NavList:

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 !!!"

*******

Best Regards

Kermit

Antoine M. Couëtte

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