Wednesday, 10 October 2012

Units Of Time

yoctosecond10−24 s
zeptosecond10−21 s
attosecond10−18 sshortest time uncertainty
in present measurements
femtosecond10−15 spulse time of ultrafast lasers
(100 as = 0.1 fs)
picosecond10−12 s
nanosecond10−9 stime for molecules to fluoresce
microsecond10−6 s
millisecond0.001 s
second1 s base unit
minute60 seconds
hour60 minutes
day24 hours
week7 daysAlso called sennight
fortnight14 days2 weeks
lunar month27.2–29.5 daysVarious definitions exist.
month28–31 days
quarter3 months
year12 months
common year365 days52 weeks + 1 day
leap year366 days52 weeks + 2 days
tropical year365.24219 daysaverage
Gregorian year365.2425 daysaverage
Olympiad4 year cycle
lustrum5 yearsAlso called pentad
decade10 years
Indiction15 year cycle
generation17–35 yearsapproximate
jubilee (Biblical)50 years
century100 years
millennium1,000 years
exasecond1018 sroughly 32 billion years, more than twice
the age of the universe on current estimates
cosmological decadevaries10 times the length of the previous
cosmological decade, with CÐ 1 beginning
either 10 seconds or 10 years after the
Big Bang, depending on the definition.


Time is a dimension in which events can be ordered from the past through the present into the future,and also the measure of durations of events and the intervals between them. Time has long been a major subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars. Nevertheless, diverse fields such as business, industry, sports, the sciences, music, dance, and the live theater all incorporate some notion of time into their respective measuring systems.Some simple, relatively uncontroversial definitions of time include "time is what clocks measure" and "time is what keeps everything from happening at once".

Sunday, 7 October 2012





The First Tick

The first mechanical clocks had a weight that would slowly lower, moving gears which moved a hand which showed the hour. They could only be build in tall towers because the weights needed to fall a great distance or else the clocks would only work for a short amount of time. People were amazed that these clocks were only off about 2 hours a day. Think if our clocks today were off by that much? If we were 2 hours late for school, we could blame it on the clock. While these clocks were inaccurate long ago, some of them were created with such care that they still work today. In Normandy, France, a big clock exists that was built in 1389. In Salisbury, England you can see the oldest clock in the world, built in 1386. Today, cuckoo clocks are still built using a weight-dropping mechanism.

The Sands Of Time

The major fault with sundials and shadow clocks is obvious...They don't work at night! Amenophis I, the king of Egypt, wanted to know what time it was all through the night without having to check the position of the stars. As you can imagine, it would be inconvenient to get up and out of bed every time you want to know the time. So, Prince Amenemhet made the king a clepsydra or a water clock. He took a big bucket of water, filled it with water up to a specific line. He then cut a small hole in the bottom of the bucket and marked off lines on the bucket after each hour had passed. There were, of course, some problems with this water clock as well. Water would flow more slowly or quickly when the temperature changed. This is where sand came into effect. The inventor of the sand clock is unknown but the sand clock or hourglass was commonly used in ancient times and is still used today. They are often found in board games or are used as kitchen timers. Is there an hourglass in your home?

The Beginning Of Time

The Egyptians were the first people who created a twenty-four hour day. Time was a little bit different in those days. The night was divided up into twelve hours, which were designated by the position of stars in the sky. The day was divided into ten hours and a shadow clock was used to keep track of these hours. The twilight hours were the hours before dawn and after sunset. The Egyptians thought they were the first to invent the shadow clock, but they were mistaken. At the same time, the Chinese, Babylonians, Greeks and the Romans were using instruments to tell time. Sundials were used in some of these groups, not because they work better, just because that's how they decided to tell time. After a while, the Egyptians and other ancient societies realized that the sun rose and set in different places in the summer and winter. In fact, the sun never took the same course on any one day throughout the year! They tried everything, until they realized that if they would just put the post of the sundial in at a special angle, it would work all year.

