Wednesday, 6 September 2017

Blinded by the Light (of the full Moon)

My first attempts at glimpsing the nearly full moon through the Digges-Bourne telescope were a dismal failure. Too early in the evening against a blue sky and there was insufficient contrast to show any surface detail on the Moon. Too late and the Moon against a dark sky was far too bright for my camera and I ended up with a badly flared image.

However, inspiration came from an unlikely source. I found a copy of Della Porta's 1593 book "De Diffractione Optices" on line (http://lhldigital.lindahall.org/cdm/ref/collection/color/id/34799) and on page 86 was the image below.



In Della Porta's book he describes how a concave mirror and convex lens can be used to produce magnified images so by moving further away from the mirror I could enlarge the image. This is shown above with the two diameters EF and CD. Della Porta had worked out the theory of the telescope nearly 20 years before Galileo but unlike Galileo he never built a telescope.
On 5th September there was a clear sky in the early evening. I captured the Moon with my Tudor telescope when it was just over the horizon around 8pm. I first held my phone camera close to the telescope mirror to get it in focus and within the field of view as shown below



and then I slowly pulled the camera lens back until the image of the Moon filled most the screen.
I cropped the image and inverted it to show the Moon the right way around.

The Tycho crater is clearly visible at 8 O' Clock as are two others (Copernicus and Aristarchus). Of course in the Tudor era no one used the term craters and even Galileo in his "Sidereus Nuncius" in 1610 mentions only "cavities with dark walls" and "ancient spots" on the Moon. In the event of someone in the Tudor period pointing a Digges-Bourne telescope at the Moon the figure above shows exactly what they would have seen.

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