A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Date: 2014 Oct 24, 17:53 +0100
It gets better. I already have a good digital SLR and 200mm lens (zoom though ,not prime, does that alter things?) Maybe this ok for lunar eclipses but will probably need telescope for Jupiter moons. My 4 inch refractor also works as a 400mm prime lens, so I guess that would do for both.
What filters did you use for the 200m? Polariser +/- solar filter, not sure?
What software to work out the number of pixels?
How do you think the old guys (eg Cook et al) did it in the 1770s without photography and computers? Did they measure the thing real time with lens objective graticles or whatever they are called? Any information on those? I’ll probably try to do it the old way if I could find out how.
Anyway, very good fun. Just need the clear weather at the right time.(not easy in wintery, wet and windy UK. I suspect that is why we sent hundreds of ships south to find clear skies and warm weather.)
Thanks a lot, I’ll get to work asap on this.
A 200mm fixed prime lens on a 10 MP digital camera was used for the partial eclipse images. This lens has been previously calibrated for infinite focus so that each pixel = .0975'. The number of pixels from the observed Sun limb to the Moon limb shadow is 179 pixels. 179 x .0975' = 17.45'. An image taken of the Sun before the eclipse is attached and can be used to calibrate given the Sun diameter of 32.15' for the time and date of the image (CalSky web sight).
As for Frank's lunar calculator it is not clear what limb settings to set for a partial eclipse. I went with the setting that gave the more accurate result. The Moon diameter for this partial eclipse is 30.5' geocentric and 30.7' topocentric (CalSky) which is smaller than the Sun's diameter of 32.15'. There is then some asymmetry if calculating distances to nonilluminated or shadowed limbs. Maybe Frank can diagram how his lunar calculator handles the eclipse special case.
P.S. In a previous post I stated using a 400mm fixed lens which was incorrect. A 200mm lens was used.
From: Francis Upchurch
Date: 2014 Oct 24, 06:18 +0100
(Sun-Spot-Jupiter-Size.jpg: Open and save)