Mapping the Cosmos on a Ceiling: Reflection Sundials from the Seventeenth Century to the Present

Ceiling reflection sundials employ a small horizontal mirror, say on a south-facing window sill, to cast a spot of sunlight to the ceiling and/or walls of a room or gallery. In this way the linear scale of the daily and annual motions of the sun are greatly amplified, allowing a plethora of information to be displayed and read. Besides the time of day and the date, typical quantities included the altitude and azimuth of the sun, the declination of the sun, the number of hours since sunrise, the length of daylight, the sign of the zodiac, the sidereal time, etc. The principles for planning and calculating these sundials were first laid out in detail in 17th century Italy by the Jesuit scholars Athanasius Kircher and Emmanuel Maignan: two reflection dials of the latter still survive today in Rome, at Trinità dei Monti (1637) and Palazzo Spada (1644). A third extant example can be found at the Lycée Stendhal in Grenoble, built by a Jesuit priest named Bonfa in 1673.

This talk will describe and illustrate these complex sundials, as well as a recently completed ceiling dial, inspired by their example, in the New World (Seattle).