Ilan Papini s VIRTUAL SAILOR 7 TUTORIAL USING THE VIRTUAL SEXTANT WITH VS7

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1 1. Introduction Ilan Papini s VIRTUAL SAILOR 7 TUTORIAL USING THE VIRTUAL SEXTANT WITH VS7 By Frankie the Funky Sailor Frankie_the_funky_sailor@yahoo.co.uk 1.1 Purpose of this tutorial: The purpose of this tutorial is to give some information about using the virtual sextant (the Virtual Sextant ) designed by Ilan Papini for his Virtual Sailor 7 ( VS7 ), and how to use the information obtained with the Virtual Sextant. This tutorial will endeavour to be practical. The theory of celestial navigation and sextants will not be dealt with here. I will write a more exhaustive and comprehensive tutorial on that subject later. 1.2 why use a sextant: It may sound weird nowadays with GPS and satnav to want to learn how to use a sextant. The reason is very simple: the system can breakdown, there s no power or a nasty wave floods the electric system. What do you do in such a case? If you can t determine your position, you have a problem, and probably a big one. So, although Virtual Sextant will not teach how to use, in practice, a sextant, it may help and facilitate the learning process with an actual sextant!!! 1.3 how does it work: the Virtual Sextant and any actual sextant work according to the same principle: it measures the elevation of sun (or any other celestial body) above the horizon, such an elevation being expressed in degrees.

2 Practically speaking, Virtual Sextant and an actual sextant will bring the sun down to the horizon. If you take a good note of the time at which you measure that angle (also call the elevation ), you can derive extremely useful information about where you are at a certain time. 1.4 Notice: Users of the Virtual sextant and readers of this tutorial should bear in mind the following: (a) (b) (c) this tutorial will only deal with virtual sightings of the sun in VS7; other celestial bodies such as the stars, planets or the Moon will not be dealt with: it is much more complicated; it s also customary to begin with the sun, which is a lot simpler to use in celestial navigation; learning how to use the Virtual Sextant may facilitate learning celestial navigation or at least some of its theoretical aspects. However, one should not go at sea on the basis of the information contained or derived from this tutorial, but should satisfy him/herself that he/she has sufficient, proper and adequate training, instrumentation and equipment before and when going at sea, all in accordance with applicable laws and regulations; I am in no way an expert in this area and everything I know, I have learned it by myself, through my readings and personal experience (e.g. spending hours on the beach training and people looking at me thinking what a weirdo: is he that lost to use a sextant ON a beach??? ). Therefore, there may be some mistakes or inaccuracies or my approach may be unconventional. Please feel free to comment, criticise, discuss etc by contacting me at frankie_the_funky_sailor@yahoo.co.uk. Or by putting your questions on any good VS fori (once you get the authorisation og the webmaster!!!); let me know by and I ll have look and try to answer. 1.5 Structure of the tutorial: So without further ado, let s see, with a Virtual Sextant, what one can do!!!! The tutorial will be divided in two main sections: How do I use Virtual Sextant (section 2); How do I use the information obtained with Virtual Sextant (section 3).

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4 2. How do I use Virtual Sextant? 2.1 Description of the Virtual Sextant Screenshot 1 Most of you are familiar with Ilan s VS7. You can see that there is a new icon: it represents a sextant. Double click on it or press shift + T. Some of the Virtual Sextant s functions are similar to those of the Telescope: Most of you are also familiar with the presentation below (see Screenshot 2): the Virtual Sextant window is very similar, at least in terms of presentation, to the virtual telescope. It has similar functions: Azimut indicator: on top you can see the Azimut indicator: you operate it exactly as you do with the virtual telescope: right click and drag. You can also use the keypad (just like for the telescope function); Zoom: on the right hand side, you can see the zoom function. Again, this works exactly as the zoom of the telescope function; Vertical angle: You can also vary the vertical angle by right clicking and dragging the mouse (without clicking on any other functions) These are the only similarities. The differences are huge because a sextant is designed to measure the elevation of a celestial body over the horizon.

5 Screenshot 2 Other functions are very different: Elevation Knob: This knob is used to bring the sun down to the horizon. In section 2.2, I ll explain how to do this. As you turn the knob (and consequently bring the sun down to the horizon), the window called elevation shows the elevation of the sun (or whatever celestial body you may be sighting). Elevation is measured in degrees, minutes of arc and seconds of arc. Here the elevation is also indicated in degrees and decimal degrees. We ll see later on that this will avoid stupid (and fatal) errors. Filter: The filter is also a very important function of the sextant. On an actual sextant, the filter is used to protect your eye when sighting the sun and to reduce the glare: in order to do a proper sighting of the sun, you must see a perfectly neat circular disk and its edges without any glare. Digital Watch: Last but not least, the digital watch. You always have to note precisely, by the second, the time at which you do your sighting: one second of time can make a material difference in your result. Be precise!!!

