Post by Ron Walker on Apr 5, 2024 11:45:56 GMT -7
With the recent discussion of Henry C. King's book, "Geared to the Stars", I must admit that I had kept my viewing to the last chapter (21) on "Stars by Optical Projection" as my interest was in planetariums and not necessarily clocks. Since Orreries do interest me I decided to peruse parts of the book I hadn't viewed before. One thing we all must admit is that the design of the Zeiss planet projectors and their accuracy has never been surpassed in a clockwork device. I marvel at the offset driving plates for Mercury and Mars so that the speed of these planets motions varies as they move about the sun in their more eccentric orbits (page 248 in the King book). Most of the other manufactures of projection planetariums have ignored this effect and gone for the circular orbit as it is easier and cheaper to do. Imagine my surprise and delight when I found my Minolta copied this mechanism from the Zeiss II literally verbatim. I doubt that any viewer of a show would notice the slight displacement caused by these different drives, but it is typical of the Zeiss engineers.
I also have found that many of the old clichés I grew up with are indeed coming true, "The more things change the more they stay the same" comes to mind. When the Zeiss VI came out this drive was improved upon by using two elliptical gears (page 364) making for an even more accurate positioning of the planet in the sky. While an improvement in the accuracy of the projected position, it is not a new idea. On page 173 we note the use of eccentric "drive gears" back in the 1700's.
So how accurate is accurate? While a projection driven by circular orbits would be something like an inch on my thirty foot dome, I can say the my Minolta which copied the Zeiss II design will hold the proper position of Mercury within two tenths inch on my thirty foot dome during the planets eighty eight day year. The new eccentric gears on the Zeiss VI would hold the anomaly within a tenth of an inch, less then the projected dot.
I also have found that many of the old clichés I grew up with are indeed coming true, "The more things change the more they stay the same" comes to mind. When the Zeiss VI came out this drive was improved upon by using two elliptical gears (page 364) making for an even more accurate positioning of the planet in the sky. While an improvement in the accuracy of the projected position, it is not a new idea. On page 173 we note the use of eccentric "drive gears" back in the 1700's.
So how accurate is accurate? While a projection driven by circular orbits would be something like an inch on my thirty foot dome, I can say the my Minolta which copied the Zeiss II design will hold the proper position of Mercury within two tenths inch on my thirty foot dome during the planets eighty eight day year. The new eccentric gears on the Zeiss VI would hold the anomaly within a tenth of an inch, less then the projected dot.