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.
• The dial plate should be parallel to the Earth's Equator (perpendicular to the gnomon), inclined at an angle equal to the complement of the observer's latitude.

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°N latitude, therefore the gnomon of the Boulder sundial is inclined at an angle of 40 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. [Notice that a picture of the dial plate of the St. Petersburg sundial shows that 12 noon (bottom) and 12 midnight (top) are slightly offset from an imaginary vertical line passing through the center of the dial face. In contrast, notice that the meridian line (or noon gap) of the Boulder sundial is exactly perpendicular to a horizontal surface.]