptian shadow clock or sundial, possibly the first portable timepiece, came into use around 1500 BC. This device divided a sunlit day into 10 parts plus two “twilight hours” in the morning and evening. When the long stem with 5 variably spaced marks was oriented east and west in the morning, an elevated crossbar on the east end cast a moving shadow over the marks. At noon, the device was turned in the opposite direction to measure the afternoon “hours”.





I. Development of Clocks—I. Development of Clocks—Water ClocksWater Clocks


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Water clocks were among the earliest timekeepers that didn’t depend on the observation of celestial bodies. One of the oldest was found in the tomb of the Egyptian pharaoh Amenhotep I, buried around 1500 BC. Later named clepsydras (“water thieves”) by the Greeks, who began using them about 325 BC, these were stone vessels with sloping sides that allowed water to drip at a nearly constant rate from a small hole near the bottom. Other clepsydras were cylindrical or bowl-shaped containers designed to slowly fill with water coming in at a constant rate.




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Markings on the inside surfaces measured the passage of “hours” as the water level reached them. These clocks were used to determine hours at night, but may have been used in daylight as well. Another version consisted of a metal bowl with a hole in the bottom; when placed in a container of water the bowl would fill and sink in a certain time. These were still in use in North Africa in the 20th century.




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More elaborate and impressive mechanized water clocks were developed between 100 BC and 500 AD by Greek and Roman horologists and astronomers. The added complexity was aimed at making the flow more constant by regulating the pressure, and at providing fancier displays of the passage of time. Some water clocks rang bells and gongs; others opened doors and windows to show little figures of people, or moved pointers, dials, and astrological models of the universe.




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In the Far East, mechanized astronomical/astrological clock making developed from 200 to 1300. Third-century Chinese clepsydras drove various mechanisms that illustrated astronomical phenomena. One of the most elaborate clock towers was built by Su Sung and his associates in 1088. Su Sung’s mechanism incorporated a water-driven escapement invented about 725. The Su Sung clock tower, over 30 feet tall, possessed a bronze power-driven armillary sphere for observations, an automatically rotating celestial globe, and five front panels with doors that permitted the viewing of changing manikins which rang bells or gongs, and held tablets indicating the hour or other special times of the day.





I. Development of Clocks—I. Development of Clocks—Pendulum ClocksPendulum Clocks


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In Europe during most of the Middle Ages (roughly 500 to 1500), technological advancement was at a virtual standstill. Sundial styles evolved, but didn’t move far from ancient Egyptian principles. During these times, simple sundials placed above doorways were used to identify midday and four “tides” of the sunlit day. By the 10th Century, several types of pocket sundials were used. One English model identified tides and even compensated for seasonal changes of the sun’s altitude.




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Then, in the early-to-mid-14th century, large mechanical clocks began to appear in the towers of several large Italian cities. There is no evidence or record of the working models preceding these public clocks that were weight-driven and regulated by a verge-and-foliot escapement. Verge-and-foliot mechanisms reigned for more than 300 years with variations in the shape of the foliot. All had the same basic problem: the period of oscillation of this escapement depended heavily on the amount of driving force and the amount of friction in the drive. Like water flow, the rate was difficult to regulate.




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Accurate Mechanical ClocksIn 1656, Christian Huygens, a Dutch scientist, made the first pendulum clock, regulated by a mechanism with a “natural” period of oscillation. Although Galileo Galilei, sometimes credited with inventing the pendulum, studied its motion as early as 1582, Galileo’s design for a clock was not built before his death. Huygens’ pendulum clock had an error of less than 1 minute a day, the first time such accuracy had been achieved. His later refinements reduced his clock’s errors to less than 10 seconds a day.




