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coralline algae wrote:

> The thing I can't quite understand though, is what standard the
> clockmakers used to decide on the length of a second, in building
> their timepieces.
> Here is my first guess, the solar day length on the equinoxes along
> with the suns meridian transit set the day length standard.
> Then build a gear train dividing the day into 24 by 60 by 60 parts.
> Recalling that Harrison used sidereal time to calibrate his clocks, it
> must have been well known by his time ( or much earlier ?) The "exact"
> difference in time
> between the solar day and the sidereal day so the clock needed to run
> slower by about 4 minutes. Wikipedia says 86164.1 seconds.  If the
> exact difference in time was well known and transit times could be
> measured to tight accuracy this would seem to be the best option.

In fact, measuring the sidereal day is the *only* option. The mean solar
day is a computational fiction that is not directly measurable. It
results from the combination of earth rotation and annual solar motion,
the latter being non-uniform. The true solar day varies throughout the
year by a few seconds. At the equinoxes, the difference between true and
mean solar day is particularly large because the effect of the
inclination of the ecliptic is at its maximum.

Herbert Prinz

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