NavList:

Getting back to that sundial discussion... Thanks to Chuck V and Adrian F for recent detailed contributions!

Chuck Varney, you wrote:
"The question is what might be the source of an error averaging about 4.5 minutes fast over the range of hour angles and solar declinations that existed when this set of photographs was taken? One possibility is that if the set of time marks on the equatorial ring were rotated 1.125° clockwise, when viewed from the south, it would increase the displayed times by 4.5 minutes. Physically, what would 1.125° look like?"

I think a rotation of the scale like that would be fairly obvious, and I don't see one when I look at this armillary sphere. But perhaps a slight rotation of the whole sundial? That might produce a similar error, at least for the few points we have available... I'm tempted to get more photos, but we have had long spells of cloudy, rainy weather recently. Longer term, in the decades that have passed since the sundial's location was selected, little saplings have become great trees, and the thing is in shade for many hours of each day. After the leaves fall, it might be worth getting more data points [for our entertainment, of course].

You also wrote:
"On 25 July I sent this query to the Mystic Seaport Museum: “At your museum there’s an armillary sphere sundial near the Treworgy Planetarium. Can someone tell me what the diameter of the sphere is?” Less than 45 minutes later I received this reply: “Thank you for the email. We just had a member of the planetarium team measure. It is approximately 42 inches in diameter.”  (A prompt thank you was sent to the museum folks for their help.)"

That's a fun story! This past Thursday I stopped by the planetarium to say hello, and based in your story here I asked 'who measured the armillary sphere?' No one seemed to know. So that suggests a unique and accidental "favor" --accidental in the sense that someone in museum communications had a few minutes to call over to the planetarium, and whoever was in the planetarium office had a few minutes free time to step outside with a tape measure and get a number. I'll try again to find out who made the measurement for you and get a photo. :)

I haven't had a chance to sit down and look at the math you've added to the puzzle. It would be nice to test it out. In my 'sundials' workshops at Mystic Seaport, we build small, functional armillary sphere models. They can be easily set to any inclination and orientation, so it would be possible to get some crude data. Maybe good enough to assess the mathematical fundamentals...

Thanks again,

Frank Reed