“Why is the time different on my watch from the sundial?”

 

Site Location: N27.77° Latitude x W82.64° Longitude

The nearly 30’ bowstring sundial was constructed and aligned specifically for this exact location in St. Petersburg, Florida to keep what is called Solar Time, so several factors affect what time it tells.

Solar Noon is when the sun is at the highest point above us (due south, because we’re north of the equator), and casting the shortest shadow over the course of the day. Standard Noon, is a man-made adjustment from “nature’s” Solar Time to achieve a common consistent time. There are three primary reasons for the difference in time between Solar Noon and Standard Noon.

  • The “Equation of Time” which accounts for the variation throughout the year between when the sun is at its peak in the sky and when our clocks display noon.
  • The distance from Sundial St. Pete to the Standard Meridian (SM), or the center of our time zone
  • Daylight Savings Time

Standard Time divides the year into equal hours, minutes, and seconds. Solar Time varies throughout the year as a function of earth’s elliptical orbit around the sun combined with the “wobble” that creates our seasons. The Equation of Time variation ranges from 14 minutes slow to 16 minutes fast during the year, with zero difference four times a year on the solstices and equinoxes.

Sundial St. Pete sits about 7.64 degrees of longitude from the center of our time zone. Earth turns about 15 degrees per hour, an offset of roughly 0.5093 an hour or 30 minutes and 34 seconds, making our Solar Noon over half an hour behind Standard Noon on average.

And our man-made Daylight Savings Time further complicates an accurate reading, depending on the day and month of the year.

So next time you meet some friends at Sundial St. Pete and they’re a few minutes late, just remember that according to Solar Time, only the SUNDIAL statue is running right on time!

 

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