On Monday, April 25, 2022 at 8:28:28 AM UTC-6,
kellehe...@gmail.com wrote:
> The second is not a unit of time, it is a unit of timekeeping
It is true that changing to the use of atomic time as the basis for our current official
system of timekeeping has led to inconveniences like leap seconds. Historically,
the second was derived from the solar day.
Why didn't we keep things that way? Why did scientists and engineers insist on tying
the second to things like a particular spectral line of a particular substance, so that the
second and the solar day can actually go out of sync?
I can't blame you for *asking that question*, but I do blame you for wilfully rejecting the
answer.
Tidal forces, caused by the gravity of the Sun and Moon, cause the Earth's rotation to
slow down by a minute amount over the centuries. For purposes of science and engineering,
a second that changes in length is intolerable. It must be possible to state physical laws in
an accurate fashion that is not complicated by correction factors taking the changed rotation
of the Earth into account.
Having a fixed, unalterable standard of time makes the mathematical formulation of physical
laws simpler. From that foundation, we can predict how electronic devices work, how
mechanical devices work, and we can predict the motions of the Earth, the Moon, and the other
planets.
Before there were "leap-seconds", the need for an unchanging standard of time was
recognized specifically in connection with the movements of the planets. Simon Newcomb
used such a scale of time in his _Tables of the Sun_ in 1895, and it became the basis
for the official time standard of Ephemeris Time in 1952, when the discrepancies became
noticeable.
The SI second, a unit of time on the basis of which units like the ohm, the farad, and the henry
are defined, was based on the length of a second of Ephemeris Time, which is why we have
leap seconds now that we no longer use a second that changes its length every year within our
civil time.
John Savard