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.

Type of hour	Duration in days	Duration in minutes
Lunar hour	Lunar day / 24	57 min. 58.0925 seg.
Sidereal hour	Sidereal day / 24	59 min. 50.169 seg.
Mean solar hour	Solar day / 24	60 minutes
True solar hour	Variable	Variable
Official hour	Solar day / 24	60 minutes

As for the official hour, it has the same duration as the mean solar hour and differs from it by a fixed number of minutes for each point on the Earth's surface, depending on its geographical longitude.

Suppose that, at a given time, the sun is over the Greenwich meridian. For simplicity, we will assume that it is one of the four days of the year when the passage of the sun through the meridian takes place exactly at twelve noon. Just then, on the antipodes of Greenwich it is midnight. At 90ºW (in New Orleans, for example) it will be dawn, six in the morning. At 90ºE (in Dhaka, the capital of Bangladesh) it will be six in the afternoon. As the apparent movement of the sun makes it make a complete revolution (360º) every 24 hours, in one hour it advances 15º (360/24) and in four minutes it travels one degree. Knowing the geographic longitude of a place, it is easy to calculate its time difference with the Greenwich meridian. Thus, the longitude of New York is 73º59'39"W. As New York is west of Greenwich, when it is 12 noon in Greenwich, in New York it will be 7 hours, 4 minutes, 1.4 seconds in the morning.

In practice, time differences are not strictly applied. If this were done, each point in a country on a different meridian would have to have a different hour. This was possible a few centuries ago, when each town and village had its local time (measured by the church clock). But since the 19th century, with the railroad, it was necessary to establish an official time valid for an entire country, or even for several countries. Otherwise, a person would miss the train if its arrival was announced for 16:30 (time of the railway company) and the local time was 16:15.

Time Zones

In theory, the distribution of time zones should be simple. The surface of the Earth would be divided into 24 regions 15 degrees wide (geographic longitude), each of which would have a different time. However, this does not happen for a number of reasons:

·         Each country usually sets a unique official time, applicable to the entire country, ignoring differences due to geographic longitude. Bordering countries adopt the time of their neighbors to facilitate exchanges by rail, as Spain and France have done by following Central European time rather than Greenwich Mean Time, which Portugal does follow.

·         When a country is very large in an east-west direction, so that the time differences are appreciable, several official hours are used applicable to different regions. This occurs, for example, in the continental United States, with four different time zones, while Russia, the largest country in the world, has eleven.

·         A country may have dependencies far from the main territory, islands or regions where it is convenient to follow their own official hour. This happens in the United States with Alaska and the Hawaiian Islands, and in Spain with the Canary Islands.

·         Also, some countries change the official time along the year for practical reasons. Thus, in summer the hour is advanced to save energy.

·         Finally, to complicate matters further, some countries have adopted fractional official times. Thus, when it is 12 noon in Greenwich, in India it is 17:30; in Iran, 15:30; in Korea, 20:30; and in Liberia 11:16 in the morning.

The combination of these irregularities has made the map of official hours almost as intricate and complex as the political map of the world.

The same post in Spanish

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Manuel Alfonseca

Happy Christmas!