"Skylight Aperture Sundial"
"Skylight Aperture Sundial" features a row of five circular openings in the ceiling, which are covered by colored glass discs of purple, teal, green, aqua and orange. The glass discs project colorful spotlights that float through the library as the sun moves across the sky. At night, artificial lights illuminate the colored discs.
Between the spring and autumn equinoxes, circles of light slide across the space throughout the day - and hang and float on beams, tables, posts, bodies and walls. The shape and position of the spotlights change, at times elongating and splaying down the staircase. After the summer solstice (about June 21), the spotlights move daily north until the autumn equinox (Sept. 23). As the sun appears lower in the sky in autumn and winter, ceiling beams block the spotlights, but they reappear near the spring equinox (March 20). In the winter and on overcast days, the light from the sky causes the colored discs to glow overhead.
The orange spotlight is the time indicator for the sundial; the other spotlight colors are decorative. A dashed line of steel markings in the floor designates the "solar noon line." When the orange spotlight crosses the line, it is solar noon. A marker also indicates solar noon on Aug. 12, 2006, the day the Montlake Branch opened, and "clock noon" on the summer solstice and equinoxes.
There are three reasons why solar time and clock time differ. Solar noon occurs when the sun is due south and highest in the sky. Solar time can be up to 16 minutes faster or slower than clock time depending on the time of year, due to the earth's elliptical orbit and the 23.44° tilt of its axis. This difference is called the Equation of Time. Another time adjustment (always nine minutes) accounts for the difference between the longitude of the central meridian of the Pacific Time Zone and the longitude of Seattle. Finally, daylight-saving time causes clock noon in Seattle to occur an hour before solar noon at the summer solstice and equinoxes.
Read about the artist and get more information about sundials.