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Sundial's timepiece in St. Petersburg is accurate for solar time


Does the sundial anchoring new downtown shops actually keep time?

"This very expensive sundial is about one-half of an hour off," a reader recently complained in a letter to the Tampa Bay Times. "Is that the best St. Petersburg can do?"

Never one to waste time, developer Bill Edwards assigned his team to research the issue during construction of his aptly named, soon-to-open Sundial shops.

According to the North American Sundial Society, the sundial tracks local solar time measured by the sun at a given location. That differs from clock time.

"Many would say that sundial time is truer and provides more insight into the nature of our world than our artificially constructed clock time," says the Planetary Society, which encourages space exploration.

To satisfy rising local curiosity, an explanation of the sundial will soon appear on the shopping center's web site.

On average, the sundial will be over half an hour behind clock time. But that will vary. On the first day of daylight savings time, that difference can grow to 1 hour, 41 minutes and 22 seconds.

To see sundial time close to clock time, head to the shops on Nov. 4. That's when the sundial will only be 14 minutes, 11 seconds behind what we consider real time.

Reading the sundial

The nearly 30-foot sculpture at Sundial is a bowstring sundial, aligned specifically for its latitude in St. Petersburg and designed to keep "solar time.''

The rod in the middle, called the gnomon, points north and casts the shadow. As Earth rotates, the shadow slides along the dial from 7 a.m. to 5 p.m. In the photo above, it is 1:30 p.m. On average, solar noon (when the sun is at its highest) is more than half an hour behind civil noon for our time zone (ignoring daylight savings time), but that varies daily.

The maximum difference is one hour, 41 minutes, and 22 seconds. The smallest difference is 14 minutes and 11 seconds.

Sundial's timepiece in St. Petersburg is accurate for solar time 08/13/14 [Last modified: Wednesday, August 13, 2014 9:20pm]
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