Time

If you can fill the unforgiving minute
With sixty seconds' worth of distance run,
Yours is the Earth and everything that's in it,
And, which is more, you'll be a Man, my son.

Time is funny stuff. Sometimes you've got far too much of it, for example when you are watching EastEnders. Then, without warning, there aren't enough hours in the day, the days rush by like seconds, and the holidays are gone again.

But, did you know that some days (i.e from noon to noon) actually are longer than others?

This is why we have Greenwich Mean Time, ie, Greenwich Average Time.

This is known as the Equation of Time. It happens because of a combination two effects.

The first is a result of the elliptical orbit of the earth around the sun.

The other effect is due to the fact that when the earth is starting leaning towards or away from the sun (just after the equinoxes), the days are longer, but are shorter between the solstices and the equinoxes.

Analemma is a website which is dedicated to this effect.

The upshot is that the sun will be due South in Aston Munslow at 12.25pm GMT on the 14th February (Valentine's Day), and at 11.55am GMT on 31st October (Hallowe'en) .

That is why local sundials appear to be inaccurate - in fact they are more accurate than your wristwatch, but they do tell Corvedale Local Time (CLT).
Equality for Hours !

Nowadays, we divide the day up into 24 hours, each of 60 minutes, but interestingly, until about 1300, some people divided the day simply into Day and Night.

Each was ascribed 12 hours, and so the length of an hour depended on your latitude, and the time of the year. These are known as the Unequal Hours and were easily worked out with an astrolabe - originally Greek or earlier, then Arabic, then reimported into the west.

Why twelve hours in the day? Well, why 12 pence in the old shilling, 12 inches in a foot, and 12 in a dozen? And besides being divisible by both 2, 3, 4 and 6, you can also count to twelve using your thumb to point to the segments of your fingers. You can then work a whole day, half a day, a third of a day, a quarter of a day, or a sixth of a day.

The 12 months relate to the 12 houses of the zodiac, or for Islam, the 12 lunar cycles in a year. Jupiter seems to pass through the zodiac in 12 years, but the fact that there were 12 tribes of Israel and 12 disciples, is probably unrelated.

Also there are 12 semi-tones in a musical octave.

It was only with the proliferation of mechanical town clocks, like this one in Prague, that the concept of dividing the day up into Equal hours became established.

Which was lucky, because otherwise TV time-tabling would have to be different between Cornwall and the Orkneys.

Are we nearly there yet? - the use of time for navigation
Navigators had known how to find their latitude (i.e. how far north or south they were) for centuries, using the altitude of the noonday sun, or known stars (Orion's belt is just about on the celestial equator, and is 38 degrees above the horizon in the Corvedale, Polaris is very close to the celestial pole).

The problem was knowing how far around the globe you were (east or west), ie your longitude.

Hipparchus (64BC - 24AD) suggested using lunar eclipses - these are seen at the same time anywhere in the world.

The Chinese are thought to have set up observatories in the countries that they visited, and made a note of the stars that were directly overhead when the eclipse happened.

They then took this information back to Beijing, and made a comparison with the local observations. This told them the longitude of their colony.

What was really needed however, was a method that would be useful to sailors at sea, otherwise this sort of thing happens:

Galileo and Jupiter
In 1610, when all the celestial objects were take as revolving around the earth, Galileo Galilei was rather surprised to see four "stars" sitting next to Jupiter in a straight line, swapping from one side to another. He came to the conclusion that these were moons orbiting Jupiter, and we now call them Io, Europa, Ganymede and Callisto.

He noted that they moved about pretty rapidly and predictably, and wondered whether they could be used to synchronise time keeping in different parts of the globe. He tried to pursuade the King of Spain to use this for navigation, but failed, due to the inaccuracy of the predictions.

