Keeping Time at Landis

No gar­den is real­ly com­plete with­out a solar time­piece. Ear­ly sun­di­als were impor­tant tools for deter­min­ing plant­i­ng and har­vest dates, as well as pro­vid­ing time-of-day. The per­fect blend of sci­ence and art, sun­di­als con­nect us to the heav­ens in a quan­tifi­able way. 

The Lan­dis dial, promi­nent­ly installed on the front of the Barn, is a ver­ti­cal declin­ing dial, set to dis­play East­ern Day­light Time (March to Novem­ber). Dur­ing East­ern Stan­dard Time, sub­tract one hour from the dial read­ings. The ​‘Hour Lines’ are diag­o­nal lines radi­at­ing from a spot near the top of the dial. Lines cor­re­spond­ing to whole hours are sol­id, and half-hour lines are seg­ment­ed (to avoid con­fu­sion). Time is read where the shad­ow of the top edge of the gno­mon (called the ​‘style’) falls among the hour lines. 

This dial can be used to accu­rate­ly set one’s watch to with­in a minute or two, if one knows how to read it! To do so, read the dial and apply the ​‘Equa­tion of Time,’ a vari­able quan­ti­ty that needs to be added to (or sub­tract­ed from) the dial read­ing to obtain ​‘Mean Time.’ This is nec­es­sary because sun­di­als show ​‘Appar­ent Solar Time,’ while watch­es keep ​‘Mean Solar Time.’ The ​‘Equa­tion of Time’ (EOT) is sim­ply the dif­fer­ence between the two. An EOT plot is includ­ed in this arti­cle. It shows, for a giv­en date, the cor­rec­tion, in min­utes, to apply to the dial read­ing. If the EOT is neg­a­tive, that means the dial will be slow rel­a­tive to watch time and that the num­ber of min­utes must be ADDED to the dial read­ing. If the EOT is pos­i­tive, SUB­TRACT it from the dial reading.

In lieu of a 12:00 hour line, there is a funky fig­ure-eight curve on the dial that con­tains tick marks at the first day of each month. This is an analem­ma. It basi­cal­ly makes the EOT cor­rec­tion for you. When the cen­ter of the tip of the gno­mon’s shad­ow is exact­ly on the curve (between the appro­pri­ate month marks asso­ci­at­ed with the cur­rent date), it is pre­cise­ly 12:00 EDT (11:00 EST).

The two sweep­ing curved lines on the dial are sol­stice lines. On the win­ter sol­stice (ca. ¹²⁄₂₁) the tip of the gno­mon’s shad­ow will fol­low the upper curve through­out the day. The shad­ow tip will fol­low the low­er curve on the sum­mer sol­stice (ca. June 21). The diag­o­nal line that cuts through the mid­dle of the dial is the equinox or ​‘equinoc­tial’ line. On the equinox­es, when the Sun is in the equa­to­r­i­al plane, (ca. ³⁄₂₀ and ⁹⁄₂₁), the tip of the gno­mon’s shad­ow will fol­low that line.

Geoff Miller’s avo­ca­tion is ​“gno­mon­ics,” the sci­ence of sun­di­als. A for­mer res­i­dent of the Cap­i­tal Dis­trict, he now lives in New Mex­i­co, return­ing to the area in the sum­mer. He ran in the Arboretum’s 5K race sev­er­al years ago and offered to con­struct a sun­di­al for Landis.