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Build a Human Sundial for Winter Solstice: Tell Time the Ancient Way –

Posted by erika.d.johnson | January 8, 2016


Published: December 21, 2015 at 6:44 PM, updated December 21, 2015 at 7:21 PM

By Homes & Gardens of the Northwest staff

How will you celebrate the shortest day of the year? Winter solstice happens here at 8:49 p.m. tonight, Dec. 21, and master gardeners in Washington State University Extension Clark County have devised a cool way to mark the occasion when the sun hits its lowest point in the sky:

They built a human sundial at WSU’s 78th Street Heritage Farm in Vancouver, Washington, where a person acts as the gnomon, the part of a sundial that casts the shadow that measures true local time.

The master gardeners joined forces with Bryan Preas, a retired engineer, Master Gardener and member of the North American Sundial Society, to create the horizontal sundial using math and hard labor. Maybe you will be inspired to build one yourself?

First, Preas made blueprint plans based on the site’s features and an idea to build a sundial by Erika Johnson, WSU Extension Master Gardener Program coordinator.

Preas chose a size most appropriate for use in spring and fall, the most popular seasons at Heritage Farm, a 79-acre parcel open to the public with community gardens, food bank gardens and veteran’s farm program.

His sundial computations followed those known to the ancients – the Egyptians were building sundials in 1500 B.C. – but instead of scratching out a plan on papyrus, Preas used his computer.

While working on his own spreadsheet, he came upon the Analemmatic Sundial Generator website that enabled him input the location data and get all of the dimensions needed to measure out and draw the sundial.

The next challenge was the building materials, which needed to be attractive and durable. Preas and Johnson approached Chris Houlahan and Fred Davis at Mutual Materials hardscape and masonry supplies in Vancouver, Washington, who donated stones and provided advice.

Fred Bass of Stone Art in Hillsboro engraved individual squares with the name of each month to be used as the ground calendar. The stone cutting and engraving had to be accurate to within a fraction of an inch to assure the sundial would work.

The human gnomon stands on the month’s square and cast a shadow toward a ring of numbers.

The volunteers used a U-Haul trailer to transport the heavy stones and the Nutter Corporation donated gravel and sand for the installation’s base layers.

Once the stones were completed, a group of volunteers helped Preas in laying out the correct positions and digging the holes to place the stones. Again, a fraction of an inch off would have resulted in a few seconds of time difference.

There were two other potential sources of inaccuracy in the sundial. One is the shadow of the person acting as the gnomon. If the person doesn’t stand on the correct position on the appropriate stone, the time will be off.

The other inaccuracy inherent in using a shadow is the size of the shadow. Sundials usually use a thin pole or pointer to indicate the time. For a person to replicate this, he or she must hold their hands over their head with the palms together to form a pointer.

In spring and fall, an average-sized person’s shadow can touch the numbers on the stones, but a shorter person will just see the direction that the shadow falls and will need to guess at the place it would touch the stones.

Since sundials measure true local time, the local solar time at the exact location of the sundial, it can run ahead of or behind watches, which measure standard mean time for our time zone.

Preas indicates that solar and mean time can differ by as much as 15 minutes. He hopes to eventually have a marking at the site with this “equation of time” explained for users.

Johnson, who came up with the idea, has created a page on the Master Gardener website that gives instructions on how to use the sundial, where to position feet and how to read the clock.

Visitors will find the sundial east of the road leading to WSU’s 78th Street Heritage Farm at 1919 NE 78th St. Gates are open 8 a.m.-5 p.m. Monday through Friday.
— Susan Cox

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