Ceiling Sundial

[J Lynes]

Here's a simpler proposal. Transfer the declination lines and hour lines of a horizontal sundial onto a transparent sheet. Mark a small circle on the centre of the mirror. Support the horizontal transparent sheet, rotated from north to south, with its nodus vertically above the centre of the circle, at a distance equal to the height of the transparent sundial's gnomon. Project a laser beam through the transparent sheet onto the centre of the circle.  Make sure the beam passes through the sundial scale at a point corresponding to some chosen time and date. The reflected spot on the ceiling is the appropriate point on the ceiling sundial. Repeat for other dates and times. John Lynes