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Hi Gianni et al,
Re. the Longitude convention: note that conventions
are just that and, by definition, aren't right or wrong but accepted (or
not). When I wrote the BSS Glossary, I consciously avoided using the IAU
definition (0-360 degrees, Eastwards positive) in favour of the one almost
universally used by diallists and navigators for four centuries (-180 to +180
degrees, Westward positive). The astronomers can use what they like, but
they don't rule the world and the rest of us (like Steve Lelievre) use what is
convenient.
For the Second Edition of the Glossary (in preparation), I
have stuck to my original definition but have added a note that the IAU one is
different.
The other area where I (and Jean Meeus) are at odds with
the IAU is the sign of the EoT, but that's another can of
worms.........
The key thing is to be consistent in a document and to
make sure that the reader is given enough information to get the right
answer.
It's clear that we could do with a term for (EoT +
longitude correction). I didn't find a common one in my trawl through the
literature, though the NASS Dialist's Companion uses "Total Correction" which
seems reasonable. However, there are still chinks not defined, such as: is
the atmospheric refraction correction included?
I quite agree that a table of the mean EoT over a long
period (the lifetime of a sundial) is very useful as an addition to the exact
EoT for a particular date/time. The new Glossary will include the one
which you (Gianni) so kindly gave me.
Best regards to all,
John
--------------------------------------------------------------------------
----- Original Message -----
Sent: 01 May 2001 16:13
Subject: Solar Noon & Equation of
Time Calculator
Hi Piers ,
I have visited with a lot of interest your Solar
Noon Calculator on the web
at www.solar-noon.com and I have immediately
made some tests to compare your
values with those calculated from me and
published in an article in the
proceedings of our "X Seminario
di Gnomonica" ( X Italian Meeting on
Sundials - 2000)
Here are
some considerations of mine
1)
>From the comparison I have
immediately seen that your results are wrong
because of a banal error:
anywhere the value of the EoT has been or taken
with opposite sign or
subtracted instead that added.
An example : Long. =12d E and TZ of
Central Europe (central Meridian 15 d
East): longitude correction =+3d =
+12m
At 1/1/2001 the exact value of the Eot = - 3m41s an so :
-
Local Apparent Time (apparent solar time) = 12h
- Local mean time (local
mean solar time) = 12h 3m 41s
- Standard Time = 12 15m 41s (NASS Dialist
Companion gives the value, less
approximate, 12h 15m39s )
In
the Table calculated with the Solar Noon Calculator is written the value
=
12h08m48s, value that is obtained adding (instead that subtracting )
the
value of the Eot: 12h+12m+(-3m12s) = 12h08m48s
2)
The table
of the EoT NOT gives the values of the EoT but the Total
correction that it
is necessary to add to the Local Apparent Time to obtain
the Standard Time
: it is therefore the sum of the EoT + longitude
correction..
This value
is certainly very useful but, perhaps, it is necessary to give
some
explanations and it is opportune not to call it Eot
Moreover in this
way the table with the values of noon is useless because
these values are
equal to those of the EoT + 12h
The definition :
"Equation of Time
displays the difference between solar time and the
standard times where you
are"
(note at the foot of the page) it is not correct.
Davis' Sundials
Glossary gives the following:
Equation of Time: the time difference between
Local Apparent Time (apparent
solar time) and mean solar time at the same
location (NOT Standard Time).
Its value varies between extremes of about
+14 minutes in February and -16
minutes in October.
3)
Checking
only for the date 1/1/2001 I have found that the error between the
exact
value and the mean value of the EoT = 3m41s -3m12s=29 sec: almost
the
double of the maximum error (in the Note).
With NASS Dialist
Companion we obtain the less approximate value Eot =
3m37s: also with
this value the error would be of 25s
Does the greater error depend on the
fact that the Eot has been calculated
at 0h (UT) instead that at 12h
(UT)
In fact the Eot can also change till 20s a day.
4)
In my
opinion it is very useful, for instance in the construction of
sundials
with mean time, to have a table of the mean values of the Eot (as
of the
mean declination of the Sun), while the table that gives the
mean
local
noon (mean on 4 years) can be used in a wrong
way.
In fact these tables, that should be used only for the search of the
mean
time from the apparent solar time given by a sundial, could be
considered
right also in the search of the declination of the walls.
In
this case it is better to use the true value of the EqT calculated
with
programs as NASS Diallist Companion
Perhaps a note could clarify
the thing.
5)
As in almost all Web sites, also you take as
positive the Longitudes for
places West of
Greenwich.
Despite the opinion of the known astronomer J. Meeus, with which
also Davis
agrees in his Sundial Glossary, even if a secular tradition
justifies this
definition, it is NOT correct.
The Explanatory Supplement
to the Astronomic Almanac (USNO 1992) at page 203
affirms:
"The
geocentric longitude is defined by the angle between the reference
(or
zero) meridian and the meridian of point p, measured EASTWARD around
the
Earth from 0 to 360 deg (IAU, 1983, p.47) "
Perhaps
it would be opportune, at least in new programs and in new
Web sites , to
use this correct definition
Best wishes
Gianni
Ferrari
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