6 It is extremely important to note the Greenwich Mean Time ( GMT ) when you do your sighting: in celestial navigation, you must never take note of the time in the time zone you are but only according to GMT. In future tutorials, we ll see how we can determine local time with a sextant, but for know just think GMT!!!! GMT is given in hours, minutes and seconds and also in decimal. This will avoid a number of silly but fatal mistakes. 2.2 Using the Virtual Sextant Step One Click on the Virtual Sextant Icon (See Screenshot 1 above and 3 below). Set the time and date at approximately 9:00 a.m. or 15 p.m. (around March or September of any given year, say 2006). For the first time, set the weather with 100% visibility, no clouds, no waves, no wind. This will be explained in more detail in the next tutorial. All you need to know at this stage is that you don t want the sun to be too high or too low for your first virtual sighting of the sun. Screenshot 3

7 2.2.2 Step Two Turn the filter knob and set it to 60/65% approx - See Screenshot 4. Screenshot 4

8 2.2.3 Step Three Set the zoom to 3 and turn the elevation knob to approximately 30 degrees (30 in this example only, depending on your position and time) - See Screenshot 5. Screenshot 5

9 2.2.4 Step Four use the keypad (4 or 6) or right click and slowly turn the Virtual Sextant visor until you see the sun appear - See Screenshot 6. In the (virtual) morning, you should look towards E or ESE (approx 120/130), in the (virtual) afternoon, WSW (approx 220/230). You should see the sun near the horizon slightly above (if in the morning) or below (if in the afternoon), in the sea. Screenshot 6

10 2.2.5 Step Five centre the cross of the Virtual Sextant on the sun so the horizontal red line coincides with the horizon; set the horizontal angle to zero See Screenshot 7 Screenshot 7 The sun s lower limb should be like one hair (i) below the horizon, if you re sighting in the virtual morning (don t forget, in the morning, the sun is going up towards the zenith); or (ii) above horizon if you re sighting in the afternoon (when it s going towards the nadir).

11 2.2.6 Step Six tangent the sun on the horizon see Screenshot 8 Screenshot 8 Right click and move the sextant in this arc like motion from left to right, not too fast and have your finger of your free hand ready to press F10. press F10 as soon as the lower limb perfectly tangent the horizon. On a piece of paper take a note of the elevation in both degrees and decimal and the time of the sighting (IN GMT!!!!) in hours and decimals. Here, the elevation is 35:50:32 at 15:47:00 Hours GMT; in decimals, and 15,7857 Hours GMT. (you may find a slightly different result than me, that s because I wrote the tutorial over several days and I couldn t remember which day I referred to initially) 2.3 Making it more real Once you have a little practice, you can set things in a more real mode: Increase the wave height; Don t lock the simulation with F10 when you do you re sighting; Keep on sailing, don t dock or anchor the ship; Use a real watch (just make sure it s set exactly on the same time as the clock of the Virtual Sextant i.e. GMT of Virtual Sextant)

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13 3. How do I use the information obtained with Virtual Sextant? 3.1 General This is where it all begins. Section 2 was the easy part!!! Let s get into the nitty gritty!!! The information obtained from a sighting - elevation and time of the sighting - will be used create a line of position. There s nothing mysterious or complicated behind this apparently esoteric term. What we will try to do is to find out how wrong our assumed position is. We can do this with the line of position. In order to use that information and create a line of position, we need to do 4 things: Create a (simple) cruise in order determine our assumed position after cruising according to the parameters of that cruise (section 3.2); Determine the coordinates of our assumed position AP (3.3); Carry out a sighting of the sun at AP, at or around the time we reach AP, with Virtual Sextant (section 3.4); Carry out certain calculations by using the information obtained with the Virtual Sextant (section 3.5); Draw the line of position (section 3.6); Do another cruise from AP to AP1, reproduce all the steps from section 3.3 to 3.6 but with a new sighting and find the intersection between our 2 lines of position but that will be the subject matter of the next tutorial. Simple, a piece of cake!!! 3.2 Creating a simple cruise Let s start a cruise, let s say we re in the middle of the Atlantic Ocean. So set your boat to carry out the cruise. DON T put any waypoints. You can use auto pilot if you want. First Leg of the Cruise Initial Point or IP 45:00:00N 025:00:00W Date of Cruise: 31 st March 2006 Course (on the compass) 225 degrees Speed Whatever you want (in my example I ll use 7 Knots) Cruise starts at : 07:46 am local time Cruise ends at: 9:16 am local time Avoid using the GPS or map function during the learning process. Just use it to position yourself at IP and then turn them both off. Set the time on weather with 100% visibility, no clouds, no waves, no wind, no current (we ll deal with current in the next tutorial). If you re using a sail boat, use the engine, not the sail: we ll make this as simple as possible; later on, you can add one or more layers of complexity. During the cruise, click on panel to monitor and control speed and course. Try to stick to the parameters of the cruise. Accelerate the simulation rate if you don t want to wait that long.

