General topic-time

henny · Posted: Tue Sep 11, 2007 7:15 am Post subject: General topic-time

Hello!Please
Anyone interested-Can suggest "What is/are your best timesaving tips"?
Thanking you!

ROLCAM · Posted: Tue Sep 11, 2007 1:10 pm Post subject: TIME

Time is one of the world's deepest mysteries. No one can say exactly what it is. Yet, the ability to measure time makes our way of life possible. Most human activities involve groups of people acting together in the same place at the same time. People could not do this if they did not all measure time in the same way.

One way of thinking about time is to imagine a world without time. This timeless world would be at a standstill. But if some kind of change took place, that timeless world would be different "now" than it was "before." The period--no matter how brief--between "before" and "now" indicates that time must have passed. Thus, time and change are related because the passing of time depends on changes taking place. In the real world, changes never stop happening. Some changes seem to happen only once, like the falling of a particular leaf. Other changes happen over and over again, like the breaking of waves against the shore.

Any change that takes place again and again stands out from other changes. The rising and setting of the sun are examples of such change. The first people to keep time probably counted such natural repeating events and used them to keep track of events that did not repeat. Later, people made clocks to imitate the regularity of natural events. When people began to count repeating events, they began to measure time.

Measuring time

Units of time measurement. For early peoples, the only changes that were truly regular--that is, repeated themselves evenly--were the motions of objects in the sky. The most obvious of these changes was the alternate daylight and darkness, caused by the rising and setting of the sun. Each of these cycles of the sun came to be called a day. Another regular change in the sky was the change in the visible shape of the moon. Each cycle of the moon's changing shape takes about 291/2 days, or a month.

The cycle of the seasons gave people an even longer unit of time. By watching the stars just before dawn or after sunset, people saw that the sun moved slowly eastward among the stars. The sun made a full circle around the sky in one cycle of the seasons. This cycle takes about 3651/4 days, or a year.

For hundreds of years, people tried to fit days and months evenly into a year or a period of several years. But no system worked perfectly. Today, the calendar is based entirely on the year. Although the year is divided into 12 so-called months, the months have no relation to the moon's actual cycle. See CALENDAR.

There is no regular change in the sky that lasts seven days, as does the week. The seven-day week came from the Jewish custom of observing a Sabbath (day of rest) every seventh day.

The division of a day into 24 hours, an hour into 60 minutes, and a minute into 60 seconds probably came from the ancient Babylonians. The Babylonians divided the imaginary circular path of the sun into 12 equal parts. Then they divided the periods of daylight and darkness into 12 parts each, resulting in a 24-hour day.

The Babylonians also divided the circle into 360 parts called degrees. Other ancient astronomers further divided each degree into 60 minutes. Later, clocks became accurate enough to need smaller units than the hour. Clockmakers, following the astronomers' division of the degree, divided the hour into 60 minutes and the minute into 60 seconds. In this way, the face of a clock could easily show hours, minutes, and seconds. A clock face has 12 divisions. Each of these divisions equals one hour for the hour hand, five minutes for the minute hand, and five seconds for the second hand.

Some clock faces are divided into 24 hours. On such a clock, 9 a.m. would be shown as 0900 and 3 p.m. would be 1500. This system avoids confusion between the morning and evening hours.

Measuring time by the sun. Directly above every spot on the earth, an imaginary curved line called the celestial meridian passes through the sky. As the earth rotates on its axis, the sun crosses every celestial meridian once each day. When the sun crosses the celestial meridian above a particular place, the time there is noon. Twelve hours later, the time at that place is midnight. The period from one midnight to the next is called a solar day. The length of a solar day varies because of the tilt of the earth's axis, the oval shape of its orbit, and its changing speed along the orbit.

To make all solar days the same length, astronomers do not measure solar time with the apparent (real) sun. Instead, they use an imaginary mean (average) sun that moves at a steady speed around the sky. Local mean solar noon occurs when the mean sun crosses the celestial meridian above a particular place. The time between one mean solar noon and the next is always the same. Thus, all mean solar days are the same length.

Measuring time by the stars. Astronomers also measure time by the earth's rotation in relation to the stars. This time is called sidereal time. Each day, as the earth rotates on its axis, an imaginary point among the stars called the vernal equinox crosses the celestial meridian above every place on the earth. The time when this happens is sidereal noon. The time between one sidereal noon and the next is one sidereal day. A sidereal day is shorter than a mean solar day by 3 minutes 56.555 seconds. See STAR (Measuring time).

Devices that measure time. The sundial was one of the earliest devices for measuring time. But it can work only in uncloudy daylight. Early peoples also used ropes with knots tied at regular intervals or candles marked with regularly spaced lines. When burned, such devices measured time. An hourglass or sandglass tells time by means of sand trickling through a narrow opening. A water clock, or clepsydra, measures time by allowing water to drip slowly from one marked container into another. By the 1700's, people had developed clocks and watches that told time to the minute. Modern electronic and atomic clocks can measure time with far greater accuracy. See ATOMIC CLOCK.

