Newsgroups: rec.org.sca
Date: Tue, 17 Nov 1992 01:41:32 -0500 
From: Kevin William Ryan <kr0u+@andrew.cmu.edu>
Subject: Re: "period" wristwatches

    As to pocket sundials:

    Any decent library will have some books on sundials. I found over 20
references with one search in the electronic catalogs at the Carnegie
Library in Pittsburgh. Give it a shot - you should be able to find something.

    Ring dials: basically a napkin ring with a string attached and a small hole
drilled in one side. The hole projects a spot of sunlight onto the inside of
the ring. You need different scales for different months, as the height of
the sun changes with the seasons, but lining up the spot of light with the
proper month should show how many hours from noon it is, and if you can
remember if it's morning or afternoon you have the time. A decent book on
sundials should give enough information to calculate the scale.

    An alternate is the cylindrical 'shepherds dial'. This is a cylinder with
a projecting pointer (gnomen) extending horizontally from the top. As, again,
the height of the sun at a particular time of day varies with the season,
the cylinder is marked with scales indexed by date. Rotate the cylinder until
the proper week or month is under the gnomen, hold it by the string from the
top so that the gnomen is turned toward the sun, and the time of day will
be the curve pointed to by the shadow of the gnomen. Incidentally, the curves
of the hour lines on this type of dial look like upside down gaussian curves
wrapped around the barrel of the cylinder.
    The card style sundials are just that. A sight is rigged on one edge of
the card (pinhole and a spot to project upon), a string hangs from near the
pinhole, and there are a set of curves: hours march from projection spot to
the opposite corner, and time of year from the pinhole to its opposite
corner in lines of equal radius from the string attachment point. Slide
a bead along the string to the distance of the appropriate month, sight it
upon the sun, and the bead will hang on the appropriate hour.

    There are many more: these are the simplest I know of. I don't have the
books with me (they're library books, ya know), and the calculations are
moderately complicated. More complex pocket sundials include ones with two
different types of dial, one of which has a movable gnomen; you adjust the
gnomen until the two dials agree, and then you know what time it is as
well as what direction is true north. Others have a simple sundial of some
kind and a compass so that you can point it properly. However, these are
subject to the difference between magnetic and true north, and a table of
corrections is probably a good idea.

    To go a little further: note that a sundial will give you true sun time.
This is _not_ identical with standard time, which is true only every 15
degrees of longitude. The Pennsic site, for example, will show a sun time
roughly 14 minutes earlier than Eastern Standard Time, and throwing in
Daylight Savings time makes it 46 minutes off in the other direction. Add
in about 5 minutes for the 'equation of time', which tells how the lead
and lag of the sun affects local sun time, and it can get rather complicated.
My (potential) plans for Pennsic sundials include making the scale correct
for that latitude, longitude, and time of year with daylight savings time,
although I might also consider one with the appropriate corrections as
well. The card type dials lend themselves to that sort of thing, for
example.

    It's not as bad as it seems. The lead/lag of the sun amounts to about
15 minutes at maximum, (that's the 'equation of time', which will show up
in every book on sundials, and is associated with that figure eight you
see on a lot of the better globes for the noontime sun) longitude will
throw things off by 3 minutes per hundred miles east/west at my latitude,
with perhaps an equal amount of offset for travelling nort/south. A good
sundial you make for your area will be reasonably accurate for quite
some distance from home.

    And then there are the heliochronometers, which until recently (the
introduction of atomic clocks) used to set railroads and national timekeeping:
a wonderful sundial with the corrections for the 'equation of time',
daylights savings, etc., adjustable with verniers. Accurate to within
seconds, quite literally. Unfortunately, I don't have the machining tools
to make one...

    Hope this is helpful. Sundials are _extremely_ period. They date back
to the Egyptians, and all of the portable ones I have described were
extant in period. This discussion has been most fun, and my enthusiasm
for making up a few is renewed. You might see me pushing dials at Pennsic.

                                                   kwr
                                     aka Donnallain o'r Galaru Glais

Internet: kevin.ryan@cmu.edu