A Tale of Two Sundials

Sandburg Center for Sky Awareness A Fairfax County Public Schools Planetarium A Tale of Two Sundials "It was the best of times; it was the worst of times." - A Tale of Two Cities, by Charles Dickens A tale of two cities; two equatorial sundials. Same manufacturer (Erickson Memorial Co.); virtually the same design. So what's the most important difference between the two sundials? Look closely at the following photographs...
St. Petersburg, FL	Boulder, CO
In both pictures, observe that the gnomon (or style) is a pole (or rod) that is perpendicular to the dial plate. However, notice that the two gnomons (and therefore the dial plates) are inclined at different angles relative to a horizontal surface. Why? Like the old saying from real estate sales, it's all about "location, location, location!" Latitude. Remember the first two (of three) criteria for setting an equatorial sundial? The gnomon should be parallel to the Earth's axis, inclined at an angle equal to the latitude of the observer (, phi). The dial plate should be parallel to the plane of the Earth's Equator (perpendicular to the gnomon), inclined at an angle equal to the complement of the observer's latitude. This angle is also known as the colatitude. St. Petersburg, FL is located at 27.8°N latitude, therefore the gnomon of the St. Petersburg sundial is inclined at an angle of 27.8 degrees and the dial plate is inclined at a 62.2 degree complementary angle. In contrast, Boulder, CO is located at 40.0°N latitude, therefore the gnomon of the Boulder sundial is inclined at an angle of 40.0 degrees and the dial plate is inclined at a 50.0 degree complementary angle. Longitude. The Standard Time Meridian for the Mountain Time Zone is 105°W longitude. By coincidence, Boulder, CO is located at 105.3°W longitude, therefore Solar Time (sundial time) is virtually the same as Standard Time (wristwatch time), ignoring the Equation of Time and Daylight Saving Time. The Standard Time Meridian for the Eastern Time Zone is 75°W longitude; St. Petersburg, FL is located at 82.7°W. In contrast to Boulder, St. Petersburg, FL is located near the extreme western limit of the Eastern Time Zone, therefore the difference between Solar Time and Standard Time is approximately 30 minutes. For this reason, the St. Petersburg sundial features a built-in correction for longitude. In the following photographs, look closely at the orientation of the 12 noon hour line (or noon gap)...
St. Petersburg, FL 12 noon offset by ~30 min.	Boulder, CO 12 noon not offset.
The meridian line of an equatorial sundial is an imaginary vertical line passing through the center of the dial face--when the Sun crosses the meridian line, it is local solar noon. Notice that the 12 noon hour line of the St. Petersburg sundial is offset slightly from the meridian line because 12 noon Eastern Standard Time occurs ~30 minutes earlier than local solar noon in St. Petersburg, FL. In contrast, notice that the 12 noon hour line of the Boulder sundial is almost perfectly aligned with the meridian line because the times of local solar noon and 12 noon Mountain Standard Time are virtually coincident in Boulder, CO. Note: For both sundials, correct Solar Time (sundial time) for Standard Time (wristwatch time) by adding or subtracting the Equation of Time as appropriate (using graphs that appear on plaques accompanying the sundials) and adding one hour for Daylight Saving Time (when in effect). © Copyright 2002-2008 Walter Sanford. All rights reserved. Me and My Shadow - Making the Sun-Earth Connection \| SCSA Home