About Time
Time is a measure of the rate of change, and we use the sun to measure this rate of change, the original clock mechanism for each day that we have used and still do. It’s done by using the sun’s path or track across the sky as the earth rotates to figure the length of each day, an action which conveniently makes it look like the sun is circling around us.
A Fototime Miniature of a Hawaiian sunset, made in 1923. ANMM Collection: 00042453.
The sun’s longer term changes relative to the earth’s equator alters its height above the horizon in a regular cycle, and this pattern of movement causes the seasons that help measure a year as it cycles from solstice to solstice through two equinoxes.
Time and tide stops for no man is a quote well used and somewhat evolved, but around 1200 AD the interpretation for tide was a measurement of the continually changing seasons, and thus the words noted the obvious, the march of time is unstoppable.
So, going back to our elegant solution for a minute, it’s actually quite simple to figure out your longitude relatively accurately, which is why the method was so appealing- you need to know the time at another point or location, and your own time as a comparison.
Having agreed on London as the location for the prime meridian of 0 degrees that location also became the datum for time, which was initially Greenwich Mean Time, but is now the almost super accurate UTC.
Here is how it works relative to 0 degrees longitude: if you know what time it is in London through which 0 degrees passes, and you also know what time it is where you are, you know how many hours (or parts thereof) you are away from London. Then you convert this time difference into degrees of longitude to see how many degrees away you are from London. It takes 24 hours for the sun to go through the full 360 degrees around the world, so an hour of time is equal to 15 degrees of longitude, and the rest of maths is simple, even the fractions.
If you know the two times precisely, you can bring it down to a high degree of accuracy.
In the pioneering days of the chronometer on the ship, working out the exact time where you were by the sun could be done but only once a day, which was good enough at this point. The sun rises and reaches its maximum height in the sky at midday. If you observed the sun through a sextant, and took observations (“taking a sight”) as it passed through its maximum height above the horizon, you could calculate midday at your location, and could then refer to the chronometer carrying your datum time to see what time it was at this location that the timepiece was set too. Other stars and planets could be used too if you have the right tables for ‘reducing your sight’.
The sun traces a curved path across the sky each day, at its highest point it is midday. Images: David Payne / ANMM.
If it was midday or 12.00 noon on the datum chronometer for GMT, and 11.00am on the ship’s chronometer, then you were one hour away time wise, which is 15 degrees of longitude, and as you were one hour ahead, you were to the east. If it was midnight on the ship’s timepiece you were on the opposite side of the world. If it was midday then you were somewhere on the 0 degree line of longitude to the north or south of the equator.
It takes 24 hours for the sun to go through the full 360 degrees around the world, so an hour of time is equal to 15 degrees of longitude. Diagram: David Payne / ANMM.
A diagram shows this relationship. We have 360 degrees to make up a circle, but a clock is divided into 24 hours. So if we draw a circle with 360 degrees and then a smaller one with 24 hours with zeros aligned we can do a conversion by sight between the two, like having inches on one edge of a ruler and mm on the opposite edge.
If we then declare the time division ring as representing GMT, we can draw our own sun clock relative to where we are outside of these two circles, and can now see the whole picture.
Numbers and Units
It’s not possible to do this without the numbers for distance and for time on the clock. They are needed to represent and count distances, time spans and divisions, and in this story the number 12 keeps coming up. It’s quite useful too.
For many of us we are schooled in the decimal system working to units of ten and this is relatively easy to grasp- units, tens, hundreds, thousands and so on. Unfortunately, a lot of the elements that make up longitude are not decimal and use earlier systems that have been adopted with other numbers as their base – and the base figure of 12 appears in a number of these elements, or else 12 divides easily into another chosen base, such as 24, 36 and 60. And many of us are not that uncomfortable with 12 today- how many times do we talk of a dozen somethings, or two dozen, three dozen and so on – we are counting in 12s. The imperial foot many of us used for measurements has 12 inches- we have lived with 12, and still do, but where does it come from?
In this story the number 12 keeps coming up.
12 comes about historically from more than one source, but an obvious one for many civilisations and cultures was that there are 12 lunar cycles to the moon each year. Another useful thing about 12 is that it is easily counted on your hand. Ignoring the thumb your remaining four fingers have three phalanges each – 4 x 3 = 12 – therefore hand counting 12 was easy, and with five fingers (thumb included now) its 5 x 12 = 60 – allowing you to count to 60 quite easily, abacus style. In this system you have units and 12s, instead of units and 10s.
Units of 12 can easily be counted with your fingers. Diagram: David Payne / ANMM.
Hours, minutes and seconds, then degrees, minutes and seconds, how did we end up with this apparent confusion of shared terms with different measures? It could have been easier if we had been able to divide the earth’s surface and the time it takes to do one revolution into the divisions where they equalled each other without the need to do further maths or conversions. However their independent study and recording from multiple civilisations and societies produced a range of options, some quite similar but others are quite diverse in their logic. It was up to the path of history to define the choice of what survived and what passed into the background.
