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| 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.
