Hello David,
See BSS Bulletin 98.1, page 30, _Reflecting Sundials_ by C.M. Lowne.
The article includes a section, The Cycloid as a Reflector, mentioning
that the cusps (of the reflected caustic) are neither evenly-spaced with
time nor did they lie on a straight line. However, the edge of the
caustic can be used to mark an evenly spaced set of hour-lines. The
article, by the way, does not address reflecting ceiling dials like we
have been discussing. A caustic is a pattern of reflected light.
Regards,
Luke Coletti
David Pratten wrote:
>
> The BSS sundial glossary has the following entry under "Types of Dials":
>
> "reflecting ~: these dials have no gnomon, but reflect sunlight by means
> of a semi-cylindrical mirror, set with the axis of the mirror parallel
> to the polar axis. The mirror reflects the light to form a caustic curve
> amongst the hour points. For the special case of the mirror having a
> cycloid shape, the hour points are equally spaced. Note that the term
> reflecting dial may also be used for ceiling dials."
>
> What is the typical dialface for a 'reflecting dial'? Are the comments
> above about semi-cylindrical mirrors apply equally to ceiling and wall
> dials?
>
> If a polar ceiling dial has an inclination of phi/2 what does having a
> mirror inclined at phi do?
>
> What is a caustic curve?
>
> David
