iMariner 2.3 Quick Start Guide

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iMariner 2.3 Quick Start Guide - English

A. Settings : to be done the first time only

1. Enter the eye elevation in meters : in a sailboat this can be usually 2.5m
2. Leave the Body, the horizon, the limb at the defaults values
3. Enter the Sextant's index error (in minutes of arc)

B. Observation of the Sun

1. Select the 'Sight' tab
2. Enter the date and time of the observation (Universal Time or Greenwich Time) (you need a very good watch and you must known the exact
number of seconds your watch gives or loss)
3. Enter your estimated latitude and longitude : relatively near your position, but choose these values to be in your current marine chart, for example the intersection of a medidian and a parallel.
4. Enter the Observed Altitude : the value read from your sextant
5. Press the 'Reduce' button
6. the app will display two informations : the Intercept and the Azimut.

7. Plotting : take your marine chart. Draw a single line from your estimated position in the direction of the Azimut.
This line indicate the direction of the Sun.
In this single line, report the number of Nautical Miles given by the Intercept.
Important : if the Intercept is positive, you are much near the Sun
If the Intercept is negative, you are much far the Sun.
Draw the Point corresponding of the Intercept in the line.
From this Point, draw a double line perpendicular of the single line.
Your boat is somewere in this double line. But where ? We can't determine your exact position. This is exactly when
you determine your Sight with a lighthouse : you need *at least* 2 observations to have 2 lines, cutting in a Point (and 3 is better).
The same is true with the Sun. You only have 1 observation. The second one can be with the same body (Sun) 3 or 4 hours later.



C. An example
Try to reproduce this example.

This sight was made the Aug 8, 2011 @ 16h33m42s UT.
We were in a trip from Kinsale (Ireland) to the coasts of France.
Our estimated position is 51°N / 8°W
My eye elevation was 1.5m, watch error=0 (really precise Swiss watch !), index error=0.
The observation of the Sun give : hi=31°32,5'

Entering theses values in the application, we will read : Intercept=-3.5Nm and Azimut=256°


D. An example with stars

It was the Aug 8, 2011.
I prepare the twilight observation in the afternoon, using the 'Star Finder' tab of the application.
I enter 51N / 8W and the date and displays the stars available at the nautical twilight.
From the list : Mirfak, Deneb, Altair, Vega, Moon, Arcturus, Alcaid, Alioth, Dubhe
I choose 3 bodies :

Deneb : Az=089° Alt=17°
Arcturus : Az=259° Alt=33°
The Moon : Az=197° Alt=14°

These informations are usefull to prepare the sextant and myself, because we don't have really much time to take these 3 sights.
So, at 20:50UT I'm ready.
The 3 sights are taken :
Deneb : 21h04m11s hi=60°0.0'
Arcturus : 20h59m18s hi=39°32.5'
Moon : 20h55m28s hi=14°20'

Now to the calculations :
Go to the 'Sight' tab.
Found for Deneb : Intercept=15.2Nm Azimut=083°
and for Arcturus : Intercept=-11.5Nm Az=250°
and for the Moon : Intercept=-3.1Nm Az=187°



For any questions, please contact iMariner.Contact@gmail.com


Recommended readings :
RYA Astro Navigation Handbook - Tim Bartlett (www.rya.org.uk)