14 3.3 Determining the coordinates of our assumed position AP Step One - Distance. The distance sailed from IP to AP since 7:46 is calculated as follows: we sailed for one hour and a half at the speed of 7 knots. As 1 knot is 1 nautical mile an hour, the distance is 1.5 x 7 = 10.5 nautical miles Step 2 drawing the relevant portion of the map Picture 1 Take an A3 sheet of paper (if you don t have one, just take two A4 sheets and stick them; Indicate N-E-S-W as shown of the Picture 1; As we are going WSW, place IP on upper right hand side corner of the sheet of paper by drawing a horizontal line (latitude) about 3cm away from the top of the page and a vertical line (longitude) 3 cm away from the right hand side of the sheet;

15 the intersection of these two lines is IP write as shown on Picture 1 the coordinates of IP and the time at which you left; determine the scale of your portion of map: you must divided 1 by COSINE 45 which equal This means that 1 nautical mile on your chart is equal to cm. This also means that on your chart, a distance of 10.5 nm will be equal to10.5 x cm = cm. To calculate COSINE 45 you can use any scientific calculator: enter the 45, press COSINE. The reason you choose COSINE 45 is because you must take the closest round latitude: if for example your position was 57: 48: 32 N (or S, it makes no difference), you would choose 58 because it s the closest round latitude to yours; On the latitude line you drew, mark the longitude divisions: start on IP and divide that line by going East to West (or right to left) and mark one division every centimetre. I centimetre on your chart is one minute of arc of LATITUDE. So your portion of chart will indicate 25:00, 25:01, 25:02, 25:03 etc.. until you reach the left hand side of the sheet of paper; see Picture 2;

16 On the longitude line you drew, mark the latitude divisions: start on IP and divided that line by going North to South and mark one division EVERY CM: this is extremely important otherwise your chart will be wrong and give you wrong coordinates. So your portion of chart will indicate: 45:00, 44:59, 44:58 etc.. until you reach the bottom of the page see Picture 3.

17 Picture 3 then draw a line with a ruler and protractor or with a marine protractor that goes from IP down towards, roughly the lower left corner of the sheet (don t forget, we go WSW) with an angle of 225 if you re using the marine protractor or 45 degrees if you re using the ordinary protractor; Once you have the angle, draw a line (your course) with that angle from IP going WSW; the length of that line is equal to the value of I mile on your chart ( cm) times your

18 distance; this means that on your chart, 10.5 nautical miles represent cm make that line a little longer than 14.84cm, say, 17 cm - see picture 4; Picture 4 Draw a line parallel to the longitude line, across the sheet in such way that it crosses your course see picture 5; this will give you your assumed latitude: 44:51:30

19 Picture 5 Draw another line perpendicular to the one you just drew starting from the intersection of your course and the line you drew as per the above bullet point see picture 6;

20 Picture 6 The intersection of these 3 lines give you your assumed position AP and you should find the following coordinates approximately: 44:51:30 N 025:10:30 W.

