01942 366211
Chronomaster FacebookChronomaster_Twitter
FRIDAY, 16 JULY 2010 UPDATED

Practical Thoughts On Time

 

 

What is Time?

From the dawn of civilization, philosophers and scientists have pondered and debated this apparently simple question. Yet no one has so far managed to provide a full and plausible answer acceptable to all.

Since ancient times, efforts to reconcile the solar calendar and the Julian calendar, created by Julius Caesar in 45 BC, have resulted in quirks with which we are familiar - leap years and months of different lengths. The challenge was met successfully by Pope Gregory in 1582 and taken even further 300 years later by ingenious watchmakers with the creation of perpetual calendar watches.
 
In 1582, an extraordinary event occurred in the month of October. That year, there was no 5th October, nor was there a 6th or 7th October. The day after the 4th October was... the 15th. There was no mystery - this "leap" in time was simply a necessary adjustment needed to bring the calendar back into line with the sun and the stars. Pope Gregory XIII instituted the Gregorian calendar, which made up for the shortcomings of the Julian calendar.
 
The Julian calendar was based on a year that was believed to be exactly 365 and 1/4 days long. It consisted of three consecutive years of 365 days, followed by a leap year with an extra day at the end of February. But the solar year is shorter than the Julian year by around 11 minutes, which comes to three days every 400 years.
 
By 1582, the Julian year was already ten days behind the solar year. Pope Gregory XIII corrected the error and, that year, the 4th October was followed by the 15th October. To avoid a repeat of this inconvenience, he decreed that three out of every four centennial years should be common years instead of leap years; the accuracy was such that in the future, the calendar would only be out by one day every 4000 years.
 
This means that in the year 2100, watches with a perpetual calendar will need a slight adjustment to move straight from 28th February to 1st March.
 
 

 

Greenwich Mean Time (GMT)

The official international date and time line

GMT is an imaginary line that runs through Greenwich, London and seven other countries, which is established at zero degrees longitude. This is the line by which all dates and times are set, and is the critical facto for world and satellite navigation.
 
It all began at the International Meridian Conference Washington DC, USA, October 1884. The conference was held for a number of reasons. One was to establish a single meridian so that time and dates could be standardised across the globe. As a result, the Prime Meridian was set to pass through Greenwich, which is now established as the "centre of time and space", as well as the home of the new Millennium.
 
The conference was held at the request of the President of the United States of America. Forty one delegates from 25 nations met in Washington, DC, USA for the International Meridian Conference.
 
At the Conference the following important principles were established:
 
  1. It was desirable to adopt a single world meridian to replace the numerous one's already in existence.
  2. The Meridian passing through the principal Transit Instrument at the Observatory at Greenwich was to be the 'initial meridian'.
  3. That all longitude would be calculated both east and west from this meridian up to 180°.
  4. All countries would adopt a universal day.
  5. The universal day would be a Mean Solar Day, beginning at the Mean Midnight at Greenwich and counted on a 24 hour clock.
  6. That nautical and astronomical days everywhere would begin at mean midnight.
  7. All technical studies to regulate and extend the application of the decimal system to the division of time and space would be supported.
 
Resolution 2 of the conference, fixing the Meridian at Greenwich was passed 22-1 (San Domingo voted against), France & Brazil abstained.
 
The International Date Line
 
The International Date Line is an imaginary line which runs from the North Pole to the South Pole and is 180° away from the Greenwich Meridian. There is much confusion about why each new day starts in Greenwich but not at the International Date Line.
 
The International Conference in 1884 deemed that there would be a single Universal Day and that this would begin at mean midnight at Greenwich.
 
Twenty five time zones were established to the east and west of Greenwich with the International Date Line lying along the 180° line of longitude. The line deviates in places to avoid crossing any land. Along this line the calendar moves into a new day but only in local time, which is measured relative to Greenwich Mean Time.
 
People who live on the International Date Line move their clocks (and calendars) into a new day 12 hours before the Universal Day officially begins in Greenwich.
 
Some small islands moved their territory into a new time zone so that they could claim to be the first into the millennium. Any government is able to define their time and time zones at any time relative to Greenwich. Countries like China only have a single time zone. Changing time zones is no different to 'clocking' the mileometer (odometer) on a car so that it appears different to what it actually is.
 
The official world millennium began in Greenwich. The millennium actually starts in 2001, so the millennium has not yet begun.
 
What is the Meridian Line?
 
The Meridian Line is an imaginary line which runs from the North Pole to the South Pole. By international convention it runs through "the primary transit" instrument (main telescope) at the Royal Observatory in Greenwich.
 
It is known at Zero Longitude and it is the line from which all other lines of longitude are measured. This includes the line that runs 180° away from Greenwich also known as the International Date Line.
 
There have been many meridian lines during the course of history including 9 lines at Greenwich! The Ordnance Survey maps started in England before the current meridian line was defined. New editions of the Explorer series maps do feature the Greenwich Meridian Line.
 
The Ordnance Survey website has a description of the history of Meridian lines.
 
