Ocean College

mOCNAST (monthlyish Ocean College News Article about Sailing Theory)

Date: 8th of March 2021
Author: Fabian
Geographical position: On the way to the Azores
Position: 36° 21,4′ N 040° 21,6′ W
Total: 11.856 nm
Average speed during the last 24h: 6,46 kts
To go: 603 nm

Fabian erklärt

“Eine Sekunde Geschichte und wie wir nicht einfach in die nächste Meeresschildkröte krachen…” (daily report 03.02.2021) was the first article in the mOCNAST (monthlyish Ocean College News Article about Sailing Theory).

If you want to read about some interesting facts about sailing history, shanties and how we avoid crashing into the next sea turtle, go ahead and check it out. It provides some background information for more modern navigation systems, as well.

Astro Navigation

And what we do if the Americans decide to break the GPS

“In the 21st century, satellite technology is now ubiquitous at sea as elsewhere, and is the default technique by which almost every mariner establishes their position on the oceans.

Together with the increasing adoption of electronic chart display and information systems, AIS\footnote{test} and modern radars, the amount and quality of information available to the mariner has never been greater.

However, along with the increasing sophistication of the technology we all use every day, has come the realisation that such systems are not immune from problems, whether due to errors in a satellite or its ground control systems, defects in the equipment aboard ship, or interference (accidental or otherwise) with the satellite signals.

Even today, therefore, the celestial chart remains an essential tool for mariners navigating out of sight of land.”

(Vice Admiral Ben Key CBE – Fleet Commander, Foreword of “Astro Navigation – The Admiralty Manual of Navigation Volume 2”)

So, modern devices as GPS, Radar, etc. are great, but there is the risk of errors and interferences. Some of them are accidental, some of them are less. GPS was an invention by the American military and therefore they kind of have the control over it.

That means that you should reconsider your position on your little boat if it says that you are currently in downtown New York.

Basic definitions

Before we start with the actual calculation of our position, we need to know about some basic definitions and structures used for that. The problem at hand: Stars are not the closest objects in the world and their distances differ from only a few to multiple hundred light years.

To compare and calculate with them, we create a thought sphere (aka. clestial sphere) with the same center as the earth’s, but with a much greater radius. Think about it as a cherry: in the middle we have our little home and outside of it, we have the shell, the celestial sphere. Both have poles and equators as the celestial sphere is just a bigger version of the earth.

So, let’s get our sextant (the instrument to get the angle between the horizon and the object) and go out, but wait! We can’t see the stars… it’s 1300. So let’s wait until it’s dark. Sheet. Now it is too dark and I can’t see the horizon anymore, but how do we solve this problem?

When to use the sextant

There is a small period of the day called the Twilight in which the sun is just behind the horizon so that we can see the stars, but the horizon, as well. There are three different twilights: the Civil Twilight (CT), the Nautical Twilight (NT) and the Astronomical Twilight.

Now guess which one is most relevant to us (Clue: it is not the Astronomical Twilight). The Nautical Twilight is the time when the sun is 12° below the celestial horizon. As the sun rises in the morning and sets in the evening, there are two of them: the morning nautical twilight (MNT) and the evening nautical twilight (ENT). The timeslots for those can be looked up in the respective table found in another book called the “Nautical Almanac”.

Now the only question left is when to stand up for the nautical twilight? To solve this, there is a simpleish method. For that, you need the latitude time of your event (when it will happen on your latitude) that can be read in a table in the nautical almanac.

You need the longitude converted to time. For conversion, you can use another table found in the last pages and you need your timezone. From that, it is quite straight forward:

X————————————————————X
| Latitude time                         | 0524               |
| Longitude (W+ or E-) 037.33 °E)       | ~ – 2h 28 min      |
| Local Mean Time UT (GMT)              | 0256Z              |
| Zone(-x) (+ = subtract) (- = add)     | -4                 |
| Zone Time                             | 0656B              |
| Human Readable Time                   | 06:56 a.m.         |
X————————————————————X

So now we are ready to shoot for the stars and now we do actually know when to get out, but how do we actually do that?

How the sextant works

The sextant basically works in a way that you are able to move the stars with mirrors built in up and down. As with a microscope, there exist two wheels used for moving the stars as it needs to be quite exact as we want our position not to be +-100 nm. You position the star in a way that the under edge touches the horizon.

Then you note the time and the exact angle given by the sextant. You repeat it as often as you like (hopefully with different heavenly bodies as we need multiple to do something similar than to a 3-point-fix (further described in the previous article)).

Sadly, getting the angles and from that the position is not too easy. Therefore, I will take a break, get a cup of tea and relax (and hope that I don’t spill it everywhere as it is moving all the time). If everything goes right, I will post a follow-up in a week or two. Until then, have a good day (or night) and see you soon in the next episode of mOCNAST!

Best wishes to the navigator turtle under us and if we’ve done everything correct, we will see us in a few weeks when we will see that we are still sailing on the Atlantic (or in New York if we’ve made a mistake),

Fabian

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