Sundials measure time differently than clocks. When the sun is the highest in the sky it is noon (which is not necessarily 24 hours from the previous day). Clocks, on the other hand, measure time with noon tomorrow exactly 24 hours from noon today (which is not actually true). The variations between solar time and clock time is due to the earth's elliptical orbit around the sun.
Actual day lengths vary from 23 hours, 59 minutes, and 40 seconds long to 24 hours, and 20 seconds long. These differences build up over time. Throw in daylight savings time and solar time and clock time get even farther apart. If you happen to be reading the armillary sphere and think that it is not correct, just remember that sundials and clocks are both accurate timepieces, but that they tell two different kinds of time.
If you want to set your watch by the sundial, it is possible at noon on April 15th, June 14th and September 2nd (don't forget to compensate for daylight savings time), or December 25th -- the only days when solar time and clock time coincide. From early April to early September sundials and clock time are no more than 5 minutes apart (after compensating for daylight savings time), and are no more than 15 minutes apart for the rest of the year.
Technical considerations notwithstanding, the armillary sphere is a timeless addition to the Rose Garden that will delight Arboretum visitors for generations to come.
Mike Tuszynski
Horticulturist
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