This sundial will measure the time year round to within a minute, and the way to read it is very straightforward. All you need to know is the current date. Simply look up the date on the sundial’s outer date ring and then directly below it read the corresponding time correction. In the image at the left, you can see that the correction for December10th would be -7 minutes. This value should then be added to the time indicated by the Sun’s shadow on the sundial’s time ring below the Roman numerals, here indicated as 1:50. So the actual time is 1:50 minus seven minutes or 1:43. And that’s it, except to remember that, like any other sundial, you need to add an hour to the time during daylight savings time.
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An interesting side note: The time correction values comprising the entire year are known as the equation of time. Its graphic representation was originally shown on a clock’s face rather than on a sundial. It was used to correct the clock time to what was then considered true time, the time kept by a sundial. Now, however, as we have become used to looking at clocks rather than sundials to tell the time, the correction is added to the sundial rather than the clock, but this is not how the real world works. There is actually only one day of the year in Cambridge (February11th) when the Sun is at its highest point in the sky at exactly twelve noon, so it's really the clocks that should be corrected not the sundial.