Perhaps time is eternal and infinite. Perhaps time is just God's way of stopping everything from happening at once. Although we look at such philosophical and theological questions when we study cosmology (theories of the origin and fate of the Universe), in this article I set out to give no more than a brief summary of the practical side of time; how it is used and how it is measured.

Contents

The Day and the Hour • The Week • The Month • The Year

Sidereal, Solar and Mean Time • Time and Longitude

Time Zones • References

The Day and the Hour

The Sun's rising and setting gives rise to day and night, two of the most powerful natural features of the world we live in. The length of the day is traditionally measured from sunrise to sunrise (although astronomers measure it from noon to noon). The passage of the day is marked by the sun crossing the sky, and the passage of the night by the circum-polar stars turning in the sky like the hands of a clock.

For most people living prior to the Industrial Age a machine for measuring time was unnecessary. "I will meet you when the sun is there" set the time with sufficient accuracy. "In five fingers time" defines a shorter period. (Five fingers held at arms length mark out approximately 10° of the sky. The sun crosses 10° in 40 minutes.) Daylight time was fixed by a glance at the sun, and after dark the position of the stars marked the lonely watches of the night to anyone who was familiar with the night sky.

Divisions of the day

The only natural divisions of the day are morning, afternoon, and night; marked by sunrise, noon, and sunset. Any further divisions are arbitrary.

Around 3000 BC the Sumerians divided the day into twelve periods of two hours each, which were further divided into units of four minutes length.¹ Around the time of Christ the Romans divided the daylight hours into 5 periods, and the night into watches of three or four hours long. About 600 AD the Roman system was extended by the addition of two more periods giving the seven canonical 'hours' that ruled the day in Europe for the next 1,000 years: matins and lauds (together making one hour), prime, tierce, sext, nones, vespers, and complin. Meanwhile the Saxons divided the daylight hours into three tides: morningtide, noontide, and eventide.² The division of the day into 24 equal hours came into practice only about 1600 AD.³

Sundials

The Sumerians may have used sundials in 3000 BC; Chinese and Indian sundials may have been in use as early as 4000 BC, but there is no clear evidence of this. There is evidence that the Greeks were using sundials around 500 BC. An example of the type of sundial that they may have used, called a hemicyclium, has been found and is believed to have come from around the time of Christ.⁴ A sundial marked in Saxon tides is preserved in the Kirkdale Church in Yorkshire England, and is believed to originate from the year 1064 AD.⁵ The earliest sundials marked with modern hours date from about 1600 AD.⁶

Clocks

The water clock is basically a vessel out of which water runs at a constant rate. Simple water clocks were used in India and China as early as 4000 BC. Around 150 BC the water clock took over from the sundial as the official timepiece of Roman law. The water clock had at least two advantages over the sundial: it worked when the sun was not shining, and it could be made to read the time more accurately. The water clock remained the most accurate of timepieces until the development of the the pendulum clock about 1700 AD.⁷

This century the atomic clock has superseded the pendulum clock and is so accurate that tiny fluctuations in the Earth's rotation can be detected.

However accurate a clock is, it nevertheless must be set so that it is in time with the daily passage of the sun across the sky. Traditionally clocks were set from a sundial at noon, and more recently this century they have been set from very accurate measurements of stars crossing the meridian of the sky. Even atomic clocks have to be set this way if they are to stay in time with the Sun, as they actually keep better time than either the Earth's rotation or its orbit.

The Week⁸

Babylonian and Jewish cultures have used a seven day week for thousands of years. However, there is no natural phenomenon to define the length of the week and various cultures have at various times used a week of four, five, eight and ten days, the passage of the week often being marked by market day. The seven day week was officially introduced to Europe by Constantine the Great in 321 AD.

The names of the seven days of the week originate from the seven planets of the ancients. The Sun and the Moon were recognised as planets, along with the five naked eye planets Mercury, Venus, Mars, Jupiter and Saturn.

Day	Planet	Saxon Origin
Sunday	Sun	Sun's Day
Monday	Moon	Moon's Day
Tuesday	Mars	Tiw's Day
Wednesday	Mercury	Woden's day
Thursday	Jupiter	Thor's Day
Friday	Venus	Frigg's Day
Saturday	Saturn	Saterne's Day

The Month

The moon's cycle is 29.531 days long. In that period it goes from new, to first quarter, to full, third quarter, and back to new again. This cycle forms a natural unit of time.

The moon's cycle is important as it affects the amount of light available for nighttime activities. It has also been used to indicate good times for planting or fishing. The number of moons that have passed is also a good way to measure time and is much easier to keep track of than the number of days.

There are 12.37 lunar cycles in a year. In creating a calendar of exactly 12 months the length of the month has been altered slightly giving a calendar month of 30 or 31 days. The result is that the months no longer keep in step with the moon's cycle.