Time zones

Local and standard time. Clocks in various parts of the world do not all show the same time. Suppose they all did show the same time--3 p.m., for example. At that time, people in some countries would see the sun rise, and people in other lands would see it high in the sky. In still other countries, the sun could not be seen because 3 p.m. would occur at night. Instead, clocks in all locations show 12 o'clock at midday.

Every place on the earth that is east or west of another place has noon at a different time. The time at any particular place is called the local time. At noon local time in one town, the time might be 11 a.m. in another place west of the town or 1 p.m. in a place to the east.

If every community used a different time, travelers would be confused and many other problems would be created. To avoid all such problems, standard time zones were established. These zones were set up so there would be a difference of one hour between a place on the eastern edge of a time zone and a place on the western edge if each were on its own local time. But under the time zone system, each of these places is not on its own local time. The local time at the meridian (line) of longitude that runs through the center of the zone is used by all places within the zone. Thus, time throughout the zone is the same.

Time zones in the United States and Canada. The United States and Canada each have six standard time zones. Each zone uses a time one hour different from its neighboring zones. The hours are earlier to the west of each zone and later to the east. The Newfoundland Time Zone is not a true standard time zone because it differs from its neighboring zones by only a half hour. The boundaries between the zones are irregular so that neighboring communities can have the same time.

The United States has not always had standard time zones. Every locality once set its own time by the sun. Various railroads tried to make their schedules simpler by establishing railroad time along sections of their routes. But in 1883, there were still about 100 different railroad times. That year, all the railroads divided the United States into four standard time zones.

Each zone is centered on a meridian of longitude 15° apart. In the United States and Canada, the Eastern Time Zone is centered on the 75° west meridian, and the Central Time Zone on the 90° west meridian. The Mountain Time Zone is centered on the 105° west meridian, and the Pacific Time Zone on the 120° west meridian. The central meridians of the other U.S. and Canadian zones are 60° west for the Atlantic Time Zone, 135° west for the Alaska Time Zone, and 150° west for the Hawaii-Aleutian Time Zone. The Newfoundland Time Zone is a separate zone and has no central meridian.

Worldwide time zones were established in 1884. The meridian of longitude passing through the Greenwich Observatory in England was chosen as the starting point for the world's time zones. The Greenwich meridian is often called the prime meridian. The mean solar time at Greenwich is traditionally known as Greenwich Mean Time (GMT) or Greenwich Civil Time (GCT).

An international conference in 1884 set up 12 time zones west of Greenwich and 12 to the east. These zones divide the world into 23 full zones and two half zones. The 12th zone east and the 12th zone west are each half a zone wide. They lie next to each other and are separated by an imaginary line called the International Date Line. The line is halfway around the world from Greenwich. A traveler crossing this line while headed west, toward China, loses a day. A traveler who crosses the line while traveling eastward gains a day. A few places do not use standard time zones. For example, the polar regions have weeks of constant sunlight or darkness.

In the 1940's, experts began to realize that time based on astronomical measurements was not completely smooth, since the earth slowed down and speeded up in an irregular fashion. As a result, in 1958, the length of the second was redefined in terms of the natural vibration frequency of the cesium atom. However, the length of the year continued to be determined from astronomical observations. This time scale based on both atomic and astronomical measurements is called Universal Time Coordinated (UTC).

Scientific ideas about time

Physical time. Scientists think of time as a fundamental quantity that can be measured. Other fundamental quantities include length and mass. The noted physicist Albert Einstein realized that measurements of these quantities are affected by relative motion (motion between two objects). Because of his work, time became popularly known as the fourth dimension. See RELATIVITY (Special theory); FOURTH DIMENSION.

Many physicists believe that the apparent nonstop, forward flow of time is not a property of the basic laws of nature. They consider it a result of the fact that the universe is expanding and becoming more disorganized. Some physicists have considered the possibility that, under certain circumstances, time might flow backward. But experiments have not supported this idea.

Biological time. The activities of many plants and animals are timed to the cycle of day and night. These natural rhythms are called circadian rhythms. The most obvious example is the sleep cycle.

Many plants and animals are sensitive to other natural time cycles. Certain plants do not start their next step of growth until daylight each day lasts a certain time. Some sea animals time their activities to the changing tides. These creatures even seem to know such times away from their home waters.

Geological time. Geologists have found clues in the earth's crust that indicate how many billions of years ago it was formed. One of these indicators is the element uranium. Uranium changes slowly into the element lead by means of radioactive decay. By measuring the amount of lead in a sample of uranium ore, scientists can estimate when the rock was formed.

A second clue to geological time is radioactive carbon. This form of carbon is absorbed by every living plant and animal. The rate of the carbon's decay can help a geologist estimate how long ago the plant or animal died. See RADIOCARBON.
_________________
Roland Camilleri

Moderator

Sydney , Australia.

ROLCAM · Posted: Tue Sep 11, 2007 1:12 pm Post subject: Atomic clock.

Atomic clock is a device for measuring time intervals by measuring the frequency of electromagnetic waves given off or absorbed by atoms or molecules. In an atomic clock, such frequencies are extraordinarily stable, so the clock is very accurate. Some clocks would gain or lose no more than a second in 1 million years. Atoms and molecules used in atomic clocks include cesium atoms, hydrogen atoms, and molecules of ammonia gas. Atomic clocks control the time signals sent out to the world from national laboratories, such as the National Institute of Standards and Technology. In 1958, scientists throughout the world adopted the vibration rate of an atomic clock as the standard for defining time units.
_________________
Roland Camilleri

Moderator

Sydney , Australia.