Wednesday, 3 October 2012

History of Telling Time

Prehistoric man, by simple observation of the stars, changes in the seasons, day and night began to come up with very primitive methods of measuring time. This was necessary for planning nomadic activity, farming, sacred feasts, etc.. The earliest time measurement devices before clocks and watches were the sundial, hourglass and water clock. Find out more about these types of clock here The forerunners to the sundial were poles and sticks as well as larger objects such as pyramids and other tall structures. Later the more formal sundial was invented. It is generally a round disk marked with the hours like a clock. It has an upright structure that casts a shadow on the disk - this is how time is measured with the sundial. The hourglass was also used in ancient times. It was made up of two rounded glass bulbs connected by a narrow neck of glass between them. When the hourglass is turned upside down, a measured amount of sand particles stream through from the top to bottom bulb of glass. Today's egg timers are modern versions of the hourglass. Another ancient time measurer was the water clock or clepsydra. It was a evenly marked container with a spout in which water dripped out. As the water dripped out of the container one could note by the water level against the markings what time it was. A huge advance occurred in the 1300’s when mechanical clocks, which used weights or springs, began to appear. At first, they had no faces, and no hour or minute hands; rather, they struck a bell every hour. Later, clocks with hour, and then minute hands began to appear. These early mechanical clocks worked by using an escapement, a lever that pivoted and meshed with a toothed wheel at certain intervals. This controlled the movement, or "escape" of either the weights or the springs that were powering the clock, in order to regulate the speed at which the gears and wheels which measured the time turned. In the 1400’s, another important discovery in timekeeping was made: it was learned that coiled springs, which used small coiled springs unwinding at a speed controlled by an escapement, were able to move the hands on a clock as well as weights or springs of previous, larger clocks. This discovery made smaller clocks, and later watches, possible. Then, in 1656, Christiaan Huygens invented the pendulum clock, which used weights and a swinging pendulum. These clocks were much more accurate than previous clocks, off by less than a minute a day, compared to the 15 minutes a day of earlier clocks. The bigger the pendulum, the more accurate the clock was. In 1714, the British Parliament offered a cash reward to anyone who could invent a clock accurate enough for use in navigation at sea. Thousands of sailors died because they were unable to find their exact position, because the exact time was needed to find longitude, and pendulum clocks would not work at sea. For every minute lost by a clock, it meant that there would be a navigational error of 15 miles, and sailors died because they were lost or smashed against rocks because they were unable to figure out their exact position. Then, in 1761, after 4 attempts, John Harrison finally succeeded at inventing a small clock accurate enough to use for navigation at sea. This tiny pocket watch lost only 5 seconds in 6 and ½ weeks. In the early 1800’s, one of the most important events in clock making occurred. Eli Terry developed machines, patterns, and techniques that produced clock parts that were exactly alike, so they could be mass-produced and interchanged from one clock to another. This drove the price of clocks way down, and allowed common people to own at least one, if not many, timekeeping devices. At the dawn of the 20th century, only women wore wristwatches. No self-respecting "real man" would wear one. However, in the first World War, soldiers wore wristwatches because taking out a pocket watch to check the time was difficult or impossible in battle. After the war was over, it was considered "socially acceptable" to wear wrist watches, and they became popular. Half a century later, digital watches, which used electrical currents running through quartz crystals to cause vibration and tell the time very accurately, began to appear. The next great advancement in timekeeping was in 1967, when the atomic clock, which used the oscillations of cesium-133 atoms to tell time, was invented. This clock had an error ratio of 1 second for every 1.4 million years. Recently, in 1999, scientists developed the cesium fountain atomic clock, which is off by only one second every 20 million years. This clock is the most accurate in the world.

