A horizontal sundial is located on the grounds of historic Christ Church in Old Town Alexandria, Virginia USA. The North American Sundial Society Dial Registry listing says, the “dial appears to have been designed for 32 degrees N.” In order to determine whether the sundial is in fact aligned properly, I examined a couple of photos that were geotagged by my Apple iPhone 4. (See “Tech Tips” for details, below.)
Similar to setting the correct time on an analog clock or wristwatch (by moving the hands of the timepiece into proper position), properly orienting a horizontal sundial will move the shadow of the gnomon (or style) into position so that the dial face displays the correct time.
- The dial plate should be horizontal.
- The shadow-casting edge of the gnomon should be parallel to the Earth’s axis, inclined at an angle equal to the latitude of the sundial.
- The tip of the gnomon should point toward the North Celestial Pole (i.e., Polaris, the North Star). More simply, the dial face should be aligned so that 12 noon points toward geographic north and the 12 noon hour line is aligned with your local meridian.
Photos 1-2 of 8 (shown below) verify that the dial plate is horizontal. Photos 3-4 show the gnomon is inclined at an angle of 31.86 degrees (~32 degrees); Photos 5-6 show the latitude of the sundial is 38 degrees 48 minutes 22.2 seconds. Photos 7-8 show the image direction is 218.4602 degrees, meaning the tip of the gnomon is pointing southwest rather than true geographic north (0, 360 degrees).
Bottom line: The Christ Church sundial appears to have been made for another location and is aligned improperly for its new location. In other words, the sundial is strictly ornamental and will not tell time correctly.
Tech Tips: The iPhone Camera app works seamlessly with two built-in devices to geotag photos: the GPS sensor measures position on Earth; the digital compass measures “image direction.” PixelStick, an application for Mac OS X, was used to measure angles in one of the photos (see Photos 1-4, above). Apple “Preview” was used to display GPS info for both photos (see Photos 5-8, above).
Photos © Copyright 2012 Walter Sanford. All rights reserved. www.wsanford.com