Practical Operation 3
Allowance for the Equation of Time
This allowance is directly related to Date. To establish the resultant correction needed for each day of the year, reference must be made to Graph portrayed at the southern end of the Dial Plate.
In example A, the date is assumed to be 16th November. Enter the bottom axis of the graph where it relates to 16 November and proceed vertically until encountering the curve. Then proceed horizontally, either to right or left, until reaching the vertical axis. At this point the correction for the day will be indicated. In this case – 9 minutes. Apply this to the reading already indicated by the gnomon’s shadow (2.15pm) and you have established the exact GMT as being 2.06pm. (2.15 - .09 = 2.06).
Had the same reading of solar time been obtained during Summer Time, on, say, 15th June the same process with the graph, as illustrated by example B, would have indicated a correction for that day as + 6. Apply this to the local solar time of 2.15pm and you have established a precise GMT of 2.21pm. However, as indicated in the footnote to the graph, a whole hour must be added to this in order to establish the correct Summer Time. Which consequently is determined as being 3.21pm.
Further reading
A history of the heliochronometer
The importance of Longitude
The equation of time
Full installation instructions for the Gunning Heliochronometer