Dear Walter and Membership, Accuracy again.
Increasing the surface movement of the shadow or spot of light by the means of optical levers allows very fine time measurements. In our mind's eye we can fix a small mirror so that it reflects a small part of the sun's image far out into space. In seconds the reflected image can move from star to star. At that vast distance the surface rate of movement of the reflection is thousands of lightyears per second! In general when we magnify the sun's image size on a surface we increase the rate of movement of the image on that surface. If we choose to only reflect a very small portion of that image, it still moves very fast, being a sensitive indicator of the angle of the sun's rays. Thus, if the mirror were about 300 meters away from the surface, the spot reflection would move about 2.2 centimeters per second if the path of the refection were in the equatorial plane. Of course that distance could be folded by reflecting from optically flat first surface mirrors so that a smaller device could measure small increments of time. The larger sundials use an optical lever with it's fulcrum at the tip of the gnomon, thus increasing the rate of surface movement of the shadow. A small opening, acting as a pinhole lens, can focus a spot of light and sharpen the image. Enjoy the Light! Edley McKnight [43.126N 123.357W]
