FCPS/NOVAC EarthDial Glossary

Photo Credit: The Planetary Society & The EarthDial Project.

gnomon: (pron. no-mon) the physical structure of a sundial which casts the shadow (from the Greek for "indicator"). The gnomon today is most-often polar pointing (sometimes described as an "axial gnomon"), although it may also be horizontal or vertical. The distinction between gnomon and style made (and encouraged in the NASS Glossary) in modern dialling literature is not the one used in early works, and the two words are still sometimes used interchangably.

azimuth: the angular distance clockwise around the horizon from the northern direction, usually expressed in angular measure from 0 degrees for an object in the northern direction, to 180 degrees for an object in the southern direction, around to 360 degrees. Note that sundialists measure azimuth from the southern direction.

nodus: a point which casts a shadow to indicate the time and/or (more often) the date on a dial face. It may take the form of a small sphere or a notch on a polar-pointing gnomon, or it may be the tip of a gnomon with an arbitrary (usually horizontal or vertical) orientation.

hour lines: the lines on a dial face indicating the shadow position at a particular time of day (includes fractional as well as whole hours).

declination (of the Sun): the angular distance of the Sun above or below the Celestial Equator. Its value follows an annual sine wave like-curve, varying between 0º at the equinoxes and ±23.4º (approx.) at the solstices. It has positive values when the Sun is above the Celestial Equator (summer in the Northern hemisphere) and negative when below (see a table of the Declination of the Sun).

declination lines (date curves): the lines on a dial face indicating the shadow position at a particular time of year, usually for astronomically significant dates such as the equinoxes and solstices.