ROLCAM · Posted: Tue Sep 11, 2007 1:14 pm Post subject: Biological clock.

Biological clock refers to a timing mechanism that operates in living things. Biological clocks control the rhythms of functions and processes in organisms. They keep accurate time during each 24 hours and over days, weeks, months, and even years. Biological clocks keep the activities of living things in harmony with regular changes in the surroundings.

Birds migrate, fish spawn, and flowers blossom on schedules that are set by their built-in clocks. In human beings, biological clocks time periods of sleep and wakefulness and of body activities. The science that deals with the study of biological clocks and rhythms is called chronobiology.

No one is certain where biological clocks are located or how they work. Experiments indicate every living thing inherits timing mechanisms. Most scientists believe biological clocks occur in several forms and regulate processes in such simple structures as cells, as well as complex organs and organ systems. Research indicates that the pineal gland and the hypothalamus in the brain may be the master clocks in animals.

Importance of biological clocks

Biological clocks keep track of cyclic variations in the environment, including day and night, movements of the ocean tides, phases of the moon, and seasons of the year. Most, if not all, living things have internal cycles--called biological rhythms--that are controlled by biological clocks. The biological rhythms of each particular species are timed to enable the organism to efficiently meet the demands of its environment. Biological rhythms continue on schedule even in laboratories where the organism is shielded from all evidence of passing time and of outside change. But the rhythms can be shifted--and the biological clock reset--by changing the time at which the organism gets light or by changing other critical time cues from the environment.

Daily rhythms. Many biological rhythms are based on a day-night cycle. They are called circadian rhythms because they occur about every 24 hours. Circadian comes from Latin words that mean about a day. For most living things, the day-night cycle is broken into periods of activity and periods of rest. But these periods do not occur at the same time of day for all living things. Human beings are most active during the day and rest at night. Apes, bees, butterflies, monkeys, and many other kinds of animals also follow this schedule. On the other hand, bats, cats, moths, owls, rats, and others are active at night. The genetically inherited traits of biological clocks in each species set the schedule.

Plants also show daily rhythms. For example, they raise their leaves in the day and lower them at night. These rhythmic changes, called sleep movements, continue even when the plants are kept in caves or in other places where light and temperature do not change.

Other rhythms. Fiddler crabs and other seashore animals show complex rhythms. The skin of fiddler crabs normally darkens at dawn and gets pale at dusk. Their running activity adjusts to the tides, which rise and fall about 50 minutes later each day. Fiddler crabs kept in constant darkness in laboratories continue to change color rhythmically, as if responding to the tides of their home beach. But when they are moved to a beach that has different tidal times, they adjust activities to the new tides. Their biological clocks have been automatically reset.

Many living things, including the grunion, a small fish found along the California coast, have monthly or semimonthly breeding rhythms. From February to September at maximum high tide, every 14.8 days, grunions ride a wave to shore. The wave recedes, the females drop their eggs in the wet sand, and the males fertilize the eggs. The next wave carries the grunions back into the ocean, but the eggs remain on the beach. At the next high tide--14.8 days later--a wave comes in, breaks the eggs, and carries the young fish out to sea.

Biological clocks set the schedules for yearly rhythms in living things. They control the sprouting of seeds and the hibernation and migration of birds and other animals. These clocks also seem important in helping birds, fishes, crustaceans, and insects to navigate. The clocks, used in conjunction with the sun, moon, and stars, help them to correct continuously for the earth's rotation, and stay on the proper course. Biological clocks also coordinate breeding cycles in animals to seasonal changes in the amount of daylight. Some species of animals mate in the fall, when the period of darkness exceeds the period of light. Other species breed in the spring, when the period of light is greater than that of darkness.

Biological clocks in people

Biological clocks in people work on schedules essential to life and health. Human beings have daily, weekly, monthly, and seasonal biological rhythms. The level of hormones and other chemicals in the blood varies dramatically over each of these time periods. Most vital body processes have a circadian rhythm. The activities of the cells, glands, and organ systems are coordinated with one another and with the day-night rhythm of the environment.

The rate at which the body processes work varies rhythmically throughout the day and night. For example, in most people who are active during the daytime, body temperature varies about three degrees during a 24-hour period. The temperature is lowest during sleep and greatest during the afternoon and early evening.

You become most aware of the biological timing system when you fly by airplane to a different time zone. A flight from Chicago to London that leaves Chicago in late afternoon will arrive when Londoners are just starting their day. Your rhythmic system will still be working on Chicago time. By London time, you will have insomnia at night and be sleepy during the day. Your biological clock will reset itself, but it may take several days. As a result, your body functions are out of rhythm, your efficiency drops, and you feel tired. This is called "jet lag" or "jet exhaustion." Many workers who switch from the day shift to the night shift also experience jet-lag symptoms. Scientists hope to find so-called chronobiotics, medications that can quickly reset the body's biological rhythms.