21 IMPORTANT: DON T FORGET THAT WHEN YOU DETERMINE YOUR SCALE, YOU MUST DIVIDE THE UNIT OF LONGITUDE (E.G. 1 IN THIS EXAMPLE, OR ANY OTHER UNIT YOU PREFER, DEPENDING ON THE SIZE OF YOUR SHEET AND THE DISTANCE INVOLVED) BY THE COSINE OF THE NEAREST ROUND LATITUDE OF YOUR ASSUMED POSITION. THE RESULT OF THAT DIVISION WILL TELL YOU IN CM (INCHES WHATEVER YOU CHOOSE) THE VALUE OF 1 MINUTE OF LATITUDE WHICH IS EQUAL TO ONE NAUTICAL MILE. If you don t get that, drop me an . You can also calculate your assumed position with the formulae below. Let s try it and see what result we get. Please note that this formula only works for distances less than 300 nautical miles. Please open the Excel Spreadsheet Assumed Position Tab. Before I give the formulae, let me briefly explain what they will allow to achieve: we need to know by how much our latitude and longitude will vary as we navigate. Prima facie, this exercise may seem simple but, as you all know, the earth is not flat (if you have any doubts about that, well...what can I say...buy a trip to space and find out for yourself). So we need to take into account the fact the earth is a sphere. There are 2 formulae: one to calculate the variation in longitude and one for the variation in latitude. Variation in Latitude: l = (m/60) x COS course where l is the variation in latitude we are looking for, m i the distance in nautical miles and COScourse is the cosine of our course. In our example m = 10.5 nm, our course is 225 so COSINE of 225 = minus l = (10.5/60) x minus = x minus = minus Our Latitude when we left was 45, we travelled minus degrees in latitude towards south west (225), so our assumed latitude is 45 + (minus ) = = or 44:52:34N which is very close to what we get by using the chart method (44:51:30N). Don t worry about the difference: (i) it s less than a nautical mile so this is very good for an assumed position; (ii) both methods give in essence approximate results. PLEASE NOTE THAT AS WE ARE GOING SOUTH, THE LATITUDE DECREASES. DON T GET THIS WRONG OTHERWISE YOUR RESULTS WILL BE WRONG. IF YOU DON T GET IT, DROP ME AN . Variation in Longitude: g = (m x Sin course) divided by ( 60 x COS delta latitude), where g is the variation in longitude we are lookng for, m is the same as above i.e. distance travelled, SIN course is the sine of our course and COS delta latitude is the cosine of of l we found above. In our example, we get the following result: g = (10.5 x SIN 225) divided by (60 x COS minus ) = (10.5 x minus ) divided by (60 x ) = (minus ) divided by (59.99) = minus However WE ARE GOING WEST SO OUR LONGITUDE IS. DON T GET THIS WRONG OTHERWISE YOUR RESULTS WILL BE WRONG. IF YOU DON T GET IT, DROP ME AN . Our longitude when we left was 25W, we travelled minus in longitude towards south west (225) so our assumed longitude is = = or 25:07:25W which is very close to what we get by using the chart method (25:10:30W). Don t worry about the difference: (i) it s about 3 nautical mile so this is very good for an assumed position; (ii) both methods give in essence approximate results. Use whichever method you like. I personally use the chart method when I m actually sailing becasue honestly, doing all these calculatin when you re sailing, just makes me seasick: the chart is quicker and faster, so no risk of vomitting all over the cabin!!!!

22 But this is not a problem, because our sighting, which will allow us to determine our line of position, will tell us quite accurately how wrong our assumed position is. This is the whole point of using a sextant!!!! 3.4 Sighting of the sun at AP To do this, simply follow the procedure set out in section 2.2. For your first virtual sighting, I d recommend you stop the boat, put the anchor down and use the wheel of the mouse to get out of the boat so the structure of the boat will not bother you when doing your sighting. When the sun is tangenting the horizon as explained above, press F10 and take a note of the elevation of the sun and the time of the sighting in GMT. You should find approximately: Elevation: 30:55:17 degrees and 10 hours : 56 minutes : 40 seconds GMT Elevation: 30:55:17 GMT : 10:56:40 Now we will do a few calculations. 3.5 Carrying out certain calculations Screenshot 9 We need to calculate 2 values: (i) the elevation we would get if we really are located at AP and (ii) the azimuth of the sun. The above elevation will be referred as to the Assumed Elevation ( ASE ), the one actually measured, the Actual Elevation ( ACE ); the azimuth of the Sun, Az. The idea is that if our Assumed Position is correct, in theory the difference between ASE and ACE should be equal to zero (although, in practice this is never the case). The smaller that difference is and the closer to AP your Line of Position will be. As you will be somewhere on that Line of Position, the closer it is to AP and the more accurate is your Assumed Position.

