[ATMoB-discuss] Longitude Dial

[George Roberts]

This "longitude dial" is just a sundial.  I've spent alot of time
looking at sundial designs and building some in the past and
the shown design is not very accurate because of the equation
of time.  There are more accurate sundials that take into
account the equation of time and these also give you the
date at the same time as a bonus.

There is some software called "shadows".  It works very well 
but the free version isn't as good as it used to be - you have 
to pay a bit now:
http://perso.orange.fr/blateyron/sundials/shadowspro/gb/index.html

This software will let you plot out extremely good and exteremely
accurate sundials customized for one's longitude and latitude.

Here are pictures of what I'm talking about:
http://www.doctor-roy.com/Sundials/sundial2.jpg 
http://gr5.org/sd1.png 
http://gr5.org/sd2.png

Notice that there is a half or full analemma for each hour (or even each half hour).
This is all controllable in the software (but you must pay for this feature).
Notice that the first of the month in sd1 and sd2 are circles and every 5 days
have a tick mark.  Having analemmic hour lines gives you two or three minute
accuracy in a sundial (instead of 15 minutes).  The half analemma hour lines 
only work 6 months of the year.  The months can be labelled over on the side 
with horizontal curves showing where each month starts.  Usually you can see 
the date on these dials with 2 or 3 days accuracy.


Also it might be nice to do an analemmatic sundial.  This is a large sundial
that needs about 15 feet square.  I have a full size template made out
of 6 mil plastic that the club can use to make one if they want.  Here's
a picture of one I made:

http://gr5.org/dsc02554sm.jpg

The cool thing about this type of sundial is that the person using it is
physically the gnomon (the part that casts the shadow).  You adjust
where you stand depending on the month (february is shown).  In
the picture you can see the time was 3:45pm (use white numbers
when standing on white months).  This type of sundial is good for
education.  It's very accurate except like the "longitude dial" doesn't
account for the "equation of time" which can be as much as plus or
minus 15 minutes.

These sundials aren't pretty but can be made pretty with a little effort
and planning.

- George Roberts
http://gr5.org