Wednesday, March 28, 2018

Among many other, terribly important things

Here’s a question that anyone should be able to answer. What happened in Rome on 10 October 1582?

While you try to work it out, I’m going to tell you a little story. It’s about the solar clock on the floor of Milan’s cathedral, which my father showed me when I was a boy. It’s a thin bronze strip of that runs through the floor of the left nave and climbs up the lower section of one of the walls, lined up with a hole on the façade of the church. Every day at midday, the sun shines through the hole and crosses the strip. This indicates both the time of the day – midday, evidently – and the day of the year, as measured by the point of the strip that is illuminated.

These kinds of clocks work better if the hole that the sun shines through (or gnomon) is positioned quite high, which is why historically in Europe they were often located inside cathedrals. The gnomon of the cathedral in Milan – which is a very late one (1786) – is placed 24 metres up the façade. Whereas the gnomon of the solar clock in Santa Maria del Fiore, built in 1468, is the highest in the world, being drilled into the very drum of Brunelleschi’s famous cupola, 90 metres above the cathedral’s floor.

Would it help if I told you what happened in Rome on 10 October 1582 is also what happened in Stockholm on 25 February 1753 and in a small town in Russia on 16 February 1923? I think it probably would.

It may seem counter-intuitive to claim that the solar clock of the cathedral in Milan tells time, since the only time it tells is midday, but this – and not the calendar part – was in fact its primary function: a law enacted on the year the clock was installed required Lombardy to adopt so-called ‘French time’, which marked the beginning of each day as the ‘true’ solar noon, instead of the traditional bell chime for the vesper prayers, shortly after sunset.

The solar clock in Santa Maria del Fiore, by contrast, may initially strike us as rather less useful. For one thing, the sun beam hits the floor of the cathedral for a small portion of the year, from late May to late July, again on either side of the true noon.

I said the hole was drilled into the cupola, before, but it’s not quite true. It sits on a small bronze plate inside one of the windows, and it has a diameter of about two centimetres. From there, the beam – which by the time it reaches the floor has become a fairly sizeable circle – hits another graduated strip, and on certain days superimposes itself on one of two finely drawn circles.

What happens if you track the position of the sun from late May to late July is that you’ll be able to time with absolute precision the summer solstice (or the winter one, if you’re in the Southern hemisphere. But let’s stick with Florence.) A Florentine living in those times wouldn’t have cared one little bit to know when midday was, but they would have cared a lot about the timing of the solstice. If you know when the summer solstice is, then you know when the 21st of June is. If you know when the 21st of June is, you can count the days backwards and forwards for the entire, ‘true’ solar calendar. And if you can mark out the solar calendar, you can establish the correct date in which to celebrate Easter.

I used to find it quite curious, that we celebrate the birth of Jesus on a regular fixed birthday, whereas his death and resurrection fall on days that change from year to year. This is obviously a relic of a different, ancient way of measuring time and anniversaries, from a time when the solar and lunar calendars were intertwined. As decreed at the council of Nicea of 325 CE (when it became decoupled from its Jewish counterpart), the Christian Easter falls on the Sunday following the first full moon of Spring. To know when Spring begins, you need to know when the Spring equinox falls. And for that you need a reliable solar calendar.

You all know what happened here: the calendar used in Christian Europe had been established by Julius Caesar, who did a reasonably good job of accounting for the fact that the solar year does not last a round number of days. But even so, over the span of fifteen centuries the Julian calendar had started lagging quite badly, which led to Christians celebrating their holiest of days at a noticeably wrong time of the year. Hence the need to start measuring the ‘true’ position of the sun via devices such as the clock in Santa Maria del Fiore, and to create a new, more precise calendar. Enter pope Gregory XIII, who asked his scientists to study the problem and published the solution in a decree (or ‘bull’) known (as per the custom) by its opening words, in this case ‘Inter gravissimas’, or ‘Among the very serious matters…’.

The sentence continues: ‘…entrusted upon our pastoral office, not the least is to see to it that the tasks which the holy Council of Trent reserved to the Apostolic See are conducted, with God's help, to a desirable conclusion.’ One of those tasks concerned precisely ‘the annual recurrence of Easter and the feasts that depend on it, to be measured by the movement of the sun and moon’.

So, to answer our initial question: nothing. Nothing happened in Rome on 10 October 1582, because the day never existed. In Rome and in all the territories under the direct control of the Pope, Thursday, 4 October 1582 was followed by Friday, 15 October 1582, in order to make up for the time lost by the Julian calendar. Interestingly, the Bull also specifies how the celebration for the saints honoured in those missing ten days were to be rearranged for that particular year, as well as setting new rules for the 28-year cycle of the so-called ‘Sunday letters’ which helped to locate Easter and every other ‘moveable feast’ (I didn’t know the origin of that expression until I looked it up just now).

This package – the rules for the new Gregorian calendar, plus the ‘martyrology’, or list of martyrs, which I assume means in this case which saints were celebrated on which days – was to be printed exclusively under license by the Holy See, under the penance of a fine of 100 gold ducats plus forfeiture of the publications (in territories controlled by the Pope) or excommunication (in the rest of the world). Given the difficulty in delivering the Bull ‘to all Christian places in the world’, it was decreed that it would be affixed to the doors of the Basilica of Saint Peter’s in Rome and displayed in Campo de’ Fiori, the square where 18 years later Pope Clement VIII would have Giordano Bruno burnt alive at the stake.

The new calendar was adopted almost immediately by Catholic countries and territories, and with various delays by Protestant and Orthodox countries (as hinted above, for instance, Sweden made the switch in 1753, the Soviet Union in 1923). The history of how the Gregorian calendar spread throughout nations that had never been touched by Christianity and became truly global – along with the adoption of 24 time zones starting at the Prime Meridian in Greenwich, London – was driven by very different factors and is told in a book by Vanessa Ogle entitled The Global Transformation of Time, which I warmly recommend.

As for the solar clocks in those old churches, they tell time from a different era, as if the sun that shines through those holes and onto those floors were still trapped in the Renaissance, or the Baroque period, reminding us that human time, too, has a history.