Today is the winter solstice! It’s a time for optimism—from here on, the days will be getting longer.
At 12:11pm EST today, the precise time of the winter solstice, the sun angle is more than 45 degrees lower than on June 21, the summer solstice. Because of this angle, it goes below the horizon earlier than on any other day. Today will be almost 5 hours shorter than the summer solstice.
These two simple factors, the sun angle and the length of day, have impacted building design since Woodhenge (Stonehenge’s little brother). I’m probably a bit of an astronomy fan because of it. The solstice reminds me of extremes, how buildings are deprived of heating sunlight during cold winters and subjected to an enormous amount of heating radiation during hot summer days. The astronomical reality makes quite the impact on energy usage and utility bills during both seasons. Especially when it isn’t considered.
Fortunately, we can design the building envelope to use solar angles in our favor. Simply considering them can both inform design, make more comfortable spaces, and save energy. Since the passive energy design studio I took from John Nelson at UNC Charlotte back about 1988, it is interesting to see this correlation play out in practice. I strongly recommend Sun, Wind, and Light: Architectural Design Strategies by G. Z. Brown for anyone interested in an introduction to these environmental design concepts.
As a personal example, aiming a heavy brick screen with small punched windows toward the southwest in my design of the medical office building resulted in HVAC equipment an entire size smaller. So it saved money on the construction of the building as well as all future cooling costs. Plus the smaller windows reduce the southern glare for the occupants inside. That’s a lot of wins without any drawbacks just by considering the sun at the beginning of design.
Today makes a great day to illustrate exactly how low the sun really is—with an accurate sundial. The typical sundial can be inaccurate up to one or two hours depending on several factors. So I designed one inspired by the Brown book1.
In the instructions below is a specially tuned sundial you can download. It is tuned precisely to the center of Raleigh-Durham. (Technically, Latitude 35.8° and Longitude -78.8°, exactly at the Raleigh Chinese Christian Church in Cary, on Hwy. 54 just west of NW Maynard Rd.)
It is also tuned to the wobble of the earth, which varies the shadow of sun quite a bit throughout the year. As you can see, the curve of the red and magenta lines indicates this correction. (If it didn’t wobble, these would be straight lines.)
This sundial is also based on standard local time, meaning it is adjusted to Raleigh-Durham for the sun’s position within the entire width of the Eastern Time Zone. (Which extends from Maine to Western Michigan.)
Finally, it is set to Eastern Standard Time. The United States has been adopting some form of clock adjustment since World War 1 in the implementation of Daylight Savings Time. This means that from March 9 and November 2 you’ll have to add an hour for the dial to read correctly.
One final tip: For accurate readings, the sundial must be dead flat and pointed toward true north, as in the North Pole and the North Star. But not magnetic North, such as with a compass or smartphone, because here in the Triangle it will be 9 degrees farther left than true north due to magnetic declination. This is the considerable gap between the magnetic axis and the rotational axis of the earth, which can equate to about a month and an hour to a sundial.
- Download my accurate Triangle sundial (PDF, 123k).
- Cut the tip of a toothpick to the length for the gnomon indicated and attach.
- Level the sundial surface with a bubble level. Accuracy matters!2
- Point the sundial to True North. (With a compass, True North will be 9° right/clockwise from Magnetic North.)
- In March-October, subtract an hour to correct the 1 hour addition by Daylight Savings Time.
Accuracy is affected by the length and straightness of the gnomon, the levelness of the dial, and how close alignment is to True North. Please comment below or contact me if you can’t figure it out, I’m more than happy to help!
- My sundial calculations were generated with the Sundial Program from the University of Oregon Solar Radiation Monitoring Laboratory.
- Smartphone apps can work in a pinch, but a true level is more accurate. Same goes for the compass. I recommend the Swiss Army Knife on Android for the phone. Please send me an email or comment below to recommend a good one for iOS.