The symptoms or occurrence of many diseases follows biological rhythms. For example, cerebral hemorrhages are most common late in the evening. Most heart attacks occur in the morning. Most people who suffer from asthma feel worse in the evening and overnight. The study of influences that biological rhythms have on human diseases is called chronopathology.

Biological rhythms also influence the effects medications have on illnesses. Many medications, such as those used to treat allergies, arthritis, cancer, and heart disease, are strongly affected by circadian rhythms. The branch of chronobiology that deals with the study of biological rhythms and medications is called chronopharmacology. Greater knowledge of biological rhythms in the treatment of diseases could result in important changes in the practice of medicine.
_________________
Roland Camilleri

Moderator

Sydney , Australia.

ROLCAM · Posted: Tue Sep 11, 2007 1:16 pm Post subject: Calendar .

Calendar is a system of measuring and recording the passage of time. A major scientific advance occurred when people realized that nature furnishes a regular sequence of seasons. The seasons governed their lives, determined their needs, and controlled the supply of their natural foods. They needed a calendar so they could prepare for the hardships of winter.

Before the invention of the clock, people watched the sun, the moon, and the stars to tell time. The daily rising of the sun provided a short unit of time, the solar day. The cycle of seasons roughly indicated a longer unit of time, the solar year. But early people did not know that the earth's revolution around the sun caused the different seasons. The changing position and shape of the moon was easier for them to observe. As a result, the early calendars used the interval between the successive full moons, called the lunar month, as an intermediate unit of time.

We now know that the lunar month lasts about 291/2 days. Twelve such months amount to about 354 days. This interval is almost 11 days shorter than the true solar year, which has 365 days, 5 hours, 48 minutes, and 46 seconds. But a year of 13 lunar months would amount to about 3831/2 days and would be more than 18 days longer than the solar year. The solar year, therefore, does not equal any whole number of lunar months.

The discrepancy between whole lunar months and days in a solar year explains the confusion over calendar keeping during thousands of years. A calendar based on 12 lunar months becomes out of step with the seasons. Some people who used lunar calendars kept them roughly in step with the seasons by making some years 12 months long and other years 13 months long.

Some calendars today

Most people in the Western world use the Gregorian calendar, worked out in the 1580's by Pope Gregory XIII. It has 12 months, 11 with 30 or 31 days. The other month, February, normally has 28 days. Every fourth year, called a leap year, it has 29 days. However, century years that cannot be divided evenly by 400 lose the extra day, though they are leap years. For example, February had 28 days in 1900 but will have 29 days in 2000.

The Gregorian calendar is based on the year of Jesus Christ's birth, according to some calculations. Many people refer to dates before that year as B.C., or before Christ. They use A.D., or anno Domini (in the year of our Lord), for dates after that year. Some people--especially non-Christians--write B.C.E. for before common era or before Christian Era and C.E. for common era or Christian Era instead of B.C. and A.D. See B.C.; A.D.; CHRISTIAN ERA.

The Christian church calendar is regulated partly by the sun and partly by the moon. Immovable feasts include Christmas and such feasts as the Nativity of the Blessed Virgin. They are based on the solar year. Such days as Ash Wednesday, Palm Sunday, and Easter are called movable feasts, because their dates vary from year to year, according to the phases of the moon.

The Hebrew calendar begins with an estimated moment of the world's creation. Hebrew tradition has placed this moment at 3,760 years and 3 months before the birth of Jesus Christ. To find a year in the Hebrew calendar, we must add 3,760 to the date in the Gregorian calendar. For example, 2000 in the Gregorian calendar is 5760 in the Hebrew calendar. But this system will not work to the exact month, because the Hebrew year begins in September or October in the Gregorian calendar. By November 2000, for instance, the Hebrew year will have become 5761.

The Hebrew year is based on the moon and normally consists of 12 months. The months are Tishri, Heshvan, Kislev, Tebet, Shebat, Adar, Nisan, Iyar, Sivan, Tammuz, Ab, and Elul. They are alternately 30 and 29 days long. Seven times during every 19-year period, an embolismic or extra 29-day month, called Veadar, is inserted between Adar and Nisan. At the same time, Adar is given 30 days instead of 29. These additions keep the Hebrew calendar and holidays in agreement with the seasons of the solar year.

The Islamic calendar begins with Muhammad's flight from Mecca to Medina. This flight, called the Hegira, took place in A.D. 622 by the Gregorian calendar.

The Islamic year is based on the moon, and has 12 months, alternately 30 and 29 days long. These months are Muharram, Safar, Rabi I, Rabi II, Jumada I, Jumada II, Rajab, Shaban, Ramadan, Shawwal, Zulkadah, and Zulhijjah.

The Islamic year is much shorter than the solar year, with only 354 days. As a result, the Islamic New Year moves backward through the seasons. It moves completely backward in a course of 32 1/2 years. The Islamic calendar divides time into cycles 30 years long. During each cycle, 19 years have the regular 354 days, and 11 years have an extra day each. This method of counting time makes the Islamic year nearly as accurate in measuring the lunar year as the Gregorian year is in measuring the solar year. The Islamic calendar would be only about one day off every 2,570 years with respect to the moon. The Gregorian calendar would be only a little more accurate with respect to the sun.

