Celestial navigation with Navigator for iPad
In this first celestial navigation day I had to get acquainted with the stars and planets suitable for observation at morning twilight time. The date was 03/dec/2013. Assumed position was 18░N / 36░W. Time zone +1. Sunrise twilight time was 7:17 local. In order to obtain this information, select the Sun and than click [Object data]. It shows sunrise, sunset, meridian passage and the twilight times.
Tap the Star Finder page. Tap [Pos/Time] button to set star finder position and time. Set approximate position, date, time and time zone ( in this case, time zone was +1 hour. Use positive to West longitudes). Tap [Calc!] button. This will show the sky chart below.
This is my notebook page for 03/12/2013.
Before entering the observations, go to the Astronomic
Position page and click [Clear All].
Tap the Line of Position page (use buttons in the page foot to navigate between app pages).
First set the Sextant corrections. Click [Sextant
Click [Sextant Corrections..] button again to close.
Enter the first observation: Spica / time
7:10:00 / altitude 37░ 10.2'
Click [Calculate LOP] when finished typing the altitude.
Tip: After each LOP, click [Save LOP] to save the observation for Astronomical Position calculation.
Second observation: Arcturus / time 7:13:35 / altitude 36░ 40.3'
Third observation: Sirius / time 7:17:00 / altitude 24░ 21'
Last but not least: Jupiter / time 7:22:25 / altitude 44░ 18'
I also took a LOP for Procyon, but end up deleting it, as it seemed misplaced and was almost parallel to Arcturus. Anyway, since I had 4 good lines already, I decided to delete Procyon.
Then I tapped the Astronomical Position page.
Four LOPs saved. I clicked [Calculate Position] and got the result shown below.
The calculated Astronomical Position was 17░43.3' N / 38░13.7W.
The real (GPS) position was 17░38.8'N 38░14.3'W, an error of about 4.6 NM.
The Running Fix
The next set of observations is from date 05/dec/2013. This one is a Running Fix using Sirius twilight altitude combined with two Sun observations made during that day. The whole observation extended for 9 hours, so the boat movement during that period had to be considered, by transporting the early LOPs to the latest time.
Time zone increased to +2 hours. Assumed position was 17 N / 44 W. Used the same sextant corrections as previous observation.
First LOP: Sirius at morning twilight time: 6:49:10 local / altitude 21░05'
Second LOP. Nice and hot tropical Sun at 10:48:25
local time / altitude 40░ 35.6'
Last LOP. Again the Sun , lower limb, almost 4 PM
Below is the resulting Astronomical Position calculation.
Note the Auto transport option (top-right below) is set to [x]To latest LOP time. This means that all LOPs are transported to latest time and the calculated Astronomical Position is for 17:55:40 GMT. For the transport to work, I had to inform average boat speed ( 8.5 Knts ) and true course ( 270░ ) during the period.
The calculated Astronomical Position was 17░05.8' N / 45░19.8W
at 15:55:14 local.
On 06/dec/2013 I did two sets of observations:
The first set was at sunrise twilight (around 6:50 AM local). Time zone was still +2.
Below are the LOPs:
In the end, I choose to exclude Arcturus and Capella from
the final calculation.
The calculated Astronomical Position was 16░43.1' N / 47░10.3W.
The real (GPS) position at 7:00 was 16░43.8'N 47░08.4'W, an error of about 1.8 NM.
Latter on that same day I made a Running Fix for the Sun with 2 observations.
Again, since this is a running fix, I used auto transport, with our boat sailing at 7.2 knts in direction 270░T.
The calculated Astronomical Position was 16░42.6'
N / 48░01.3W at 15:04:55.
Next set of observation was taken on 08/dec/2013. Another running fix.
By this time, the time zone had increased to +3 hours. I took two Sun sights and one Moon sight.
Mid morning Sun:
Sun in the afternoon:
Moon sight. Since the Moon disk was incomplete, I choose to read the altitude of the upper limb of the Moon. Make sure to change that in the Sextant Corrections panel before calculating this LOP.
Take a look at this chart. The blue lines are the original LOPs. The red ones are the transported LOPs. Note how the 3 red lines form the position triangle, with the astronomical position on the center. The calculated Astronomical Position was 15░32.3' N / 54░24.4.3W at 14:46:45. The real (GPS) position at 14:36 was 15░29.3'N 54░23.8'W, an error of about 3.0 NM (to the S).
More celestial navigation information - www.tecepe.com.br/nav/
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