An hour forwards, an hour back

In my last post I talked about the Albrecht Dürer House that is at the end of my history of astronomy tour of Nürnberg. My tour starts under the rather spectacular sundial painted on the south side of the church of St Lorenz.

Lorenzkirche Sundial

I start my tour by talking about the way the day was divided up in the past; the observing, quantifying and recording of the passage of time down the ages being a particular interest of mine. I mention that it was the ancient Egyptians who initially introduced the 24 hour day, first dividing the night into twelve segments based on the rising of stars and the by analogy the day was divided into twelve segments. Adopted by the rest of the ancient world this became the European norm.

However during the Middle Ages our equinoctial system in which all twenty-four hours are equally long was not used but one in which the day starts at dawn and ends at dusk meaning that the twelve daylight hours and the twelve night time hours vary continually in length throughout the year. Nürnberg had its own special system, the great Nürnberg clock, which I explained earlier in an extra post. The sundial on the church displays various system of calculating the hours including Nürnberg’s special system making it one of the most complex sundials in Europe.

Moving around the corner we come to two more sundials, one south facing and one fairly rare east facing one that only shows the afternoon hours. Here on my tour I explain that a sundial usually displays local time i.e. time measured from a midday that is marked by the sun being directly overhead. This was the way that time was recorded, almost universally, down to the end of the nineteenth century. This means that every settlement has its own time. One-degree difference in longitude means a difference of four minutes in time. Prague is 3° 20’ east of Nürnberg so on local time when it’s twelve noon in Nürnberg it’s already 13’ 20’’ past twelve in Prague.

Nassauer Haus Sundials

It was the invention of the railways and the telegraph that required and made possible the standardisation of time. A train timetable between two towns some distance apart requires that both towns are on the same clock, so to speak. This necessity led to the introduction of our system of standardised time zones, which are in reality purely convenient fictions, although based around some point, whose twelve-midday is actually local solar time such as Greenwich for the UK’s time zone. This system was discussed at the 1884 Washington longitude conference and finally accepted in the early twentieth century. When the clocks change, as was the case a couple of weeks ago in the EU and last week in North America it is purely an arbitrary redefining of the twelve-midday point within that particular time zone, largely divorced from local solar time. Some countries actually operate outside of the time zone system. Spain, which geographically is in the GMT time zone, is in the CET time zone one hour to the east.

Having set the scene, what I’m now about to say will probably convince many of my readers that I lack a sense of humour but when I see cartoons of the following type during the clock change period I don’t find them funny I just think of them as being horribly wrong. The prejudices of the expert.

Repositioning the Stones at Stonehenge today ready for the end of British Summer Time this weekend.

This is how they change the clocks for summertime at the prehistoric Avebury Stone Circle in England

What’s the problem? Firstly the Neolithic stone circles such as Stonehenge and Avebury are not and never have been clocks. They probably are constructed along solar alignments. I say probably because although they almost certainly are, because the builders are long dead and we possess no construction plans we can never be one hundred per cent certain. Stonehenge appears to have been constructed to align not with the summer solstice but with the winter solstice. These alignments are based not on time zones but on local solar time.

Stonehenge Winter
Solstice

This is actually a major problem for archeoastronomers searching for possible alignments in Neolithic and other ancient monuments because due to the precession of the equinox local solar time changes over time. From year to year the effect is almost undetectable but is noticeable over a period of several thousand years. What might have been well aligned in 3000 BCE is now 5000 years later a bit off.

All of this, means that the alignments of Neolithic monuments such as Stonehenge and Avebury, being based on local solar time, don’t actually change in anyway what so ever when we change our clocks arbitrarily in our artificial time zones. The so-called jokes display a fundamental misconception of the conventions that we apply to keep track of time.

One thing I did learn this year during the European time change is that The Maritime Museum in Greenwich has a modern sundial, the Dolphin Sundial that has changeable plates for GMT and BST. Cool! You can watch a video of somebody changing the plates here.