Sundialers,
A convenient way to mark the center of the sun's image made in a reflected ceiling dial is as follows. Punch a hole approximately 1 mm in diameter in the center of a 3" x 5" file card. Hold the card in the beam of reflected sunlight a few inches from the ceiling. Adjust the card perpendicular to the beam to get a circular image of the sun with the pinhole centered in the image. The light coming through the pinhole will indicate very closely the desired point on the ceiling. Mark this point and label it with the appropriate time and/or date. The accuracy of this method depends on how accurately you can hold the pinhole in the center of the circulate image of the sun. I can get consistent results within about 10 seconds.
If one thinks of the inverse of the above procedure: that is, forming a pinhole image of a nodus by holding a 1 mm diameter hole in the fuzzy penumbra of its shadow, one will see a pinhole image of the nodus, and, by centering the nodus's image in the circular image of the sun, very precisely mark the center of the fuzzy shadow of the nodus. This is one very useful interpretation of what has often been referred to as a Shadow Sharpener on this list.
Bill
41d 58m N, 70d 41m W
- RE: Marking Ceiling Dials, Addenda Wuwalton
