Precision
tall case clock with a 30-day movement signed by Ferdinand
Berthoud
Classic Louis XVI case signed by Balthazar Lieutaud
THE EQUATION OF TIME.
The solar day is the interval between successive passes
of the Sun across an observer's meridian and it is
not constant. Apparent
solar time is 14seconds less than mean time in mid-February
and 16seconds more in early November. The variation,
the 'equation of time', had been known for many years.
The
most impressive demonstration was perhaps that provided
by Cassini with the meridian line set out on the floor
the basilica of San Petronio in Bologna in 1652. The
line was illuminated by light from the Sun shining
through a hole in the roof above the line on the aisle
which was long enough for the variation in the length
of the day to be evident.
Although
it was well known by the middle of the 17th century
that the length of the day varied throughout the year,
the explanation was obscure. In 1672, Flamsteed argued
that the length of the solar day varied because the
angular velocity of the Sun around the Earth varied
with the position of the Earth in its orbit about
the Sun.
Flamsteed
could solve the equation of time because the clocks
he had at Greenwich, made by Thomas Tompion, were
the most reliable of their day. He could compare clock
time with solar time over a whole year, but it was
not until almost 1688 that he had sufficient data
to demonstrate that the clocks kept time with the
rotation of the Earth on its axis.
Flamsteed
used solar time corrected to mean time by the equation
of time because, as he said, it is more familiar to
more people. Sundials, which show apparent solar time,
sometimes carried tables of the equation of time,
so that mean solar time could be read off.
View
- Movement
Movement
- Maker's Signature
The
Hood
Cast Finial
in detail
Front - Quarter
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- Case Open View
- Dial
Front
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