A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bill Ritchie
Date: 2019 Feb 10, 15:04 +1300
What a joy to see such an excellent set of observations. Thank you for publishing them. They reveal a very interesting topic – perhaps this email should be entitled, with apologies to an ancient Greek sculptor, “Observing Venus’ limbs”?”
Can you recall, Ed, how you saw Venus during those sights with your 7 x 35 scope? Her phase was about 60% with a SD of 0.164’. Did you have sufficient definition to have been able to use her illuminated (far) limb rather than the centre of light?
The reason I ask is that I am doing some checking of “Astron Version 2.1” prior to its impending release in its first non-Beta version. I used your 10th lunar sight, Venus on January 29, 2019 from 28°41.1'N 082° 18.2'W at 11:20:09 GMT with your OLD (after IC of +0.2’) of 24° 47.5'.
First, I checked your result against Frank’s Lunars Calculator. Tweaking his “Distance” input to give an “Error in Longitude” of exactly 0° 0.00’ resulted in 24° 47.463’. [Images 1A & 1B]. That’s very close to your 24° 47.5’.
Using Astron’s option to show 2 extra decimal places, that 24° 47.463’ gave a GMT of 11:20:20, 11 seconds late, together with a longitude difference of 2.8’ West. That was with Venus’ limb set to “Far”. [Image 2 & 2A]. With her limb set to “Center”, the GMT became 11:20:00, 9 secs early and a longitude difference of 2.2’ East. [Image 2B]. This prompted me to restudy Frank’s excellent post of Sep 24, 2018, 12:14 PM showing that Venus’ centre of light is always about 0.1’ from her illuminated limb. Indeed, with a far limb value of 24° 47.551’, (corresponding to an offset of 0.088’ from Lunars Calculator’s zero position), Astron gives your exact GMT of 11:20:09 and a longitude within a few yards. [Image 2C].
However, in case the above was a glitch in my software, I made a further check with a stellar lunar to isolate the body limb issue. I used a hypothetical Spica lunar at the same location and instant. Using the same 2 extra decimal places, this agreed with Lunars Calculator exactly. [Images 3A-D].
Astron is unusual in offering a choice of limbs for the body as well as the Moon. With your lunars experience, and having programed four different lunars algorithms, how would you recommend that I treat the limbs of planets? I could add ‘center of light’ as an additional option or make it the only option.
Usually 50N 003W, but presently 38S 176E.https://www.vigilanceofbrixham.co.uk/Astron/AstronUserNotes.docx