** I read about this list on the "Sundials on the Internet" page by the
BSS. **
Hello Frank, and all,
As for the _name_, I think "equation" can mean something done, or
needed, to equate a thing to something else.
As Fer said, ' Can it be a translation of "Equatio Temporis"? '
Personally I would rather suspect it was the other way 'round, the Latin
expression being coined for international traffic.
As for the sign, that is an interesting point. It used to be the way you
would like to see it. Before the 1930's, you took the dial reading,
added the Equation, and wound up with clock time. The equation was
positive in February and negative in November.
By 1948, W.L.Kennon described the Equation the way we use it now. You
take clock time, add the variable Equation, and get the variable dial
time.
In 1940, dr. Minnaert, in his excellent "Natuurkunde van 't Vrije Veld"
("Open Air Physics"), used the old sign for the equation. And as late
as 1953, P.Terpstra still did; really by then he should have known
better..
Towards the end of the 1930's there was one periodical (the name of
which escapes me momentarily) that carried two articles each using the
other sign of the Equation.
There is something to say for the modern standard. If you look at the
graph, what you see is Mean Time having a zero difference from day to
day, every day having equally many star seconds in it; and Apparant
(Dial) time meandering, slow and fast, about it. This is of course
really the way it works.
You could call the modern way the "Error curve" of the sundial, except
that this is not a very nice name for it.
The old way, then, could be called the "Correction curve" for the
sundial.
One last remark, of theoretical value only: there is a very slight
difference between the old and the new method, even if we reverse the
sign of one of them.
Example: the "old" way, we read, say, 9 AM on the dial, apply the
correction, say +10 minutes, and get 9:10 AM clock time.
If we reverse this to get the new method, we see that the Equation is
-10 minutes not for 9 AM clock time, but for 9:10 AM clock time.
Granted, the difference will be very very slight- in fact we happily use
the same Equation for the entire day, and would hardly compute fresh
Equations every 10 minutes. Still, a conceptual difference is there.
Happy dialling!
Rudolf
