I'm sure there are more than two of us and my Latin is pretty minimal.
The leap year date is a double-six (in French it is called "bissextile").
It is a second sixth date. The "sixth" means six days before the March
calends (beginning of March). Therefore there is a second February 24 so to
avoid the obvious confusion of having two dates with the same number, we
call the second one February 25 and increment the the rest of February
dates. s.
... or something like that.
Jack
-----Original Message-----
From: sundial [mailto:[email protected]] On Behalf Of Richard
Mallett
Sent: Thursday, August 28, 2014 5:28 PM
To: Frank King; Jos Kint
Cc: [email protected]
Subject: Re: leap year
On 28/08/2014 14:02, Frank King wrote:
> Dear Jos,
>
> You ask:
>
>> Has anyone an idea how to use a sundial for
>> calculating if the current year is a leap
>> year?
> Yes, been there, done that, got the T-shirt.
>
> The Noon Mark on the new London Stock Exchange
> in Paternoster Square more or less does what
> you want but once you understand it, you will
> probably decide to move on to a different
> project!
>
> The FIRST thing you should do is to get a
> feel for the leap year cycle and how it
> affects solar declination.
>
> Using some kind of solar calculator, such
> as:
>
> http://www.esrl.noaa.gov/gmd/grad/solcalc/
>
> you should do the following:
>
> Choose a date when the declination is
> changing quite rapidly. We are near
> the autumnal equinox and I suggest you
> choose 22 September 2014.
>
> Then choose 12:00:00 as the time. It
> really doesn't matter what time you
> choose, or what longitude, so long as
> you stick to the same time and place.
>
> If you are on the Greenwich Meridian
> then you will see that the declination
> is +0.23 degrees at 12h on 22 September
> this year.
>
> Then change the year to 2015, but don't
> change anything else. You will see the
> declination is +0.33 degrees.
>
> Continue like this for about 36 years.
> Here are the first few values:
>
> 2014 +0.23
> 2015 +0.33
> 2016 +0.04
> 2017 +0.13
> 2018 +0.22
> 2019 +0.32
> 2020 +0.02
> 2021 +0.12
> 2022 +0.21
> 2023 +0.31
> 2024 +0.01
>
> OK, do you see the pattern? Starting in a
> leap year, say 2016, the declination increases
> for the next three years and then jumps BACK to
> (but not quite to) where we started. In 2020
> we are not quite where we were in 2016.
>
> If you have a large enough nodus height you
> can track tiny changes in declination.
>
> If you mark out a giant analemma you can
> divide it up into individual days, 366
> days including 29 February.
>
> If you concentrate on 22 September you can
> see that the declination is lowest in a
> leap year and then gets higher for the
> next three years.
>
> All this will work without ambiguity for
> about 36 years. Then, the long-term
> drift [the Julian drift] results in you
> drifting into the previous day and the
> whole thing breaks down.
>
> I could get away with this on the new
> London Stock Exchange since the design
> life of the building is only 40 years
> and the plan will work most of the time.
>
> This scheme works, in theory, for almost
> any date but near the solstices the
> changes are too small to notice and you
> won't be able to detect them. Worse, on
> the day of a solstice, the declination
> changes direction and it's a mess!
>
> Fortunately, 29 February is at a time of
> year when the declination is changing
> quite fast so you can mark that in quite
> nicely.
>
> For completeness, Leap Year Day is really
> 24 February but there are probably only
> two other people on this list who agree
> with that assertion!
And they both speak Latin :-)
>
> [I distinguish here between the extra
> DAY and the extra DATE in a leap year.]
>
> Very best wishes,
>
> Frank
>
> Frank King
> Cambridge, U.K.
>
> ---------------------------------------------------
> https://lists.uni-koeln.de/mailman/listinfo/sundial
>
>
--
--
Richard Mallett
Eaton Bray, Dunstable
South Beds. UK
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