It wasn't until Giovanni Cassini succeded in producing accurate tables predicting the times that Io and Europa passed into Jupiter's shadow that this became a useful procedure. It was then used to map the kingdom of France, which was discovered to be much smaller than previously thought.
The observations needed are impractical for seafarers due various factors, including the fact that for much of the year Jupiter is too close to the sun, the magnification needed precludes sights being taken on board ship, and weather considerations. However they were used by land-based surveyors, including Charles Mason and Jeremiah Dixon when surveying in America, and the Military Topographical Engineers in the American Civil War.

Captain Cook also took Jupiter moon sightings when he made a landfall.

There is a super moving image of the Galilean Moons here.
Lunar Distances

The Moon actually moves eastwards against the back drop of the fixed stars at a rate of 0.5 degrees an hour (i.e. the width of itself).

Due to this eastwards movement, sailors could tell the time by the moon (without magnification), before the chronometer was invented. This was important for Celestial Navigation.

This was a technique pioneered by Jean-Baptiste Morin in 1634, and improved by the Astronomer Royal, Nevil Maskelyne, who was given rather undeserved bad press recently in the book Longitude.

(One of the main reasons to choose Greenwich as the Prime Meridian was that all Maskelyne's lunar tables were based on the Royal Observatory there.)

They used a sextant, (or an octant), to measure the angle between the moon and a known star or planet. After a bit of mathematics, they compared this distance with some precalculated answers for specific times, and worked out what time it was back in good ol' Blighty (England).

If you know what your local time is (through a noon sun sighting), and compare it to the time in Greenwich, (through the moon sighting) you can work out how far around the earth you have sailed (six hours difference = 90 degrees around).

This Lunar Distance system was a bit crude and laborious, and can only be accurate to about three minutes (which is 45 nautical miles at the equator), but it was better than nothing, especially on big oceans.
This method of finding the time was also used by Sir Augustus Charles Gregory who mapped Australia, to check his chronometers were reading correctly
Chronometers
The idea of constructing a very accurate on-board clock to compare locally measured time to a known standard was certainly mentioned by the Dutchman Gemma Frisius in 1530, but he was aware that such a clock would be very difficult to make.

It was John Harrison - an Englishman (Huzzah!!) - who made such a clock, and revolutionised the safety of ships at sea. This revolution was a slow one however, and most ships still used the lunar distance method due to the horrific expense of a chronometer.

In fact, you needed three of them for comparison, to be sure that one of them wasn't on the blink.

This is the Royal Observatory in Greenwich, and in the background is the Thames. The scene has changed little in the intervening years, and it is a fantastic place to visit. They don't do any astronomy there anymore, due to the light pollution, but there are tons of interesting things to do and see, including Harrison's clocks.
The Prime Meridian

A sailor's day always started at high noon, because that's when he could see that the sun was due south - on the Meridian. It did mean, however, that everyone had a different concept of when noon was, excepting if you were on the same line of longitude (north - south).

This didn't really matter, until people started travelling about, needing international agreements on sea-charts, and clocks and gun-fights.

So at the request of U.S. President Chester A. Arthur, 41 delegates from 25 nations sat down together in October 1884 in Washington D.C., and agreed to use the Greenwich Meridian (the line running due north and south through Greenwich) as the Prime Meridian - i.e 0 degrees longitude.

Except for the French of course.

The French continued to use Paris as their Prime Meridian until 1911 for timekeeping and 1914 for navigation.

Because they are like that.

But now the world takes its timezones from England (Huzzah!!), (although what was GMT is now UTC). With Greenwich as 0 degrees longitude, every degree westward that you travel, your local noon (ie when the sun is due south) will be 4 minutes later.

Here in the Corvedale, we are 2 degrees 43 (arc)minutes west of Greenwich, so the sun is due south here 11 minutes after it is due south in London.

Greenwich

From a star-gazing point of view, this means that both the sun and the imaginary celestial globe rotate westwards at the speed of one degree every four minutes, or fifteen degrees an hour.

This is the angle that your fist and thumb subtend when your arm is outstretched. So get out there, and give the thumbs up to the stars.
If you want more information on celestial navigation, get Tony Crowley's book here.

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