To get the current time from the sundial indication, first of all adjust for daylight saving time if necessary:
o If the sundial was drawn for standard time, add 1 hour in summer
o If the sundial was drawn for daylight saving time, subtract 1 hour in winter
If you chose straight hour lines, you also need to compensate for seasonal variations using the equation of time given by the curve below:
Or, if knowing the time within five minutes is good enough, just use to the following table:
From December 30 to January 10 | +5 min |
From January 11 to 25 | +10 min |
From January 26 to February 28 | +15 min |
From March 1 to 19 | +10 min |
From March 20 to April 5 | +5 min |
From April 6 to 27 | 0 min |
From April 28 to May 29 | -5 min |
From May 30 to June 24 | 0 min |
From June 25 to August 23 | +5 min |
From August 24 to September 8 | 0 min |
From September 9 to 22 | -5 min |
From September 23 to October 8 | -10 min |
From October 9 to November 26 | -15 min |
From November 27 to December 9 | -10 min |
From December 10 to 19 | -5 min |
From December 20 to 29 | 0 min |
Finally, if the sundial is drawn for the solar hours, it is necessary to add a correction that depends of the longitude and the time zone. This correction can be negative that is to say it must be subtracted. It is computed by Calcad and displayed in the numeric data.
The following picture was taken on January 19, 2005. This sundial is not of solar hour type, what time was it?
The answer may be found in this section.