Dear David, Living in the mountains most of my life, I early on thought sunset and sunrise was when the sky turned colors other than blue. I've never met anyone that has observed the sun actually rising or setting at the published times for such occurances, except for one astronomer whose job was providing such figures for publication. Atmospheric refraction effects can vary considerably within a ten foot section along a roadway. ( varying over 40 seconds in time due to an atmospheric lens created by the off shore flow of cooler air exiting from a canyon.) In desert places the rapidly changing temperatures at sunrise can cause the shadows to appear to jump around like trying to tell time by a sundial 10 feet from a bonfire. Often quoted and computer derived sunrise/sunset times are for a topospheric calculation that assumes the earth to be a smooth sphere with no atmosphere at all! Average refraction figures that seem to be published are taken from observed data at observatories which are of course chosen in the first place because of their good viewing conditions and are at elevations missing most of the really neat atmospheric refraction phenomena.
Observatories measure these phenomena versus the topospheric calculated values by the minute, over many years and fit them to a complex integro/differential equation involving weather conditions, atmospheric pressure, time of the year, etc. which kind of works for them most of the time. If your sundial site is blessed with repetative, predictable, sunrise/sunset refractive behaviour, then yes, modifying the hour lines by average refractive error can be quite helpful. Many both sundial programs and better yet, navigational programs provide good figures for these corrections. > > > Dear Dialists, > > I have been pondering the effect of refraction on dial performance. > > Refraction quoted at .1 degree to .5 degree apparent difference in > altitude below 10 degrees of altitude, will impact the hour angle to > some degree. See http://www.gcstudio.com/calcuse.html These are average > values - refraction (according to Meeus quoting someone) can vary by up to > .3 degree at the lower altitudes. > > 1. It would seem to me that dials that attempt to tell time during the > hour after sunrise and hour before sunset could usefully take refraction > into account. However altitude for a given hour angle changes through out > the year. Are there dials laid out with reference to average refraction > effects? Yes, quite a few. > And > > 2. We know that Mean time + EoT = Apparent Time > > However because viewing the sun through our atmosphere's refraction is > unavoidable - Is there established terminology to distinguish between > Apparent Time (geometric) as just defined above and Actual Observable > Apparent Time (after refraction)? The apparant time (geometric) is called topospheric rather than geospheric ( from the center of the earth ) or heliocentric ( from the center of the sun ) and is without atmosphere. Your last term is what a lot of folk call the actual observed time. Some folk call it topospheric ( with refraction). > David Edley McKnight [43.126N 123.357W]