The Chinese calendar begins at 2637 B.C., the year in which the legendary Emperor Huangdi is said to have invented it. This calendar counts years in cycles of 60. For example, the year 2000 in the Gregorian calendar is the 17th year in the 78th cycle. The years within each cycle are broken down into repeating 12-year cycles. In these cycles, each year is named after 10 Chinese constellations and 12 animals. The animals are the rat, ox, tiger, hare, dragon, snake, horse, sheep, monkey, rooster, dog, and pig. The year 2000 is the year of the dragon.

The Chinese year is based on the moon and generally consists of 12 months. Each month begins at new moon and has 29 or 30 days. A month is repeated seven times during each 19-year period, so that the calendar stays approximately in line with the seasons. The year starts at the second new moon after the beginning of winter in the Northern Hemisphere. Thus, the Chinese New Year occurs no earlier than January 20 and no later than February 20.

History

Early calendars usually represented some sort of compromise between the lunar and solar years. Some years lasted 12 months, and others lasted 13 months.

The Babylonians, who lived in what is now Iraq, added an extra month to their years at irregular intervals. Their calendar, composed of alternate 29-day and 30-day months, kept roughly in step with the lunar year. To balance the calendar with the solar year, the early Babylonians calculated that they needed to add an extra month three times every eight years. But this system still did not accurately make up for the accumulated differences between the solar year and the lunar year. Whenever the king felt that the calendar had slipped too far out of step with the seasons, he ordered another extra month. However, the Babylonian calendar was quite confused until the 300's B.C., when the Babylonians began to use a more reliable system.

The Egyptians were probably the first to adopt a mainly solar calendar. They noted that the Dog Star, Sirius, reappeared in the eastern sky just before sunrise after several months of invisibility. They also observed that the annual flood of the Nile River came soon after Sirius reappeared. They used this combination of events to fix their calendar and came to recognize a year of 365 days, made up of 12 months each 30 days long, and an extra five days added at the end. But they did not allow for the extra fourth of a day, and their calendar drifted into error. According to the famed Egyptologist J. H. Breasted, the earliest date known in the Egyptian calendar corresponds to 4236 B.C. in terms of the Gregorian calendar.

The Romans apparently borrowed parts of their earliest known calendar from the Greeks. The calendar consisted of 10 months in a year of 304 days. The Romans seem to have ignored the remaining 61 days, which fell in the middle of winter. The 10 months were named Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, and December. The last six names were taken from the words for five, six, seven, eight, nine, and ten. Romulus, the legendary first ruler of Rome, is supposed to have introduced this calendar in the 700's B.C.

According to tradition, the Roman ruler Numa Pompilius added January and February to the calendar. This made the Roman year 355 days long. To make the calendar correspond approximately to the solar year, Numa also ordered the addition every other year of a month called Mercedinus. Mercedinus was inserted after February 23 or 24, and the last days of February were moved to the end of Mercedinus. In years when it was inserted, Mercedinus added 22 or 23 days to the year.

The Julian calendar. By the time of Julius Caesar, the accumulated error caused by the incorrect length of the Roman year--and by the occasional failure to add extra days at the proper times--had made the calendar about three months ahead of the seasons. Winter occurred in September, and autumn came in the month now called July.

In 46 B.C., Caesar asked the astronomer Sosigenes to review the calendar and suggest ways for improving it. Acting on Sosigenes's suggestions, Caesar ordered the Romans to disregard the moon in calculating their calendars. He divided the year into 12 months of 31 and 30 days, except for February, which had only 29 days. Every fourth year, it would have 30 days. To realign the calendar with the seasons, Caesar ruled that the year we know as 46 B.C. should have 445 days. The Romans called it the year of confusion.

The Romans renamed Quintilis to honor Julius Caesar, giving us July. Sextilis was renamed August by the Roman Senate to honor the Emperor Augustus. According to tradition, Augustus moved a day from February to August to make August as long as July.

The Julian calendar was widely used for more than 1,500 years. A Julian year lasted 3651/4 days. But it was actually about 11 minutes and 14 seconds longer than the solar year. This difference led to a gradual change in the dates on which the seasons began. By A.D. 1580, the spring equinox fell 10 days earlier on the Julian calendar than its appointed date.

The Gregorian calendar was designed to correct the errors of the Julian calendar. In 1582, on the advice of astronomers, Pope Gregory XIII corrected the difference between sun and calendar by ordering 10 days dropped from October, the month with the fewest Roman Catholic holy days. The day that would have been Oct. 5, 1582, became October 15. This procedure restored the next equinox to its proper date. To correct the Julian calendar's error regularly, the pope decreed that February would have an extra day in century years that could be divided evenly by 400, such as 1600 and 2000, but not in others, such as 1700, 1800, and 1900.

The Gregorian calendar is so accurate that the difference between the calendar and solar years is now only about 26 seconds. This difference will increase by 0.53 second every hundred years, because the solar year is gradually becoming shorter.

The Roman Catholic nations of Europe adopted the Gregorian calendar almost immediately after Gregory XIII devised it. Various German states kept the Julian calendar until 1700. Britain and the American Colonies changed to the Gregorian calendar in 1752. Russia and Turkey did not adopt the Gregorian calendar until the early 1900's.

