Happy solstice, Wallingford! Want to know the best place to enjoy it?
Sure, sure, you’re an old Wallingford hand, you know all about the sundial on top of Kite Hill at Gas Works, right? Well, unless you’re a sundial nerd, you guarantee you we’ve got something new to impress your friends with.
The sundial was created in 1978 by local artists Chuck Greening and Kim Lazare and features beautiful wrought natural imagery as well as embedded rocks, shells, marbles and other visual delights. What makes it truly unusual, though, is that, unlike the common horizontal dial, which features a triangular gnomon whose shadow edge denotes the time, the Gas Works sundial is what’s known as an analemmatic sundial: the gnomon is straight up and down and must move with the season to adjust to the change in position of the sun over the year.
In this case, the gnomon, the thing that casts the shadow, is you: stand on the spot corresponding to the date, and your shadow will point out the time on the edge of the dial1.
First, let’s visit how a traditional horizontal sundial (like the one of the right) works.
In order for it to work correctly, the angle of the gnomon to the earth must be equal to the latitude at which it is being used (in Seattle, that’s roughly 47°) and pointing up to the North.
Ready for some geekery? If you dust off your high school geometry and follow along, you’ll see that this means the gnomon will be be parallel to the axis of rotation of the Earth. Thus, the sun will appear to spin around it. Our poorly drawn diagram below illustrates why: the line dc is tangent to the Earth’s surface at your latitude. ∠bac is your latitude.
∆bac is a right triangle, so ∠bac and ∠bca are complementary. Since ∆dac is also a right triangle, ∠bca and ∠cda are also complementary, so ∠bac equals ∠cda. The Tranversal Postulate tells us that if ∠adb and ∠dbe are congruent, than |ad and |be must be parallel. ∠dbe is your gnomon.
If we were Tom Lehrer, that would have come to music.
To understand why the hours lines are splayed out from the base of the gnomon, let’s first take a look at a different kind of sundial, the equatorial sundial. The one pictured below happens to lie on the prime meridian in London, but that part doesn’t matter. What’s important is that instead of setting a gnomon to match its latitude, the entire face of the dial is angled to the latitude and the gnomon is perpendicular to the face. The hour lines aren’t splayed, they are parallel lines set across the dial face.
For a horizontal dial, then, imagine taking a plane tangent to the Earth’s surface and inserting it into the dial above, then extending the shadow lines down. They’d be splayed as you see above.
So, how do you get a sundial to work if the face of the dial is flat and the gnomon (you!) is perpendicular to the surface? You have to account for the fact that as the Earth revolves around the sun throughout the year, the angle of the sun at a given time of day at your particular latitude will shift.
Now, it’s a bit more geekery than even we can handle at the moment, but if you’ve got a head for trig and really want to get into it, check out this fine explanation of analemmatic sundials. Long story short: you can lay out hour lines on an ellipse and adjust the position of the gnomon (that’s you!) based on the date, which is exactly what Greening and Lazare have done at Gas Works. If you examine the ground, you’ll see that the dates are marked out. Stand on today, check out your shadow and POW!, you’ve got the time.
Sort of. See, it’s even a bit more complicated than that. The Earth doesn’t actually revolve around the sun in a circle, it travels in a ellipse. As it gets further from the sun, it slows down; as it gets closer, it speeds up. Not only that, but the plane of the rotation of the Earth around the sun is different than the plane of the rotation of the Earth around its axis by about 23°. You layer those two effects together, and you get what’s called The Equation of Time, the difference between “apparent solar time” and what your watch would tell you.
The Summer Solstice is the moment when the axis of rotation of the Earth points directly towards the sun. That happens today at 10:16 am PT.
So hike yourself up to Gas Works hill today, stand yourself at the right spot on the dial, take a look at your shadow and wish the city around you a Happy Solstice.
1 Remember, though, that sundials can’t adjust for daylight savings time, so it will be off your watch by one hour throughout the summer.
(Gas Works Sundial photo by Joe Mabel)