 

 

Longitude

How to navigate at sea by using just a watch and the Sun

John Harrison perfected watch making to the extent that watches could be used at sea to navigate longitude. Prior to this mariners used the stars and the sun. This however was very unreliable and just one degree out could put a ship fifty miles off course. This could obviously have devastating effects in unknown waters, since rocks, reefs and unexpected land made hazardous shipping.
 
John Harrison was born over 300 years ago and was one of the worlds greatest horologists. He convinced the world that navigation by a watch was more superior than by stars (in those days reliability was an issue and watches often could not tolerate damp and rough sea journeys). Harrison developed the chronometer - a mechanism still used today in the worlds most accurate watches!
 
He had a struggle because he was a mechanic, and maritime boards were run by astronomers (seen to be more academic). These astronomers refused to accept that a mere mechanic could lead the way in accurate marine navigation - since the position of stars required much complicated mathematic calculations.
 
Harrison was right though, and tests proved his chronometer to loose about a second a day making long 3000 mile journeys accurate to within several feet, rather than several hundred miles.
 
Harrison's four major timepieces that he developed spanned his entire life. Think about it. You spend your entire working life building four timepieces. All in the pursuit of perfection. All four timepieces can be seen at the Greenwich Royal Observatory in London.
 
 

 

The Equation of Time

Why we have a calendar

As the Earth rotates around the sun, it travels an irregular elliptic course. Its wayward orbit inevitably results in days of uneven length, varying in duration from 23 hours 44 minutes to 24 hours 14 minutes. This is apparent solar time and is not really practical for an organized society. So the calendar was born. The one in use today assumes a perfectly circular orbit around the sun, with days of equal 24 hour length. This is mean solar time, the time of day read off every watch and clock. The equation of time expresses the difference between the two times, mean and apparent.
 
Apparent solar time thus runs up to 16 minutes behind mean solar time, as on November 3rd, and up to 14 minutes ahead, as on February 12th. Four days a year both times match precisely. Some watchmakers such as, Abraham-Louis Breguet built a number of watches showing the equation of time. His successors have recently gone a step further with the design of a now patented movement featuring both the equation of time and a calendar. Both these auxiliary mechanisms are perpetual, i.e. automatically and fully self-correcting for over a century.
 
 

 

Relativity

Why objects in time and space are relative to one another.

When we say that one object is moving relatively to another we mean that it is moving when viewed from the other object or compared with it. Only relative movement can be detected and measured because there is no object in the universe that can be said to be absolutely at rest. Starting from this simple truth, the great scientific thinker Albert Einstein worked out his Theory of Relativity. This theory caused important changes in scientific thought and progress. This is theorised in his famous formula.
 
e=mc2
 
In 1905 Einstein announced the first part of his Theory of Relativity, called the Special Theory of Relativity, based on something that everyone has noticed. For example, you sitting in a train at a station and watching another train alongside you which is beginning to move. At first you are puzzled to know whether your train or the other train is moving, and must look at a station building or a telegraph pole or something else he knows to be at a standstill in order to be sure. Before you look at the telegraph pole, you can say only that one train is moving relative to the other. Similarly, if two motor cars are racing alongside one another and one begins to draw ahead it looks to a person in that car as if the other one is going backwards, as indeed it is relative to the first car.
 
Before Einstein thought of his theory two scientists, Michelson and Morley had already tried to find out whether the movement of the earth around the sun could be detected without looking at the sun or stars. That is, whether the earth had any absolute motion or only its motion relative to the sun and stars. Their experiments showed that there was only relative motion. For this and other more complicated reasons Einstein suggested that there was really no such thing as absolute motion and that all motion was relative.
 
Some of the other conclusions he drew were that nothing can go faster than light, and that if something such as a ruler were moving faster and faster it would seem to get shorter and shorter as its speed approached the speed of light. Some of these conclusions seemed absurd and many scientists at the time refused to believe Einstein’s theory, although many experiments agreed with it. So what would happen to the energy of that ruler if it got shorter as it approached the speed of light? The popular notion is that it would gain in mass.
 
By 1915, Einstein had worked out some new ideas, and he announced the second part of his theory, called the General Theory of Relativity. He said that the light from a star would be slightly bent if it passed close to the sun. This was tested and found to be true. Einstein’s theory also dealt with the action of gravity, the force which makes an apple fall to the ground and keeps the earth and the other planets moving in regular paths around the sun. It explained the motion of the planet Mercury, which could not be explained exactly by Newton’s theory of gravity. Some of these matters may not sound very important, but they are the laws according to which all matter behaves.
 
For instance, it had long been known that the amount of energy in a moving object like a bullet, as measured by the work it could do by going through a certain thickness of wood, depended on the weight of the bullet multiplied by the square of its speed. Einstein’s theory, however, said that the amount of energy actually "locked up" in a substance depended also on the weight of the substance multiplied by the square of the speed of light. This was soon found to be true in laboratory experiments. Since 1945 it has been shown to be true on a large scale in the working of atomic and hydrogen bombs and at atomic power stations where large amounts of power are produced from quite small lumps of uranium. Healthy.
 
POSTED BY NEIL WOOD AT 12:31 FRIDAY 16/07/2010
UPDATED 18:24 SATURDAY 09/11/2013
COMMENTS loading