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The first atomic clock, Caesium I, was designed by Louis Essen and built at the National Physical Laboratory in Teddington in 1955. Although it was not the first machine to use atoms for timekeeping, it was the first to keep time better than the best pendulum or quartz clocks.It was also the first clock whose timekeeping was significantly more constant than the rotation of the Earth. Modern atomic clocks are even more accurate than Caesium I and time is now defined in terms of atoms rather than the Earth’s motion.


I. Development of Clocks—I. Development of Clocks—Atomic ClocksAtomic Clocks


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All mechanical clocks work by counting the vibrations of something which has a constant frequency such as a pendulum. Unfortunately, the frequency of a pendulum is not perfectly constant. It is affected by changes in temperature, air pressure and the strength of gravity. This causes the clock to run too quickly or too slowly. The frequencies measured by atomic clocks are much higher than those of a pendulum but vary much less, so atomic clocks keep time much better. Caesium I was so accurate that it would only gain or lose one second in three hundred years. Modern atomic clocks are even more accurate.




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Modern Atomic Clocks

Atomic clock technology continued to

improve. Machines which are accurate to

one second in 30,000 years, such as the

HP5071A, are now commercially available

and are often used by communications

companies to obtain precise frequencies.




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These clocks work on the same principle as Caesium I. The latest atomic clocks work in a slightly different way, which gives even greater accuracy. In 1993, the National Institute of Standards and Technology (NIST) in the U.S.A. (formerly the NBS) built NIST-7, a caesium atomic clock that uses lasers instead of magnets to separate the atoms before and after they pass through the beam tube. Initially, NIST-7 was accurate to one second in 800,000 years but has since been improved to one second in 6 million years. The next generation of atomic clocks will use other methods for controlling the movement of their atoms and will be even more reliable.

The end of Development of Clocks.



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Molecular “clocks” in the brain create natural cycles in many body traits, such as blood pressure and temperature. Scientists have learned that these clocks, which can be reset by sunlight, are controlled by special genes. Knowledge of these clocks is leading to an improved understanding of biological cycles and new ways of treating disorders such as insomnia.


II. II. Biological ClockBiological Clock


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Are you a “night owl” or an “early bird”? The answer depends on a biological “clock” in your brain. This clock controls many natural body cycles—from the time you wake up each morning to rhythmic changes in body temperature and blood hormone levels. Nearly all organisms, from bacteria to plants to humans, have biological clocks that help maintain rhythms. By studying these clocks, scientists are beginning to understand: The biological foundations of behavior. Jet lag, insomnia, mental disorders, and how to treat them. Rhythmic changes in heart rate and other traits that affect the diagnosis and treatment of many disorders, including fever and high blood pressure.


II. Biological ClockII. Biological Clock


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During the mid-1900s, scientists began to examine biological cycles in several different organisms. By the early 1960s, they showed that daily, or circadian rhythms—“circa” means around and “dia” means day—are generated internally and synchronized to the 24-hour day. How are these cycles generated and altered?

In mammals, including humans, a biological clock resides in a region of the brain’s hypothalamus, a quarter-sized structure that regulates hormone levels and plays a role in emotions. In some insects and snails the clocks are usually located in the retina of the eye. In birds the clocks can also be found in a brain region called the pineal gland or in the hypothalamus.


II. II. BiologicalBiological ClockClock


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The clocks are almost always linked to some form of light-sensing cell called a photoreceptor. This type of cell responds to sunlight in ways that help synchronize the clock with the 24-hour day.

Scientists have learned that exposure to light at certain times in the internal cycle can reset the clock in animals. In mammals, light turns on important genes and affects sleep patterns, alertness, and body temperature. In nature, this light sensitivity helps organisms synchronize their clock within the cycle of day and night.




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Researchers have found that exposure to strong artificial light at certain times can reset the clock in ways that relieve insomnia, jet lag, and other problems. Light at the wrong time of the internal cycle, however, might contribute to or intensify these conditions. By studying mold, flies, mice, and other organisms, scientists have learned that the function of the biological clock is controlled by specific genes. Research in flies with disabled or altered clock genes shows that the proteins clock cells produce often work in a negative feedback cycle. When high levels of clock proteins are present, they block further production of these molecules until the levels fall again.




