[EMAIL PROTECTED] wrote: > > I want to design an altitude dial that will indicate local clock time. I > was thinking of an oversized pillar or shepherd's dial or a large ring > dial. Usually these are portable, but I want to make one for my particular > location (Lat 38d53m48s N, Long 121d04m33s W). > > What types of corrections do I need to know about to get it to read the > local clock time? I am assuming that I will need to know whether it's > Daylight Savings time (I'm in the U.S.). Anything else? > > The Equation of Time only applies to azimuth dials, not altitude dials, > right? > > Any hints, references, or helpful suggestions appriciated. > > Tom Bower, Auburn, California > > P.S. I'm not an expert, so I really like reading the mail from such > friendly and knowledgeable people. Thanks in advance!
Tom, To read clocktime as used in the time zone you live you always need 2 corrections: for longitude and for the equation of time. This is necessary for all types of sundials. You live at lat 38.90 long 121.08W (decimal degrees) Your longitude is 1.08 W of the time zone meridian of 120W. In time this is 1.08 * 4 = 4.32 minutes (decimal), or 4m 20s. Because you live west of the zone meridian your watch is fast to the sun. If watch reads 12, the sun is at 11h 55m 40s. If it is XII suntime, with the sun due south, your watch reads 12h 4m 20s. This is the longitude correction and this would be the situation whole the year if there was no equation of time. At daylight saving time 12 becomes 13 or the longitude correction is 1h 4m 20s. In november, par example, the equation ot time at a given day is +12m. This means: the real sun is 12m fast to the mean sun, or the real sun is fast to the reading of your watch. With both corrections: If your watch reads 14h the suntime is 14h -4m20s + 12m = 14h 7m 40s If it is X suntime, your watch reads 10h + 4m20s - 12m = 9h 52m 20s In this way for any day you can convert watchtime into suntime and visa versa. At daylight saving time all differs 1 hour or use longitude correction 1h 4m 20s. Think of the rigth sign of the equation of time. + in november, - in february etc. If you want to calculate a clocktime line on your dial you have to calculate a number of points for several days. You need to know the equation of time (E) and the declination (delta) of the sun for that day. You can find them in tables, graphs or compute them. You have to convert the equation of time into degrees. (1 minute of time is 0.25 degrees) Assume you want to calculete the line u. Then the hourangle t (that is suntime in degrees) of the sun is: t = (12 - u)*15 -1.08 + E degrees. (1.08 is your long. correction) Now you can calculate the suns height sinh = cost . cosphi . cosdelta + sinphi . sindelta. h = suns height t = suns hourangle phi = latitude delta = suns declination for the same day To read daylight saving time just add 1 to the numbers on your dial or use long. correction 1.08 + 15 = 16.08 degrees. Happy dialing. Fer J. de Vries Netherlands.
