John, Isn't the problem down there in Tuscon that the water would evaporate before any algae has a chance to grow?
As Gino pointed out, if one creates a dial by calculation, there may be a problem transferring it to a room by measuring off, because walls and ceilings aren't plumb true or may be bowed. Even projecting the dial using a transparent acetate slide using a laser pointer won't work if the slide is not true vertical or not at precisely the declination used in calculation. He's right as far as I can see, so simply plotting the actual position of a reflection at known times may be the best way after all. With the latter approach, it doesn't matter if the mirror is not level. Is that not so? Steve
