"Roger Bailey" <[EMAIL PROTECTED]> asked on Monday, April 18, 2005 5:50 AM Subject: Sonne Software for Shepherd Dials <snip>...
> My only problem so far is the equation of time correction. I could not find > this option for this type of sundial. Has it been included? Did I miss it? > Has any one else included this option in a similar program to give "standard > time" on a cylinder dial? > > Regards, > > Roger Bailey > Walking Shadow Designs > N 48.6 W 123.4 I'm not aware of any cylinder dial with this correction, but, by coincidence, I have been investigating adding Equation of Time correction to a totally different type of dial which has exactly the same problem. That problem is that the same line is normally used to indicate two hours, one before noon, the other after, and on two dates when the sun's declination is the same. So, if you move the line to adjust for EoT before noon, the adjustment will be in the wrong direction for the same hour after noon. And, worse still you would think, it will be wrong for the other date with the same declination. So, instead of having hour lines for just half the day (the same line being marked, for example, both 11am and 1pm) and half the year, you really need separate 'am' and 'pm' scales, and a whole year date scale. That needs four times as much area as a standard pillar or shepherd's dial. But I can suggest a way to halve the area needed, if you can accept an error of up to two minutes. This takes advantage of the symmetry of the analemma and of the sun's altitude in the morning and afternoon. Whatever the sun's declination, the EoT values on the two dates with that declination are generally very similar, but with opposite sign. For instance, at the equinoxes the EoT is about -7:30 in spring, +7:30 in the autumn. So the 11am hour line in the spring is almost identical to the 1pm hour line in the autumn. So, while you do need separate am and pm scales for any date, these can be switched half way through the year. The 'am' scale for Jan-Jun is also the 'pm' scale for Jul-Dec. The 'am' scale for Jul-Dec is also the 'pm' scale for Jan-Jun. Using the average absolute value of the EoT for any declination is never as much as 2 minutes wrong, and usually less than 1, which is better than you can probably read the dial to anyway. Let me know if you need further clarification. Chris Lusby Taylor 51.4N, 1.3W -