Monday, 1 October 2012

The Sundial

Some people can tell what time it is by looking at the sun. But I have never been able to make out the numbers." (Attributed to an essay by a student in elementary school.) The simplest sundial is a vertical stick rising from a flat horizontal surface. [IMAGE: A Simple Sundial] As the Sun rises, passes the highest point in its path (at noon and to the south, in the northern hemisphere) and sets, the shadow rotates around the stick in a clockwise direction, and its position can be used to mark time. Indeed, it has been claimed that the "clockwise" direction in which the hands on a clock rotate was chosen for this reason. A sundial with a vertical pointer ("gnomon") will indicate noon correctly when its shadow points north or south. [North in northern middle latitudes, south in southern ones, while near the equator it can be either way, depending on season.] However, the direction of the shadow at some other time of the day may depend on the season--its value in summer, when the Sun's path is high, may differ from what it is in winter, with Sun low above the horizon. Such a sundial will however work equally well at all times if the pointer is slanted, to point towards the pole of the celestial sphere (click here for an explanation--but be warned, it is a bit complicated!). The angle between it and the base then equals the geographic latitude of the user. A Paper Sundial Ornamental sundials are often found in parks and gardens, with the pointer widened into a triangular fin, which must point northwards. A sundial of this type can be constructed from folded cardboard or stiff paper: click here to see the basic design used around latitude 38 North of the equator, here for a corresponding one in the southern hemisphere. Either can be printed and then photo-copied onto suitable sheets of stiff paper or cardboard [You may want to use the "option" menu to reduce size to 90% before printing--but make sure to return the setting to 100% afterwards!]. It is meant to be used at a latitude of 38 degrees and should work adequately in most of the continental US. Instructions: Cut the paper along the marked line: one half will serve as base, the other will be used to construct the gnomon. In the gnomon part, cut away the two marked corners. Fold the sheet in its middle, in a way that the two secondary printed lines (leading to the cut-off corners) remain visible. The line of the fold is the gnomon. Note: In stiff paper, straight folds are helped by first scoring the paper, by drawing a line along them with a black ballpoint, guided by a ruler and pressed down hard. With the page folded in its middle, cut out along the curved line, cutting a double thickness of paper in one cut. The cut begins near the top of the gnomon-fold and ends on the secondary line. Do not cut along the secondary line. No pieces come off. Score the other two secondary lines, then fold the gnomon sheet along them. The fold is opposite to that of the fold in the middle. These two folds should form 90-degree angles, so that the two pieces with the corners not cut in step 2 can be placed flat on the table, and the triangular gnomon rises above them. In cut (4), the fin of the gnomon was separated from two pieces with curved outlines. Fold those pieces so that they, too, are flat with the table. One goes above the other, and the slots they form near the secondary lines create a place for the fin to fit into. You are almost done. Take the base sheet, and note the apex where the hour-lines all meet (that is where the bottom corner of the fin will go). Carefully cut the sheet from this point along its middle line, up to the small cross-line marked on it. Do not cut any further! Slide the fin into the cut you made, so that all horizontal parts of the first sheet are below the base sheet; only the fin sticks out. Its bottom corner should be at the apex. The sundial is now ready, but you might use tape on the bottom of the base-sheet to hold the two pieces together firmly. For further stability, and to prevent the sundial from being blown away, you may attach its base with thumbtacks to a section of a wooden board or a piece of plywood. Finally, orient the fin to point north. You may use a magnetic compass; before pocket watches were available, folding pocket sundials were used in Europe, with small magnetic compasses embedded in their bases. If clear sunlight is available, the shadow of the tip of the fin now tells the time. If you want to make a sundial of more durable materials, draw the pre-noon hour lines at the angles to the fin (given in degrees) given below. These lines are meant for a latitude of 38 degrees; if your latitude is markedly different, see note at the end. 6 -- 90° 9 -- 31.6° 7 -- 66.5° 10 -- 19.6° 8 -- 46.8° 11 -- 9.4° Accuracy