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@david pierce wrote:

The science of time is somewhat complex and interesting. Because of their position in the world at that time, England had to rely on the shipping industry for their survival. The shipping industry in turn had to rely on accurate ways of keeping time so it is no accident that the line of longitude starts in Greenwich England.
The time is taken by looking at a distant star through a telescope. The actual time standard is the rotation of the earth which is accurate to one second every 100,000 years. The telescope has a retical with a grid drawn on it marked off in periods of time. As the earth rotates, the star appears to move across the grid, so by counting the grid lines, the astronomer can calculate periods of time. This is called sidereal time and is not particularly useful in this form. This is because the rotation of the earth has no particular relationship to the earth’s rotation around the sun. This has to be averaged out with a statistical mean to be of any pratical use hence the term Greenwich Mean Time. This is how the world time standard was kept until the invention of the Atomic Clock.
When Lewis & Clark surveyed the Louisiana pruchase, they carried a pendulum clock and a telescope. When they wanted to map out a surveying position, they would look through the telescope at one of the major planets (probably Jupiter) and wait for one of its moons to come out from behind the planet. At this point they would release the pendulum which would calibrete their clock. The positions of the moons were recorded on charts they carried so they were able to make accurate position markings.


Years ago I took a basic course in surveying in which we dealt with sidereal time and some ofthe celestial time calculations. I often find it interesting that the Western Church still uses these these to set the date of Easter – well, they use algorithims derived from celestial movements something like this:

Easter occurs on the first Sunday after the first full moon after the Vernal Equinox.

It good until the planets get out of alignment to suit the algorithm… < 4100 ad!


tmac1956Reply To: Horology 101