Calendar reform would simplify the present calendar. Two proposed calendars have received considerable support. In each, months and years would begin on the same day of the week every year. All months would contain the same or nearly the same number of days. The Fixed Calendar, also called The Thirteen-Month Calendar, would provide 13 months exactly four weeks long. The extra month, Sol, would come before July. A year day placed at the end of the year would belong to no week or month. Every four years, a leap-year day would be added just before July 1. The World Calendar would have 12 months of 30 or 31 days, a year day at the end of each year, and a leap-year day before July 1 every four years.
_________________
Roland Camilleri

Moderator

Sydney , Australia.

ROLCAM · Posted: Tue Sep 11, 2007 1:18 pm Post subject: Chronometer.

Chronometer, is an instrument that keeps time with extreme accuracy. Clockmakers developed it for use on ships because navigators needed an accurate clock to help determine their position at sea. Astronomers and other scientists who need accurate time measurements also use chronometers.

The marine chronometer is an accurate clock that has been specially mounted to remove the effect of a ship's motion. It is usually set to Universal Time Coordinated (UTC), an international time standard. To find a ship's position, a navigator notes the time and measures the positions of certain stars. The navigator compares these positions with tables that show the stars' positions at UTC, and then calculates the ship's position.

The first reliable chronometer was developed in 1735 by John Harrison, an English clockmaker. In 1776, Pierre LeRoy, a French watchmaker, constructed a chronometer that became the model for the modern chronometer. During the 1800's, improved metals that resisted expansion led to better clockworks and more accurate chronometers. Today, many ships rely chiefly on radar and other electronic systems for navigation. Radio time signals and electric and atomic clocks have replaced mechanical chronometers in most other applications.
_________________
Roland Camilleri

Moderator

Sydney , Australia.

ROLCAM · Posted: Tue Sep 11, 2007 1:20 pm Post subject: Clock.

Clock is an instrument that shows the time. Clocks not only measure and tell time but also serve as decorations in homes and other buildings.

The first clocks were probably developed in the late 1200's. They had no hands or dial but told time by ringing a bell. The word clock probably comes from the French word cloche and the German word Glocke, both of which mean bell.

Kinds of clocks

Modern clocks range from small, inexpensive models to large, ornamental grandfather clocks with beautiful wood cases and complex chimes. Traditional clocks, called dial clocks, have hands that show the time by pointing to numbers on a dial. Other clocks, called digital clocks, show the time in digits on the clock face. Many clocks have chimes or sound an alarm. Others have mechanical birds or dancing figurines that mark the hours or other intervals of time.

Every clock has two main parts, the case and the works, or movement, inside the case. The works performs three functions. In addition to showing the time, it supplies power to run the clock and it keeps time. Clocks differ according to how their works carry out one or more of these three functions. This article classifies clocks into two groups, mechanical clocks and electric clocks, according to how they are powered.

Timekeeping in most clocks is based on the frequency of some regularly repeated action, such as the swing of a pendulum. Clocks with extremely stable frequencies keep time more accurately than those with less stable frequencies. For example, the operation of atomic clocks, the most accurate clocks ever made, is based on the vibrations of certain atoms and molecules. Each of these particles has a natural, characteristic frequency that is extremely stable. As a result, the best atomic clocks would not lose or gain more than a second in 1 million years.

Mechanical clocks are powered by various mechanical devices that must be wound at various intervals. Some have to be wound every day, but others can run for seven or eight days without rewinding. There are two main kinds of mechanical clocks, weight driven and spring driven. Almost all of them are dial clocks.

Weight-driven clocks are powered by a heavy weight that hangs from a cord or chain. When the clock is wound, the cord or chain gets wrapped around a drum and draws the weight up near the drum. As gravity pulls the weight down, the cord or chain slowly unwinds and turns the drum. This action of the drum turns a number of gear wheels that are connected in a series called the train. The hands of the clock are attached to individual wheels in the train. Each of these wheels turns at a specific speed. A pendulum and a mechanism called the escapement prevent the weight from being lowered too fast. The pendulum and the escapement also regulate the clock's speed.

The escapement includes an escape wheel and a verge. The escape wheel is connected to the train and turns when the clock runs. The pendulum, which is the timekeeping device of the clock, swings from side to side at a steady rate. As the pendulum swings, it tilts the verge from side to side. With each tilt, two hooks called pallets--one at each end of the verge--catch on the escape wheel and stop it. When the pendulum swings back, the pallets release the wheel, and the wheel turns slightly. This process regulates the speed of the escape wheel and of the wheels in the train. It also causes the tick-tock sound of the clock.

Spring-driven clocks contain a coiled spring called the mainspring. This spring gets wound up when the clock is wound. Then the mainspring unwinds slowly, turning the wheels in the train.

The escapement in a spring-driven clock resembles that of a weight-driven clock. However, many spring-driven clocks have a balance wheel instead of a pendulum as the timekeeping device. A coiled spring called the balance spring, or hairspring, is connected to the balance wheel. This spring coils and uncoils and makes the balance wheel swing back and forth at a fixed rate. The swinging motion causes the verge to tilt. The pallets alternately catch and release the escape wheel and regulate the speed of the train.

