Chris Lusby Taylor gives an excellent rundown of portable dials, but says
the following about ring dials:
"The ring dial's main attraction is its
extreme portability and ruggedness. As an
accurate timepiece it is very limited."
I disagree! I have a ~12-cm-diam. brass reproduction of an 18th c.
ring dial that reads to ~5-min. accuracy when set and read correctly. With
a faint sun or in windy conditions, I can only get 10-min accuracy. I carry
it around the world with me and have used it even on airplanes. I've used
it at latitudes from 20 deg N (Hawaii) to 60 deg N (Stockholm) (where in
early June I was able to get a reading at ~3:30 a.m.) I've also used it in
Australia, but in the S. Hemisphere there are *three* ways that it must be
used differently -- one has to hang it "upside-down" from the usual (for N.
Hemisphere chauvinists), the sliding aperture that is set for the date
(solar declination) must be read in the opposite sense, *and* of course the
hour values increase in the opposite sense (*counter*-clockwise - another
bit of Northern chauvinism).
One disadvantage of this type of dial is that for a few days near
each equinox, the equatorial ring blocks out the spot of light, so it
can't be used at all! But on the whole it's a great companion for any
gnomonicist to have folded up in his or her pocket; when someone asks you
the time, you just pull it out!
******************************************************************
Prof. Woodruff T. Sullivan, III [EMAIL PROTECTED]
Dept. of Astronomy, Box 351580 tel. 206-543-7773
Univ. of Washington fax 206-685-0403
Seattle, WA 98195 USA