The sundial will obviously be one hour off during daylight saving time in the summer, when clocks are reset. In addition, "clock time" (or "standard time") will differ from sundial time, because it is usually kept uniform across "time zones"; each time zone differs from its neighbors by one full hour (more in China and Alaska). In each such zone, sundial time matches clock time at only one geographical longitude: elsewhere a correction must be added, proportional to the difference in longitude from the locations where sundial time is exact. (Up to the second half of the 19th century, local time and sundial time were generally the same, and each city kept its own local time, as is still the case in Saudi Arabia. In the US standard time was introduced by the railroads, to help set up uniform timetables across the nation.) Finally, a small periodic variation exists ("equation of time") amounting at most to about 15 minutes and contributed by two factors. First, the Earth's motion around the sun is an ellipse, not a circle, with slightly variable speed in accordance with Kepler's 2nd law (see here as well as the section preceding that page). Secondly, the ecliptic is inclined by 23.5 degrees to the equator, which means the projection of the Sun's apparent motion on it (which determines solar time) is slowed down near the crossing points of the two. Note on Latitude The angles listed above are intended for a latitude of 38 degrees. If your latitude is L, √ denotes "square root of" and K (=cotg2L) is K = cos2L/ sin2L then the angle between the fin and the line corresponding to the hour N+6 (N going from 0 to 6) satisfies sin A = cos(15N) / √(1 + Ksin215N) Here 15N (=15 times N) is an angle in degrees, ranging from 0 to 90, and of course, the afternoon angles are mirror reflections of the morning ones. If your calculator has a button (sin-1), if you enter (sin A) and press it, you will get the angle A. For an explanation of sines and cosines, look up the math refresher. And don't forget to adjust the angle of your fin to L, too! And by the way... The sundial described here, with a gnomon pointing to the celestial pole, is a relatively recent invention, probably of the last 1000 years. Yet sundials were used long before, often with unequal hours at different times of the day. The bible--2nd book of Kings, chapter 20, verses 9-11 (also Isaiah, ch. 38, v. 8) tells of an "accidental" sundial, in which the number of steps covered by the Sun's shadow on a staircase was used to measure the passage of time. In that story, the shadow miraculously retreated ten steps on the staircase built by King Ahaz. Exploring Further The "Sundial Bridge," with a unique design which may well make it the largest sundial anywhere, opened July 4, 2004 in Turtle Bay Park in Redding, California, at the foot of Mt. Shasta. Designed by the innovative Spanish architect Santiago Calatrava, it resembles his stunning 1992 bridge erected in Seville, Spain. It is a pedestrian bridge, connecting two parts of Turtle Bay Park, and it also operates as a sundial, using plaques set in a semicircular upper plaza. [IMAGE: Sundial bridge] For a more detailed article about this bridge, see Sundial Bridge at Turtle Bay Before the days of affordable wristwatches, people often carried a folding sundial in their pocket ("poke" below), with a small magnetic compass embedded, to show the north direction. In "As You Like It" by William Shakespeare (act 2, scene 7) one of the characters tells of meeting in the forest a fool (= witty court entertainer) carrying such a "dial": "Good morrow, fool," quoth I. "No, sir," quoth he, "Call me not fool till heaven sent me fortune:" And then he drew a dial from his poke, And, loking on it with lack-lustre eye Says very wisely, "It is ten o'clock: Thus we may see how the world wags: 'T is but an hour ago since it was nine; And after one hour more 't will be eleven; (and continues) You may also be interested to know that a North American Sundial Society (NASS) exists, with its home page at From the main page, the visitor can click on "About NASS", and/or access the many other features the site offers. And in case you wonder about the creatures drawn at the bottom of the "About NASS" page, they are toves, the whimsical invention of Lewis Caroll in his poem Jabberwocky. Concerning what toves are, see Humpty Dumpty's explanation, also reachable by clicking the winking sun icon on the top of the NASS home page. The reference is from the 6th chapter of Lewis Carrol's Through the Looking Glass. The British Sundial Society also has its sundial page. A sundial was included as part of the Mars lander mission and is shown in "Astronomy Picture of the Day" for 28 April 1999. It has a thick vertical gnomon, so that its readings may need some extra corrections