apparent time and mean time

The use of a 24-hour day, with 60 minutes (minutiae prima) in an hour and 60 seconds (minutiae secundae) to the minute does not require that all hours be the same length. In medieval times the period of darkness was divided into 12 equal hours, and the period of light also divided into 12 equal hours. This is time by sundial, temporal hours.

A night hour in the winter was much longer than a night hour in the summer, and so with the minute and second–which few or none could measure in any case. For people who can only work by the light of the sun there is much to be said for this system, quite apart from its ease of measurement. But there is another problem, as the ancient Greeks realized.

Because the Earth's orbit is not circular, and because the Earth's axis is not perpendicular to the plane of its orbit (it's “tilted” about 23½°), the sun appears to move across the sky more quickly during some parts of the year than at others. Thus some days (day in the sense of day plus night) are longer than others, and hence some hours in a year will be longer than others even if the day is divided into 24 equal hours. Time kept by dividing the day into 24 equal hours is called apparent solar time.

An alternative is to average the lengths of all the days in a year to obtain a mean solar day, and divide that day into 24 equal mean solar hours. Time thus found is called mean solar time. The difference in minutes between apparent solar time and mean solar time is given by the equation of time, shown in the chart below.

graph of minutes against day of the year

Water clocks, which in ancient and medieval times were the most common type of clock other than candles, can easily be built to tell apparent time. Instead of a single mark indicating a hour's depth, the side of the container receiving the water could be marked with a series of vertical lines for various dates in the year. It is much more difficult to make a mechanical clock tell hours of varying lengths throughout the year, although such clocks, called equation clocks, have been built.

As mechanical clocks became more accurate and available, mean time became more important. Technical advances such as the invention of the pendulum clock by Huygens in 1657 made clocks and watches more accurate. The industrial revolution made them available to more people, a process still continuing at the opening of the 21st century. By the end of the 18th century, mean time was replacing apparent time in Europe.ยน It was adopted in 1780 in Geneva, in 1792 in England, in 1810 in Berlin, and in 1816 in Paris.

1. Derek Howse.
Greenwich Time and the Discovery of the Longitude.
Oxford University Press, 1980.

Page 82.

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