John Carmichael wrote: <<My customers are thrilled when the find out that they can use their sundials at night using the moon, even if it is 45 minutes off. I just thought that I could narrow down the error with the E.O.T. (they understand the E.O.T.) and still keeps it simple for the layman.
I am still in doubt whether to use the E.O.T. Would you please reconsider your answers knowing that I'm trying to keep it simple for my sundial customers?>> Hi John, You are basically correct. I am afraid that in the replies you have had to your message we have got rather bogged down in the complexities of the moon's motion and undoubtedly if one wanted to be highly accurate you would have to consider these aspects. However, there are a few examples of moon dials - the most famous in the UK is the one at Queens College Cambridge (though I believe the Nuremburg Diptych dials carry them too). On the Queens College dial there is a table of 'hours correction' versus 'age of the moon, in days'. This is simply applied to the observed reading to convert moon time to apparent solar time. One then applies the EoT to get clock time. This process is not at all accurate and, except perhaps at the moment of full moon, is not likely to be something on which you would like to depend for an appointment! An interesting point is that the Queens College table is designed so that the correction is always added whereas I believe that on other moon dials may have to be added or subtracted. There is an interesting article in the BSS Bulletin (No.97.1 January 1997, p37) by Ing JTRC Schepman of the Netherlands entitled "How to determine time by Moonlight". In this article the author discusses these correction tables but also describes a simple analogue instrument to convert lunar indication to apparent solar time - it consists of concentric disks - and you may wish to refer to that article. Also, if you are able to get to Cambridge or can write to the Head Porter to buy a copy, you might wish to read the blurb in the Queens College leaflet on the dial - that too describes the way in which the simple table works. This dial was also discussed (albeit rather derogatorily I thought!) by the late Charles Aked in the BSS Bulletin 94.3 October 1994. There was another article in a BSS Bulletin (I think - though it may have been in the NASS Compendium, my memory is not clear! - about the ease (or otherwise!) by which a 'moon EoT' might be derived but I cannot find that at the moment. I suspect that these ideas only work on dials that use hour angle or azimuth. I am sure that one or other of these approaches would be suitable for your clients. But do stress the inherent inaccuracy! Patrick