Electric clocks can be battery powered or line powered. A line-powered clock receives power from an electric outlet. Almost all digital clocks manufactured since the 1930's have been electric models.

Battery-powered clocks. Many battery-powered clocks have a balance wheel or a pendulum that regulates their speed. Others have a miniature tuning fork or a tiny bar of quartz crystal. The battery activates the tuning fork or crystal, which vibrates with high, steady frequencies. In clocks with a tuning fork, an indexing mechanism changes the number of vibrations into the correct speeds for the gear wheels. Quartz-based clocks contain a complex electric circuit that translates the number of vibrations into time information. The circuit also controls the time display. Most quartz-based clocks are accurate to within 60 seconds a year.

Line-powered clocks. In line-powered clocks, the current from the electric outlet not only supplies power but also regulates the clock's speed. The outlet supplies alternating current (current that reverses its direction). Current supplied in most parts of the United States and Canada reverses its direction 120 times every second. In some clocks, a motor counts the changes in direction and uses that information to control the time display. In other clocks, this function is performed by a computer chip, a complex electronic circuit built into a tiny piece of silicon. See COMPUTER CHIP; ELECTRIC CURRENT (Direct and alternating current).

Most digital clocks are line powered. In some, the digits are printed on flip cards, rotating drums, or a moving tape. Other line-powered models and some quartz-based clocks have electronic digital displays, such as a liquid crystal display (LCD) and a light-emitting diode display (LED). A liquid crystal display uses digits that reflect the light around it. A light-emitting diode display has digits shaped from electronic devices called diodes, which give off light.

History

Prehistoric peoples probably told the time of day by watching shadows cast by the sun. As the sun moved, the lengths of the shadows changed. When the shadows were short, the watchers knew the time of day was near noon. When the shadows were long, they knew the day was either beginning or ending.

Sundials, which were developed more than 4,000 years ago, are the oldest known instruments designed for telling time. As the sun crosses the sky, it casts a shadow on the dial. A sundial tells time by measuring the length or the angle of the shadow. See SUNDIAL.

Other early timekeeping devices included hourglasses and water clocks. In these devices, sand or water flowed from one container into another at a steady rate. By measuring the material in either container, people could tell how much time had passed. See HOURGLASS; WATER CLOCK.

The first mechanical clock was probably invented in China in the late 1000's. However, this invention was never developed further, and later Chinese clocks were based on European models.

The first mechanical clocks in Western civilization were developed by a number of inventors during the late 1200's. These clocks were weight driven, but they had no pendulum or hands. A bell rang to indicate the hour. By the mid-1300's, the dial and hour hand had been added. The first spring-driven clocks were probably developed in Italy during the late 1400's.

Most early clocks ran unevenly and inaccurately. The pendulum and the balance spring, which were developed during the mid-1600's, greatly improved timekeeping accuracy. Minute and second hands became common. By the mid-1700's, inventors had developed most of the mechanisms found in modern mechanical clocks.

Electric clocks, introduced in the mid-1800's, were in many homes by the 1920's. Quartz-based clocks appeared during the 1930's, and scientists developed the first atomic clock in the 1940's. Digital clocks became popular in the 1970's, particularly as wrist watches. In the 1980's, the computer chip was incorporated into clock mechanisms. Besides displaying the time, watches with chips can store information, and serve as calculators and miniature game boards.
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Roland Camilleri

Moderator

Sydney , Australia.

ROLCAM · Posted: Tue Sep 11, 2007 1:30 pm Post subject: Daylight saving time.

Daylight saving time is a plan in which clocks are set one hour ahead of standard time for a certain period, so that darkness comes an hour later. The plan provides an additional hour of daylight in the evening. Most of the states of the United States observe daylight saving time. Wherever it is observed in the United States, it begins on the first Sunday in April and ends on the last Sunday in October. A state may decide to remain on standard time. States that lie in more than one time zone may use daylight time in one zone and not the other.

The chief purpose of daylight saving time is to save energy by reducing evening use of lighting. As a result, countries often first adopt daylight time during a war or other crisis. Britain, for example, went on daylight time or what it calls "Summer Time" during World War I (1914-1918). The United States adopted the plan in 1918 but repealed it in 1919. The United States also observed it from Feb. 9, 1942, to Sept. 30, 1945, due to World War II.

After the war, many states established some type of daylight saving time. Beginning in 1967, the entire nation went on daylight time from the last Sunday in April to the last Sunday in October. In the 1970's, a reduction in Arab oil exports caused a fuel shortage in the United States. To conserve energy, Congress enacted daylight time from Jan. 6 to Oct. 27, 1974, and from Feb. 23 to Oct. 26, 1975. Since 1987, daylight time has begun on the first Sunday in April.
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Roland Camilleri

Moderator

Sydney , Australia.

ROLCAM · Posted: Tue Sep 11, 2007 1:37 pm Post subject: Fourth dimension.

Fourth dimension. We usually think of space as having three dimensions: length, width, and height. A box that is 6 feet long, 4 feet wide, and 2 feet high can be described by the ordered set of numbers (6,4,2). Such a set may also be used to describe the position of a point in space--for example, the position of an airplane. But three numbers cannot represent the location of a moving plane. To indicate when a plane in flight is at a particular location, such as (6,4,2), we need a fourth dimension--time.

