The 24th hour

Melting Clock
Sculpture by Dalí

I have tweaked the title of the well-known novel by the Romanian writer Constantin Virgil Gheorghiu (The 25th hour), because in this post I am going to talk about the hours. As with the days, which we talked about in the previous post, there are different types of hours.

Each type of time is defined by dividing the length of the corresponding day by 24. Thus there are as many types of hours as there are days: lunar hour, sidereal hour and mean solar hour. But in addition to these three, there are two more: the true solar time and the official time (see the attached table). The first is a consequence of the fact that the true solar day differs from the mean solar day by up to a quarter of an hour, approximately. For that reason, when ship captains wanted to calculate the geographical longitude of their position by observing the moment when the sun passed through the meridian, they had to apply correction tables based on the date. If we divide the length of each true solar day by 24, we will get the true solar hour, which of course is equal to 60 minutes just a few times a year.

The Clock in Strasbourg Cathedral

Clock of Strasbourg Cathedral

One of the most surprising instruments used to measure time is the clock of Strasbourg Cathedral, which contains a mechanical computer, a marvel made up of gears and cogwheels, a pinnacle of the instrumentation of that time, which could be considered comparable to Charles Babbage's Analytical Engine, except that it is not a programmable computing device, but a computing machine built for the specific purpose of calculating time. More details can be found in this article in the journal The Sciences of the New York Academy of Sciences.

The clock is inside the building, rather than at a tower, like those of many other cathedrals. It has a long history, as it dates back since the 14th century, although it was completely rebuilt in the 16th. By the end of the 18th century it stopped working. Legend says that, at the beginning of the 19th century, an orderly who was showing the cathedral to a group of visitors mentioned that the clock had not been working for a long time. Then a boy who was part of the group exclaimed: I will fix it! Forty years later, he did. That child would have been Jean-Baptiste Schwilgué, who remodeled the clock around 1840.