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Re: Sun sights during an eclipse: "bad limb" calculation
From: Geoff Hitchcox
Date: 2023 Oct 6, 18:46 -0700
From: Geoff Hitchcox
Date: 2023 Oct 6, 18:46 -0700
Lars, I have tinkered a little more with this problem - and thought you might find my results interesting.
I wondered if refraction made any difference, so I ran my "simple test" (using JPL Horizons to create the ephemeris), using their refractive model, and then their "airless" model. As you can see it made no difference to the resultant times.
2023-Oct-14 17:07:48 = Geoff's "simple test" (Using Refraction with Alt).
2023-Oct-14 17:07:44 = Frank's estimate using Stellarium
2023-Oct-14 17:07:48 = Geoff's "simple test" (No Refraction with Alt)
2023-Oct-14 18:20:40 = Geoff's "simple test" (Using Refraction with Alt)
2023-Oct-14 18:20:36 = Frank's estimate using Stellarium
2023-Oct-14 18:20:40 = Geoff's "simple test" (No Refraction with Alt)
I'm still out by 4 seconds.
To try and find out why, I would love to know where Stellarium thinks the Moon and Sun are at the following.
Assumed position.
Latitude: 28.0° N
Longitude: 90.0° W
Altitude: 0m
Time Zone: 00:00 E
JPL Horizons give the following positions for an "airless" atmosphere.
Date__(UT)__HR:MN:SC.fff, , ,Azi_(a-app), Elev_(a-app), Ang-diam (arc seconds),
2023-Oct-14 18:20:40.000,*,m, 193.862553, 52.525591, 1828.926, MOON position
2023-Oct-14 18:20:40.000,*,m, 194.280049, 52.793284, 1924.160, SUN position
My "simple test" is currently:
delta_az = (sunLL_az - moon_az) * cos(((sunLL_alt+moon_alt)/2.0) * PI / 180.0);
distance = sqrt( pow( (delta_az), 2 ) + pow( (sunLL_alt - moon_alt), 2 ) );
if (distance <= moon_radius) // Moon is covering the SunLL
After spending many hours trying to compile Stellarium on my LINUX system, I cannot run the desktop version. So the only way I can use Stellarium is to use the online version However this does not allow one to wander too far 'offshore'. I cannot go to the above coordinates that Frank chose. The online version of Stellarium has a great interface to change your location, either using your 'location' or a named place. So I first chose "New Orleans" and then dragged the location icon due South, down to "Grand Isle Beach", but I cannot drag this a further 137Km to Franks given coordinates. Not sure if this is a limitation of my version of Chromium or one must never go out to sea (for there be dragons) ;-)
So I wonder if the above "4 second difference" to Frank's numbers, is due to:
1. Delta_T issues.
2. It's just not the done thing, to use "flat" Pythagoras Trigonometry on a *Spherical Trigonometry* problem!
Regards, Geoff Hitchcox, Christchurch, New Zealand.
