Gnomon's Land

Sometimes it can help to tell the local time it is using the Sun. The relative behavior of the Sun and the tilted Earth over the cycle of the year (called the declination) defines our Solstices and Equinoxes, and the transit of the Sun across the surface of Earth each hour defines our Standard Meridians (15 degrees apart each), while the arc of the Sun across our sky dome conveys time at the local level. If we apply the technology of a gnomon and apply our local solar vernacular to read the shadows, voila, we have a solar clock!

Today I visited the H. O. Botanical Gardens of the Penn State Arboretum, a fabulously beautiful space to walk about on a sunny pre-Summer Solstice day! This image shows a large installation piece, the Joel N. Myers Sun Dial, with a gnomon standing 9 feet tall. The gnomon is the vertical part of a sundial which casts a shadow onto the ground and permits us to tell solar time (defined by solar noon occurring when the arc of the sun reaches its daily maximum in the sky dome). On the day of June 21, in the Northern Hemisphere, the shadows will be the shortest of the year, and the day will be the longest–fifteen hours and three minutes long in State College, PA (no 12-hour days up North in the the Summer)! And on this particular Solstice of 2016, there will be a full moon associated with the day (relevant Slate article –credit: Phil Plait). Lots of fun in the sky these days, and wonderful solar ecology happening down here in the gardens at Penn State!

#SolarEcology embedded in our measures of #time. Big gnomon/sun dial .@PSUarboretum

A photo posted by Jeffrey Brownson (@heliotactic) on

From the Special Attractions description:

The Joel N. Myers Sundial in The Arboretum at Penn State is a landscape-scale, granite sculpture that serves as a functionally accurate timepiece and artistic attraction. Created by sculptor Mark Mennin in 2011, the sculpture consists of 25 large, custom-shaped “digit” stones and a 9-foot high “gnomon.” The upper edge of the gnomon casts the shadow that is used to tell time. Hours are indicated by variously colored inlays in the stones. While everyone can enjoy the artistry of the Myers Sundial, the functionality of the timepiece introduces a complexity that requires explanation. Detailed directions for reading the sundial are provided in an interpretive sign next to the sundial and a brochure available in the H.O. Smith Botanic Gardens.

This custom-made sundial was made possible by a gift from 1961 Penn State alumnus, Dr. Joel N. Myers, founder of AccuWeather.

Location: Joan Milius Smith Esplanade

For more detailed information on the distinction between solar time and watch time, please visit our Open Educational Resource for Solar Resource Assessment and Economics: Solar Time and Watch Time.

Yes, sometimes you need to know what time it is using the Sun. In fact, sometimes it is incredibly rewarding to be aware that time and space itself were shaped by the movement of the Sun across the hours, days, and months. For a full description of the role of the Sun, time, and the role of Longitude (and great summer reading), I highly recommend checking out Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time by Dava Sobel. For the history of sundials and gnomons used in China over 6,000 years ago, I also highly recommend reading Let It Shine: The 6,000-Year Story of Solar Energy by my friend and colleague Jon Perlin.

Happy Solstice and Full Moon everyone. -JRSB

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