>J Tallman wrote: "Yesterday evening I was driving south on the interstate and saw a fabulous sunset where the sun was incredibly magnified...even before it got right down on the horizon. There were thin clouds that cut the light enough for me to observe the disk, and it sure did look wide. How would an apparently larger solar disk like that affect the gnomon/distance ratio and the shadow width characteristics on a tubular polar style, say, with the resultant shadow landing on the inside of an equinoctial ring where the receiving surface is parallel to the gnomon? The dark umbral part of the shadow would be diminished, right? I wonder if that effect should be factored in when choosing the ratio to use in determining the optimum diameter of a tubular gnomon"<
The vertical squashing effect that you saw (which, whilst genuine, doesn't involve any form of magnification by the way - that is an optical illusion) is due to refraction which is virtually incalculable when the sun is near the horizon since its effect is often more due to the atmospheric cooling and the geometry of any grazing ray in the terrain which is only yards and miles near to where you are as an observer. Indeed, its effects on the rising/setting appearance of the moon at night can be far different to that seen a little time before, at the time of sunset, for the same reason. Any of the series of books on 'Megalithic Lunar Observatories' by Professor Thom and his son go into this effect in considerable detail - much more than I have seen elsewhere - whatever you might think of their ultimate conclusions about the stone circles of Britain and Brittany. Very well worth a read! Basically, the answer to your question is that it actually makes little difference since you just cannot rely on any sundial's reading at or near the time of sunset since there is little or no reproducibility in the effective level of refraction from one day to the next . Indeed that is the case for any dial when used for time telling below a solar altitude of 10 degs - but, as with any dial, a cylindrical gnomon makes life easier! If you are designing a dial with a cylindrical gnomon, just ignore refraction is my advice!! Patrick --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