Distance
If we look at distance around the globe first, any direct east west travel follows a circular path of latitude. You can divide a circle into equal parts or arcs radiating from the centre, and those equal arcs will remain constant relative to each other regardless of the physical size of the circle – that’s our system of degrees. A degree is usually denoted by the symbol ° and is an angular measurement, and 360 degrees make up once full circle. But as the latitude moves north or south of the equator the circle’s circumference changes, each degree measures a different length of an arc. So one degree of longitude at the equator measures nearly 110km, but gradually reduces to zero at the poles as you travel on latitudes north or south of the equator.
How did this come about, as degrees are used for any angular measurement between two lines, not just for circles? Maybe someone was thinking beyond the square when they realised the obvious – as any angular measurement between two lines increases, the lines move apart in a circular path and eventually meet up again as they complete the full circle – it happens all the time on your analogue watch.
But returning to 360 degrees. The origin is not known and only guessed at. It is thought that early astronomers had concluded that there was a circular path within the earth and sun relationship, but they had measured the full orbit as only 360 days, so one day become one unit of movement in this circular path. The Persian calendar was one example that had a 360 day year.
Equilateral triangles with 60 degrees at each corner, then divided up into 60. Diagram: David Payne / ANMM.
The Babylonians whose early trigonometry was based on circles and chords noted that a circle neatly subdivided itself with an equilateral triangle into six parts, and then went to their sexagesimal base 60 (5 x 12) method of counting to divide each part, and six of these then gave 6 x 60 or 360 divisions. The Greeks followed suit, and there is evidence the Indians were also using a 360 unit circle. 360 divides up well too, all numbers except 7 divide comfortably into 360, and this ability to carve a number up was a valuable asset to its use in society.
For many purposes a degree is sufficient definition, but it was then further subdivided with the sexagesimal base into 60 minutes and 60 seconds. In recent time a decimal based degree system was proposed but never adopted, however radians which help with calculations have been long accepted, but we seem to have avoided them in longitude, where distance is measured in degrees, minutes and seconds.
Time
Hours, minutes and seconds, that’s how we can measure time, but measuring time was taking place with broad divisions by early humans well before the concept of a spherical/circular 360 degree type world was understood by some in early civilisations. The notion that the graphical concept of the time span for both one day and one year could each be described as a following a circular path was one for the deep thinkers, and some of these early astronomers and philosophers probably concluded there was a spherical nature to earth and its relation to the sun as they pondered the whole picture. For most people life went on not over a curved surface on a sphere travelling in a huge circle, all they could see was a flat ground going nowhere and the priority was wanting to know what was happening with time during the daylight period.
A year and its divisions into days were easy enough to put start and end points to, but dividing the day did not workout easily as daylight lengthened and shortened daily to a pattern repeated each year, rather than neatly staying the same for each day. Once again the reliance on counting systems based on 12 gave rise to a number of similar approaches in different parts of the world that divided the daylight into 12 or multiples of 12 such as 24, 36, and even 60 and sometimes they were not necessarily equal divisions. Some aspects of the divisions related to stars to help mark the progress of night, but the day was locked onto the sun.
The word 'hour' has evolved from referring to the board span of the seasons to a specific unit of time.
The word hour given to these 12 divisions now used widely was originally Greek – hōra ‘meaning season, moving onto the French ‘ure’ and finally ‘hour’ in Middle English, and has gone from a connection to the broad span of seasons to a specific unit of time.
Minutes are vague in origin- they don’t seem to become recognised until around the time mechanical clocks were invented. Historically, the word ‘minute’ comes from the Latin pars minuta prima, meaning the “first small part”. It was then refined with a “second small part” Latin: parte minutae secundae and this is where the word ‘second’ comes from.
But why 60 of each? The answer is not written down in any history, and how it came to be remains speculative, but go back to a clock or watch and remember the counting system of 12 on the finger joints and 5 on the fingers, the 60 based system. The circular track of a clock face featuring 12 hours, with 5 divisions between each creates 60 divisions all round. And it works rather neatly – the hour hand labours on slowly, a minute hand can gradually move around ticking off time 60 times faster, while the second had does it another 60 times quicker again, all using the same grid around the circumference.
Creating the time span for a second based on the sun was not practical, in theory it was a fraction of a mean solar day, but solar days varied, and no mechanical clock was ever going to keep time to the second forever. However the invention of an atomic clock finally gave acceptable certainty to its length. In 1967, the International Committee for Weights and Measures defined the second for all of us as: the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom – and further refined this – it refers to a caesium atom at rest at a temperature of 0 degrees Kelvin.
That would seem to be the last word, but nanotechnology is on us now and who knows how long before this convention is changed again. Time will tell.