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Some clock genes also regulate or stabilize other genes’ activity. The way these clock proteins affect body functions is not yet known. However, learning how normal cycles affect the body may improve the diagnosis of many disorders, such as fever and high blood pressure, by accounting for daily rhythmic variation in hormone levels, blood pressure, temperature, and other traits.

Researchers believe only a few genes regulate circadian cycles in most organisms. Mutations in known clock genes usually create large changes in length of the cycle.




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Organisms with some mutations are unable to maintain any normal rhythm at all. If mutations in human clock genes act in a similar way, they may prevent some people from synchronizing their cycles with the environment, causing sleep disturbances and other problems.

Scientists have now uncovered several ways to treat clock-related disorders. Some studies suggest that the hormone melatonin, given at specific times, may be useful for resetting daily rhythms to help overcome the effects of jet lag and sleep disorders.




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Exposure to bright light at certain times in

the cycle may also help people with

depression and other disorders. These

strategies, now being tested in humans,

may brighten the lives of millions of

people.

The end of Biological Clock.



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The Julian Calendar was introduced by Julius Caesar in 45 BCE and introduced a simple leap year rule: Insert an extra day every four years. The Julian Calendar eventually standardized on 21 March as the date of the vernal equinox. Although this leap year rule is a simple one, it is does not produce a precise match to the solar year. Over the centuries the date of the astronomical vernal equinox slowly drifted away from the date of 21 March. The ecclesiastical rules to compute the date of Easter defined 21 March as the date of the vernal equinox. The Gregorian Calendar resulted from a perceived need to reform the calculation method for the dates of Easter. Nonetheless, the Julian Calendar and variations of it are still in use by some groups to set the dates for liturgical events.


III. Calendars—Solar III. Calendars—Solar CalendarsCalendars


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The Gregorian Calendar has become the internationally accepted civil calendar. The leap year rule for the Gregorian Calendar differs slightly from one for the Julian Calendar. The Gregorian leap year rule is: Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100. For example, the year 1900 is not a leap year; the year 2000 is a leap year. The centurial years that are exactly divisible by 400 are still leap years. The Gregorian dates for Easter are computed from a set of ecclesiastical rules and tables.


III. Calendars—Solar III. Calendars—Solar CalendarsCalendars


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The Islamic Calendar is a purely lunar calendar in which months correspond to the lunar phase cycle. Thus the twelve months of the Islamic Calendar systematically shift with respect to the months of the international civil calendar. The cycle of twelve months regresses through the seasons over a period of about 33 years. For religious purposes, Muslims begin each month with the first visibility of the lunar crescent after conjunction. For civil purposes a tabulated calendar that approximates the lunar phase cycle is often used.


III. Calendars—Lunar III. Calendars—Lunar CalendarsCalendars


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The Hebrew Calendar is a lunisolar calendar based on calculation rather than observation. Its current form dates from about 359CE. This calendar is the official calendar for the State of Israel, although variations on this calendar exist. The dates for Passover for this calendar are computed from a set of defined rules.


III. Calendars—Lunisolar CalendaIII. Calendars—Lunisolar Calendarsrs


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The National Calendar of India is a formalized lunisolar calendar in which leap years coincide with those of the Gregorian Calendar. The Gregorian Calendar is used for administrative purposes. The Indian religious calendars require calculations of the motions of the Sun and the Moon. Tabulations of the religious holidays are prepared by the India Meteorological Department and published annually in The Indian Astronomical Ephemeris. Many local variations exist.




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The Chinese Calendar is a lunisolar calendar based on calculations of the positions of the Sun and the Moon. Since this calendar uses the true positions of the Sun and the Moon, its accuracy depends on the accuracy of the astronomical theories and calculations.

The end of Calendars.



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