Learning by the Sun: Observing Seasonal Declination with a Vertical Sundial

We designed and constructed a sundial for the purpose of observing the declination of the sun and thus marking solar seasonal variation. The 122 × 122 cm vertical sundial on the south-facing wall of our library has two unusual features: a nodus on the gnomon that casts a shadow of a point for marking the height of the sun and a large blank working space for students to mark the shadow of the nodus at different hours of the day and to connect the marks of 1 day in a line of declination. We discuss the design of a dial that emphasizes a working space for observations on solar declination, methods for determining the position of the nodus such that lines of declination can be observed every day of the year, mathematical procedures that minimize error in laying out the hour lines for this unusual design, the type of materials that are best suited for this design, and how students in an interdisciplinary honors colloquium on seasonal rhythms used the sundial. We also include two appendices on general sundial construction that indicates the information designers need to construct their own sundials.