Equation of Time
The equation of time is the disparity between the arrival of the sun at its highest point and the clock definition of noon, this difference may be as wide as 16 minutes, and renders normal sundials inaccurate.
For more details see the Derbyshire Sundials Equation of Time page.
An analemma is the resultant shadow track if a board is stood vertically facing south, and a rod is standing squarely from the face of the board near the top.
A shadow will be cast on the board from the rod (gnomon). If the end of the shadow is marked on the board at noon by the clock time for all of the days of the year, the result would be a figure of eight shape called an analemma.
The shape would be generated by a combination of the equation of time and the variation in the altitude of the sun through the year.
In order to tell the time on the hour, you need to know which side of the figure of eight is relevant for that time of year (as dictated by the equation of time).
For example, on March 21st, the hour is read on the left side, and on September 21st it is read on the right side.
In fact an analemma can be made on any hour line as shown on the slate dial further up this page.
One method of doing this using the Vertical dial Calculator is plotting point 'F' for various times of the year for each hour, though it would be a tedious job.