Thanks to everyone who is still persevering with the Celestial Navigation notes. I hope you have picked up a few things.
You might have noticed a couple of the videos that I have linked through to belong to the Vanderbilt University who have a fantastic free online Celestial Navigation course. They whip through the topics very quickly, but if you are feeling like you need a refresher, now could be a good opportunity to check in and test your understanding of what we have covered so far. Click here to go to the course.
Today we are going to look at pulling everything together and actually plotting our position on a chart.
- We obtained and corrected a sight from our sextant.
- We used the Almanac to find out the Declination and GHA of the sun at that exact time.
- We then used our assumed position to calculate our LHA – or position of our boat in relation to the ground position of the sun.
- With our assumed latitude, LHA, and declination of the sun we entered the sight reduction tables to work out the bearing of the sun and the computed altitude from our position.
- Now in this post we use the bearing of the sun from our position – Azimuth angle and then compare the difference between our sextant angle with the one we got from the sight reduction tables – the difference is in nautical miles and our line of position can be drawn intersecting the angle line.
If you haven’t yet done the Vanderbilt University Celestial navigation online course. Here is one of their fantastic videos which will give you a pretty good understanding of what we are trying to do here.
I have watched this video now a number of times and while they run through it very quickly, it really does clarify what we are trying to do here.
As mentioned in the video, you will need a plotting sheet and your completed sight reduction form. You can get the forms from this blog post: Equipment Required
Step 1 & 2. First of all you need to mark up the plotting sheet with the latitude and longitude of your area – using your DR or AP position as the middle of the sheet. The video gives you instructions on how to do this and how to use the longitude scale on the sheet to measure and then draw in your lines of Longitude.
Remember that the numbers on your plotting sheet will increase in different directions depending on whether you are in the Northern or Southern Hemisphere or in the Eastern or Western Longitudes.
Step 3. Add your Dead Reckoning – DR and Assumed Position – AP to the chart and label them accordingly
Step 4. Using the Azimuth angle you got from the Sight Reduction forms, use the compass rose in the centre of the plotting sheet to measure the angle and then transfer that line to your estimated position towards the ground position of the sun.
Step 5. Then you calculate the difference between the corrected angle you observed on the sextant – Ho and the angle that you obtained from the Sight Reduction table. The difference in minutes of an angle are equal to nautical miles.
You measure the miles from the numbers running up the middle of the compass rose.
You can use the mnemonic Ho-Mo-To – if the Ho (your corrected sextant angle) is More than the computed angle (from the sight reduction tables) then the position line is drawn Towards the sun (or celestial body). If it is less than, then it is drawn away from the celestial body. You draw this line at a 90º angle to the azimuth line pointing towards the sun.
You are somewhere on that red line, and now need another position line – or ideally three lines to intersect to give you a position fix.
You can get your next position line from another sun sight later on in the day by advancing your first line as mentioned in this post. This is called a sun-run-sun position fix. Alternatively you could get a fix by using three stars around sunrise or sunset, or the moon which can also be seen during the day. We will learn more about how to do that in the next post.