Hello all,
on Sat, 4 Jan 1997 I sent a message to this list with the results of a
little research on the definition of the Equation of Time.
I send it again (only partly) after having revised the results (extended
to new books..)
I've searched for EoT on many books on Sundials and on Astronomy and I've
reached this conclusion : THERE IS E GREAT CONFUSON ON THIS SUBJECT
Let be
AT the Local Apparent Time (often called True Time or Solar Time) (marked
by a Sundial)
MT the Mean Solar Time (marked by standard clocks)
The results of my search are the following :
BOOKS ON SUNDIALS
n. 2 Italian books on Sundial EoT=AT-MT in
some pages and EoT=MT-AT in others
n. 2 Italian books on Sundial EoT=AT-MT
n. 4 Italian book on Sundial EoT=MT-AT
Rohr (It.& Engl. Editions) EoT=MT-AT
is said that in Anglo-Saxon Countries is EoT=AT-MT
n. 1 English book on Sundial EoT=MT-AT
n. 1 English book on Sundial EoT=AT-MT
n. 2 Spanish book on Sundial EoT=AT-MT
n. 3 French book on Sundial EoT=MT-AT
n. 1 French book on Sundial EoT=AT-MT
Mayall-1994 - pag. 86
Waugh -1973 - pag.201
Duffett-Smith-1979 - pag.171 EoT=MT-AT
Jhon Davis -The British Sundial Society Glossary EoT=AT-MT
ASTRONOMY BOOKS
n. 6 handbook on Spherical .Astronomy EoT=AT-MT
Meeus-Astronomical .Algh. - pag. 171 EoT=AT-MT
In the book is said that in older and in French texts is EoT=MT-AT
H..Mills-Positional Astronomy EoT=AT-MT when
Sun transits Meridian
(Very good explanations )
Explanatory Sup. Astronomical Almanac (ESAA) pag. 5 - 74 EoT=MT-AT
Total : 30 books or documents
RESULTS:
- in Astronomy handbooks the definition EoT=AT-MT wins (8 to 1)
( ALSO IF THIS IS NOT IN ESAA !! )
This occurrence probably because in Astronomy EoT was used ( and also now is
used at sea) to determine the True (Apparent) Time when is known the Mean
Time (from Clock)
- in books on Sundials (21) , where EoT is used to find the Clock's
Time knowing the Sundial's one (AT), the definition EoT=MT-AT wins ( 13
to 8)
------------------
For the use of EoT with Sundials I suggest that is preferred the definition
EoT=MT-AT
so EoT is the correction to "add" to Sundial's time (AT) to obtain the Mean
Time (MT) : MT=AT+EoT
see : Duffett-Smith, ESAA, Ferrari, Mayall, Rohr, Savoie, Waugh
- the values to use for tables or graphs have to be calculated in every day
in the instant of the Meridian transit of the True Sun ( True Noon) ( for
European countries also at 12h UT)
In such a way the values in the same day in different places of the world
are not equal but the differences are small ( max. 20 sec.) (and therefore
the tables published in the books should not be used all over the world)
Gianni Ferrari