The names of the months come from the Roman calendar developed over the period 738 BC to 8 BC.

Month	Latin	Origin9
January	Januarius	after Janus, guardian of the heavens, symbolising the beginning
February	Februarius	after Februalia, meaning 'repentance,' a period when sacrifices were made to the gods
March	Martius	after Mars, the god of war
April	Aprilis	from aperire, meaning 'to open,' suggesting the period of budding leaves and opening flowers
May	Maius	after Maia, the godess of growth
June	Junius	from juvenis, meaning 'youth'
July	Julius	after Julius Caesar
August	Augustus	after Augustus Caesar
September	Septembris	the seventh month of the original Roman calendar
October	Octobris	the eighth month of the original Roman calendar
November	Novembris	the ninth month of the original Roman calendar
December	Decembris	the tenth month of the original Roman calendar

The Year

Definition of the year

The year is a complete cycle of seasons. During the year the sun travels around the sky, passing through all twelve signs of the zodiac. The year is caused by the Earth completing one orbit of the Sun. Its length is measured from one northern hemisphere spring equinox to the next spring equinox.

Measuring the year

It was important to most people to know how far through the year it was. Knowing the season was vital for knowing when to plant and harvest crops, such as the Maori planting of kumara in spring and harvest in autumn.¹⁰ It was also important for other activities such as the Celtic practice of putting the cattle in the barns at the start of winter (Halloween/Guy Fawkes) and letting them out to pasture at the end of winter (May Day).¹¹ Knowing the progress of the year is also important for religion because many religious festivals happen at specific times of the year, such as Easter.

The sun is the best way to measure the progress of the year, because it is the sun that creates the year. The obelisks of Egypt, dating from as far back as 3000 BC, were used to measure the progress of the year by the length of the shadow they cast.¹² Stonehenge was apparently built for the same purpose although it measured the year by the sunrise and sunset angle on the horizon, a more useful method at higher latitudes. In this way the ancients were able to determine the progress of the year accurately.

It is possible to use other, easily observed, signs of the passage of the year. The annual disappearance and re-appearance of the stars is something that has been used by many cultures around the world. Unfortunately there is a major drawback with this method. The sun's path through the sky slowly but steadily changes place over the years (known as the precession of the equinoxes), and it becomes necessary to choose new marker stars every 500 to 1,000 years. Hence it is that natural signs such as the blossoming or fruiting of particular plants, or the migrations of birds and whales, give a better long term system for determining the year. Maori in New Zealand used a combination of the stars and the natural signs to determine New Year.

History of the calendar

The Maori year was divided into months by the lunar cycle. The New Year began on the first new moon after the rise of Matariki,¹³ which occurred a week or two before winter solstice. The months were true lunar cycles from new moon to new moon. Because there are 12.37 lunar cycles in a year, the thirteenth month of the year was dropped as required. This month was effectively a leap month. This system means that the original Maori month names do not actually match one-to-one the months of the modern calendar, not only because they were of different lengths, but also because the date of New Year shifted by up to one lunar cycle every year.

The Roman calendar was initially based on a European calendar of ten months, each of 30 days. The New Year began at the spring equinox. It ran for ten months and then stopped until the next spring equinox. This is believed to have been done because the deepest months of winter were not worthy of being counted. The Romans adopted a calendar like this in 738 BC. It is intrinsically capable of staying in step with the seasons, because the uncounted days could vary as required to bring the New Year in line with the equinox.

Later the Romans added two more months (in 713 BC) giving a year of 355 days. This year would not line up with the seasons unless more days were inserted. Julius Caesar got onto this in 47 BC and overhauled the calendar changing the number of days in the months to bring the total to 365 days with a leap day every four years. This calendar is known as the Julian Calendar. The calendar year was now 356.25 days long and capable of staying in step with the seasons for a considerable time and not needing adjustment for about 500 to 1,000 years. However, Julius Caesar's calendar was mangled after his death and leap years were taken at will. Augustus Caesar set things right again in 8 BC and the calendar then functioned beautifully until 1582 AD. At that time it was realised that the system of leap years was not perfect, and it was fine tuned by dropping three leap years every 400 years. This last refinement, known as the Gregorian Calendar, results in a calendar year of 365.2422 days, which is very accurate. The Gregorian Calendar was not adopted in England or the colonies until 1752 AD.¹⁴

New year

Strangely enough, when Julius Caesar reformed the calendar he changed the New Year from the spring equinox to just after the winter solstice. However, the equinox was still celebrated at the traditional time, March 25 according to the new calendar.