The fourth dimension need not always represent time, however. It may represent anything that we can measure, including temperature and weight.

In the early 1900's, the mathematician Hermann Minkowski realized that the special relativity theory proposed by physicist Albert Einstein described a universe with four dimensions. According to Minkowski, time combines with the three dimensions of space to form space-time. Mathematicians afterward began to study geometries of four or more dimensions.
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Roland Camilleri

Moderator

Sydney , Australia.

ROLCAM · Posted: Tue Sep 11, 2007 1:38 pm Post subject: Hour is an interval of time.

Hour is an interval of time. It consists of 60 minutes. A day, from midnight to midnight, has 24 hours. Every nation regulates its activities according to the hour. But people did not begin to use hours to mark uniform periods of a day until the 1300's, when the mechanical clock was invented.

The ancient Romans used the hour to note a point of time, such as sunrise and sunset. They later added the hour of noon. At the beginning of the Christian era, the Romans divided the hours of daylight into five periods, which they marked on their sundials. In A.D. 605, the Christian church named the seven canonical hours, or hours of prayer. They were (1) matins (morning) and lauds (praise), (2) prime (first), (3) tierce (third), (4) sext (sixth), (5) nones (ninth), (6) vespers (evening), and (7) complin (complete). These hours marked only periods of daylight, beginning at 6 a.m. The nights were sometimes divided into watches, which marked the times when guards reported for duty or were changed. The length of the hour varied with the season. The winter hours were shorter than the summer hours, because there was less daylight during the winter.

By the 1500's, many churches and palaces in Europe had installed mechanical clocks with 12-hour dials. These clocks did not keep good time, and had to be set every sunshiny day at noon, when the sun was at its highest point, or on the meridian. From this we get the letters a.m., meaning ante (before) meridiem, or before noon; p.m. means post (after) meridiem, or after noon. When people first began to tell time by the clock, they substituted the word o'clock, meaning by the clock, for the word hour.

Confusion can result if the letters a.m. and p.m. are not used. European railroads and airplane timetables use a single 24-hour system. To avoid confusion, four figures are used. Thus, 1:00 a.m. is written 01:00. 1:00 p.m. is written as 13:00, and 12:00 midnight is 24:00. The United States armed forces also use this system, but without the colon. In conversation, all four figures are used, such as "O one hundred" for 1:00 a.m., "twelve hundred" for 12:00 noon, and "twenty-four hundred" for 12:00 midnight.
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Roland Camilleri

Moderator

Sydney , Australia.

ROLCAM · Posted: Tue Sep 11, 2007 1:40 pm Post subject: Longitude.

Longitude. If one person on the equator travels directly north, and another person 69 miles (111 kilometers) west also travels directly north, their paths will meet at the North Pole. Each person will have traveled in the same direction along a different line of longitude. Lines of longitude run north and south along the surface of the earth. Mapmakers think of the earth as a huge globe that is divided into 360 equal slices. The lines between the slices on the outside of the globe are called meridians. Meridians are the main lines of longitude on maps.

Longitude and location. Most nations start counting longitude east and west from an imaginary line running through Greenwich, a borough of London. These countries have agreed Greenwich lies at 0° longitude.

The earth is divided into two parts, or hemispheres, of east and west longitude. Each hemisphere has 180 degrees. Degrees of longitude are used to measure east and west distances on maps. They are used, along with degrees of latitude, to find certain points (see LATITUDE). For example, New York City lies at 74° west longitude. This means that if a person travels west from Greenwich to New York City and counts the imaginary meridians, New York City would lie on the 74th meridian west of Greenwich. Sailors and pilots use longitude to help determine the location of their ships and airplanes.

The space between two meridians is greatest at the equator--about 69 miles (111 kilometers). This space narrows as the meridians approach the North and South poles. For example, a degree of longitude at New Orleans is about 60 miles (97 kilometers) wide. At Winnipeg, Canada, which lies much nearer the North Pole, a degree of longitude is less than 45 miles (72 kilometers) wide. At Fairbanks, Alaska, which is still closer to the pole, it is even narrower.

Longitude and time. Any point on the earth's surface traces a whole circle--360 degrees--once every 24 hours. It does this because the earth turns once on its axis every 24 hours. All 360 degrees of the earth's circumference also pass beneath the sun once in 24 hours. In one hour, 1/24 of 360 degrees, or 15 degrees, passes beneath the sun. Because it seems that the sun is moving instead of the earth, people say that one hour of time equals 15 degrees of longitude.

Each degree of longitude is divided into 60 parts called minutes. Each minute is divided into 60 seconds of longitude. These minutes and seconds of longitude measure distance, not time. But since an hour of time equals 15° of longitude, a minute or second of time equals a certain distance that can be expressed in minutes and seconds of longitude.

The following table gives the equivalent in distance for five units of time. These units range from a day to a second:

24 hours of time = 360° of longitude

1 hour of time = 15° of longitude

4 minutes of time = 1° of longitude
1 minute of time = 15 minutes of longitude

1 second of time = 15 seconds of longitude
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Roland Camilleri

Moderator

Sydney , Australia.