Casting Shadows on Time: Unraveling the Secrets of Sundials and Crafting Your Own

With the summer solstice behind us, our attention turns once again to the intricate dance of the sun and Earth – the celestial ballet that’s shaped human civilization for millennia. Among the oldest ways we’ve tracked this dance is the sundial, an elegant tool that harnesses the moving shadow of the sun to mark the passing hours.

The sundial dates back nearly 3500 years, with ancient civilizations from Egypt to China using them to divide their days. These primitive timekeepers consist of a flat plate and a gnomon, a rod or straight edge that casts a shadow across the plate. As the sun traverses the sky, the shadow moves, with its position indicating the time of day.

Creating your own sundial isn’t just a fascinating craft project—it’s a chance to connect with this ancient history and get hands-on with the principles of astronomy. To help you do that, let’s go step-by-step through building a simple horizontal sundial.

Materials:

• A flat, sturdy board or large, flat stone
• A straight rod (the gnomon) – the length will depend on the size of your sundial base
• Marker or paint

Steps:

1. Place the rod upright in the center of the board. Make sure it’s firmly attached and perpendicular to the base.
2. Now, you’ll mark the hours. This step requires a bit of math. The position of each hour line is determined by the angle of the shadow, which is dependent on your latitude (θ) and the sun’s hour angle (H).

The formula for the hour line angle from the noon line (A) is:

A = arctan(sin(θ) * tan(H))

Let’s break it down:

• sin(θ): This is the sine of your latitude. You can use online tools to find the latitude of your location.
• tan(H): This is the tangent of the sun’s hour angle. The hour angle is the number of degrees the sun moves across the sky in an hour. The sun moves 15 degrees per hour, so for 1 o’clock, the hour angle would be 15 degrees, for 2 o’clock, it would be 30 degrees, and so forth.
• arctan: This is the inverse of the tangent function, converting the ratio from the previous operations back into an angle.

The hour angle H can be calculated as 15 degrees times the number of hours before or after noon. Positive values are used for times before noon, negative for after.

3. Use this formula to calculate and mark the hour lines on the sundial, starting from the rod’s base. The line for noon should be drawn perpendicular to the gnomon.

Now, while marking the hour lines, it’s important to note they all converge at the base of the gnomon, with each line radiating out at the calculated angle from the noon line.

Note: Ensure the gnomon’s angle matches your latitude, so the tip’s shadow will track along the noon line on the equinoxes.

So, what’s next? Once you’ve built and set up your sundial, you can start enjoying it. On sunny days, watch the shadow shift as the day progresses, and see how accurately your sundial matches up with ‘real’ time.

However, remember to adjust for daylight saving time and the ‘equation of time’—the slight discrepancy caused by the Earth’s elliptical orbit and axial tilt—if you want your sundial to keep time precisely.

Creating a sundial is a practical and interactive way to appreciate the science of astronomy and the ancient quest for marking time. It bridges the gap between art and science, making abstract concepts tangible. So, whether you’re a history buff, a science enthusiast, or simply looking for a fun summer project, why not give this a try? The sundial might not replace your wristwatch or phone any time soon, but the act of creating it, understanding its principles, and seeing it in action is a rewarding experience like no other.