Later March 25 was officially recognised as the day for observance of New Years Day and this held from 532 AD until the Gregorian Calendar was adopted (1582 AD in Europe, and 1752 AD in England and the colonies). Only then came the switch to observing New Years Day on the first day of the calendar year.¹⁵

Sidereal, Solar and Mean Time

Sidereal time

The Earth completes one rotation about its axis every 23 hours and 56 minutes. This period is known as the sidereal day. Each of the stars completes one circuit of the sky, and returns to the same place in the sky, once every sidereal day.

Solar time

The sun takes a little longer than the stars to come back to the same place in the sky. This is because while the Earth completes one rotation it also moves a part of the way around its orbit of the sun. This means that it has to rotate a little bit more to get the sun back to the same place in the sky. This 'little bit more' takes about 4 minutes to complete.

The 'little bit more' is the difference between sidereal time (star time) and solar time (sun time). What's more, the 'little bit more' varies from day to day, ranging from as little as 3 minutes 30 seconds up to 4 minutes 30 seconds. This variation is caused partly by the tilt of the Earth's axis away from its plane of orbit, and partly because the Earth's orbit is not a perfect circle and the Earth travels around it at slightly varying speeds.

This means that the solar day varies in length. The cumulative effect of these variations is to put sundials as much as 16 minutes faster or slower than clocks. The pattern is very regular and tables of adjustments can be used to correct sundials so that they read clock time.

Mean time

'Mean' is another word for average. The solar day varies in length, but the pattern repeats every year. The average length of all of the solar days in a year is the mean solar day. This is exactly 24 hours long, and gives rise to the expression 'mean time.' Mean time is the time kept by clocks, and the day, according to mean time, is 24 hours long.

It was not until sufficiently accurate clocks were made that the difference between solar time and mean time was noticed. This happened about 1700 AD with the advent of the pendulum clock.

Descriptive name	Technical name	Length of day
star time	sidereal time	23 hr 56 min
sun time	solar time	24 hr ± 30 sec
clock time	mean time (or mean solar time)	24 hr

Time and Longitude

As the Earth rotates the Sun appears to pass over the surface of the Earth. The 'line of noon' travels across the Earth, completing a complete circuit every day. This means that every longitude on Earth has a different solar time.

If you know the time that the sun passed over the prime meridian (Greenwich noon), and also know the time that the sun passed over your local meridian (local noon), you can work out your longitude by calculating how much the Earth has turned in the interval. This was the only practical way to determine longitude prior to the space age.

So, go to Greenwich and set your clock from the sun. Your clock will read Greenwich Time. Now make your journey around the world. Your clock will show you when it is noon in Greenwich, and your sextant will show you when it is noon at your location. You can then calculate your longitude.

The problem of course is how to make a clock that will keep time accurately enough while you gallivant around the world. Prior to the development of the ship's chronometer in 1763 no clock would do this.¹⁶ Christopher Columbus had no way to determine his longitude and so mistook America for India. The development of the ship's chronometer revolutionised navigation because it made it possible to determine longitude.

The chronometer was set to Greenwich Mean Time, and great care was lavished on keeping it accurate.

With the advent of radio the ship's chronometer became less important. Time signals were broadcast around the world. These broadcasts still take place and can be used for setting a clock or chronometer accurately to Greenwich Time.

Modern navigation is carried out using a satellite positional system called GPS. This system enables a hand held device to tell you your latitude and longitude anywhere in the world at the press of a button. However, knowledge of other methods of navigating has value in the event of a breakdown or deliberate shutdown of the system.

Time Zones

Because every longitude has its own solar time, and because prior to 1884 clocks were normally set to local solar time, America in 1860 had 300 different times in common use¹⁷ . This made scheduling a transcontinental rail service a nightmare.

The solution did not lie in using Greenwich Mean Time. While this was fine for the navigator of a ship, the common populace would not accept setting their clocks to show 12 midday just as the sun was rising on the American continent.

The solution was to define standard time zones one hour wide. This resulted in America being divided into four standard time zones, each of which was offset from Greenwich Mean Time by a different time. This gave clock noon at approximately the time the sun crossed the local meridian, while giving sufficient standardisation to enable the railways to cope. The system was in fact adopted by the whole world in 1884 and has remained in place since then.¹⁸ Almost the only change since that time is that the term Greenwich Mean Time has been replaced by the term Coordinated Universal Time.

The boundaries of standard time zones have changed in some places, and many time zones now apply daylight saving during summer. Daylight saving time merely alters the offset from Greenwich Time to fool the populace into thinking the sun stays up longer. Under daylight saving time the sun crosses the meridian at approximately 1pm, instead of the standard time of 12 noon.

New Zealand Standard Time is 12 hours ahead of Greenwich, changing to 13 hours ahead during daylight saving.

References

Cowan, H J Time and Its Measurement The World Publishing Company, Ohio, 1958

Batten, J Celebrating the Southern Seasons Tandem Press, North Shore City, NZ, 1995

Footnotes