A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Antoine Couëtte
Date: 2011 Feb 15, 11:22 -0800
Hello to all,
On Saturday evening 12 Feb 2011, I had a first and long expected opportunity to shoot a Lunar from a (fast) moving vessel. Since my previous Jupiter Lunar ashore on Feb 09, 2011 went OK with only my wristwatch set in "elapsed time" mode, I decided to shoot the same kind of observations in the same following order as a few days earlier :
1 - Sun Height(s), with the Sun serving as a third body for a Fix LOP with good spacing in Azimuth with the Moon in order to remove all position ambiguity (one solution possible only with 3 heights), then
2 - Moon Heights, as usual in a Lunar, with the Moon also used for a Fix LOP, then
3 - Jupiter Heights, as usual in a Lunar, with Jupiter also used for a Fix LOP, with shots taken shortly after sunset (not too late to still have a "decent" horizon), and finally
4 - Jupiter-Moon Near Limb Distance (it is a Lunar after all ...), but not too late since when Jupiter becomes too bright I (personally) find it a bit more difficult to shoot a Lunar Distance.
I observed from the Navigation deck and the horizon was quite good, although not so sharp/well defined as ashore 3 days earlier. I started my wrist watch a few minutes before my Observations. Here are the data :
Feb 12th, 2011
Height of Eye : 6.40m/21ft , Outside Temp : 9°C/48.2°F , Pressure : 1011 mb/hPa / 29.85 In.Hg
All heights corrected for only Instrument error (Refraction, SD and Parallax need to be performed)
1 - For the first of observations our bottom course/speed were 255° True / 12.0 kts
I first shot 3 heights of the SunLL, averaged as follows : watch-time t1=00h08m54.6s , h1=7°46'.7
2 - At watch-time t=00h12m our bottom course/speed became 184° True / 16.2 kts
I then shot 4 heights of the MoonLL, averaged as follows : t2=01h26m57.9s , h2=61°02'.9, and
I then shot 3 heights of Jupiter, averaged as follows : t3=01h42m01.8s , h3=28°28'8
3 - At watch time t=01h47m our bottom course and speed became 135° True/ 22.0 kts
I then took 3 Jupiter-Moon Near Distances averaged as follows t4=01h50m21.4s , d4=66°12'9
Note : For those of you who wish to solve this one, when you look up a table giving the Jupiter-Moon Geocentric Centers Distance, you can see that :
For UT = 16h , Centers Distance = 65°14'4
For UT = 17h , Centers Distance = 65°45'5
For UT = 18h , Centers Distance = 66°16'9
Accordingly, and for the first approximation, you can simply start with UT = watch time + 16 hours. This will assume that the UT for averaged Lunar distance is 17h50m21s4, which seems a reasonable departure point.
We can therefore recap our observational data as follows :
***Initial course/speed : 255°/12.0kts***
UT11=16h08m54.6 s SUNLL h1=07°46'.7
***UT=16h12m new course/speed : 184°/16.2kts***
UT21=17h26m57.9s MOONLL h2=61°02'.9
UT31=17h42m01.8s JUPITER h3=28°28'.8
*** UT=17h47m new course/speed : 135°/22.0kts***
UT41=17h50m21.4s JUPITER-MOONNL distance D4=66°12'.9
I am quite happy with my calculation end results (0.5 NM off in Latitude, and a little less than 5 NM off in Longitude, which actually is almost as good as the previous lunar ashore). Certainly these results were very much "assisted" with the help of the Cruiser top-notch auto-pilot known to wonderfully follow prescribed tracks/speeds. For that reason, I would think that my "dead reckoning computation (in)accuracies" were certainly well below half a NM over this 2 hour period, although we had covered an overall distance of some 30 NM.
Observing such an excellent auto-pilot was certainly something quite new to me on a Sea Cruiser (almost as good as an Aircraft !!! ).
As regards new generation of equipment onboard, maybe some of you on NavList have some experiences of this kind to share with us. Jeremy ? other NavList Members ?
"Vive la Technique Moderne !!!"
Antoine M. Couëtte
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