Summary
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Reading the sundial

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.

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