I have a museum’s pewter replica of an 18th-century New England sundial. The inscription says it was designed for latitude 42°. Can I use this sundial in Seattle, Washington?
The iconic garden sundial, with triangular shadow-casting gnomon and horizontal plate, gives accurate readings at the latitude for which its hour lines were laid out. If you replace the gnomon with one cut for Seattle’s latitude, 48°, it won’t work any more.
Instead, you can make a wedge that will tip the north end of the entire sundial upward by 6°, the latitude difference between the two locations. The gnomon’s slanting edge now points to the north celestial pole over Seattle, and the sundial’s plate remains parallel to the original horizon for which it was designed.
As with all sundials of this type, don’t forget to correct for the “equation of time” and for Seattle’s slight distance from the Pacific Standard Time meridian. (These corrections are explained in sundial books and in the instructions to the Skygazer’s Almanac that goes out with each January’s Sky & Telescope.)
— Roger W. Sinnott