23 The tables below (from the French Almanach du Marin Breton ) will allow to do the relevant adjustments to the instrumental elevation. Calculation of ASE and Az To do this, we need to use the following formulae: First formula: to calculate the ASE: ( SinALAT SinD) + ( CosALAT CosD CosGHA) SinASE = Second formula: to calculate Az or the Azimut of the sun or the direction in which we see the sun when doing our sighting at AP at that particular GMT: CosAz = ( SinALAT SinASE) SinD CosALAT CosASE where SinASE means SINE of ASE, SinALAT means SINE of assumed latitude, SinD means SINE of declination, CosALAT means COSINE of assumed latitude, CosD means COSINE of Declination and CosGHA is the angle of rotation of the Earth from midnight GMT until the GMT of the sighting; Cos These formulae are not as scary or complicated as they seem: all you need to do, is to be precise and follow the steps. You can calculate them by hand or use the small Excel Spreadsheet I prepared. You can also download a whole bunch of freeware providing these calculations. I usually do it by hand even at sea but for my next trip, I ll bring my laptop: it should be ok as I don t need alot of power to run the calculation with the laptop. The important point is to be able to do it by hand EVEN if instrumentation fails you!!!! Please open the Excel Spreadsheet Line of Position Tab.

24 Please note that in practice your intercept i.e. the difference between actual elevation and calculated elevation, should be very small, in any event, less than 30 miles or 30 minutes of arc. You will notice that when doing sightings and lines of position with data different than that used in this tutorial, you may find much larger differences. The reason is that I m not 100% sure of all the parameters of VS celestial mechanics and there might be slight differences that I may have not taken into account. However, this is not really important; what s important is that one learns how to position itself with a sextant; I will endeavour to fix very soon my excel spreadsheet. In the meantime play around with the GHA angle at 00:00. The reason is that in the actual world the earth doesn t rotate exactly by 360 degrees in 24 hours. In order to correct this, what I do is to vary that angle. It seems that for morning sightings, that angle is somewhere between 170 and 180 and in the afternoon between 180 and 190. Try to find the GHA at 00:00 which will give you the smallest intercept, but only go by 0.5 degree increments. The target function in excel would allow to find the exact difference but if you do that, you are just making the whole exercise pointless: mind as well use the GPS!!! Keep a reasonable difference, roughly 10 nauticals, and then practice doing your line of position. This is not a very orthodox way to proceed, I entirely agree, but this will allow you training using the sextant until i sort this out once and for all (I hope!!!). I thought however that it would be just fair to mention it. 3.6 Drawing the line of position Now that we have the ASE - Assumed Elevation - and Az the azimuth of the sun we can draw our line of position as follows:

25 3.6.1 Step 1 Determine the Intercept. Compare the value of ACE (Actual Elevation) and ASE (Assumed Elevation). In our case, ACE is greater than ASE. This difference is called the intercept. In some cases, ASE is greater than ACE. Calculate that difference by doing ACE minus ASE = = 0.05 degrees or 2.75 minutes of arc or 2.75 nautical miles approximately (the Intercept ) (please see decimals for rounding). IMPORTANT POINT TO KEEP IN MIND WHEN DRAWING YOUR LINE OF POSITION: if ACE > ASE (or if the intercept is positive) that means you are closer to the sun than you though; otherwise you are further away from the sun Step 2 Draw a line that starts from your assumed position AP orientated towards the azimuth of the sun and in the direction of the sun:. Take the value of the Intercept: it is equal to approximately 2.75 nautical miles. Go back to paragraph and use the scale you determined there: 1 nautical mile on your chart is 1.4 cm. Measure a distance of 2.75 x 1.4 = 3.85 cm from your Assumed Position goint towards the sun i.e. in its direction - see below: Step 3 draw a double line perpendicular to the Intercept, about one nautical miles on each side => this line is your line of position and your are somewhere on that line. You can check this by opening the GPS window in VS: the GPS coordinates should (ideally) be somewhere on that line, but in practice it is always NEAR that line.

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27 4. Conclusion So what happens next? Well, you were able to check your line of position was (more or less) correct BECAUSE you were able to use your GPS. This is what I would do in real life. But whether or not you can check with a GPS, ALWAYS BEAR THIS IN MIND: Your line of position will only make sense if the 3 following conditions are met: (i) latitude is less than 60 degrees (so near the poles, things are different); (ii) elevation is less than 80 degrees (for sighting near the equator or in the tropical zone, we use another technique we ll talk about this in future tutorials); and (iii) the Intercept you find is less than 30 miles (if it exceeds that value, start over your calculation but use as your assumed position, the point where the intersection of the Line of Position and the line going towards the sun); You keep your first line of position, because in the next tutorial, we ll learn how to determine precisly our position by using the Line of Position we just made with the one we ll determine during the next leg of the trip. We will make these 2 lines cross each other: their intersection is our position. Frank Sarfati aka Frankie the Funky Sailor London 7 th October 2006.