All you Astro Navigators out there

[Independence]

I'm trying to brush up on my astro navigation.

1. Do I need to take both a sun sight and a sun meridian to get a proper fix?
2. Is there a web sight that might remind the how to plot the position lines on a plotting sheet? There seems lots out there on the calculations but not what to do with them afterwards!!

Many thanks

 

[Barnacle Bill]

have a look at the astro theory on www.aztecsailing.co.uk
Yuo can take two sun sights at different times, or the sun and another body (e.g. moon) to get two position lines that cross at a good angle.

 

[Wunja]

I'm trying to brush up on my astro navigation.

1. Do I need to take both a sun sight and a sun meridian to get a proper fix?
2. Is there a web sight that might remind the how to plot the position lines on a plotting sheet? There seems lots out there on the calculations but not what to do with them afterwards!!

Many thanks

How do you configure the sextant to take a web sight?

 

[Porthandbuoy]

Download this sextant user's guide: http://www.minitransat650.com/Sextant_Users_GuideV6.pdf

This will guide you through all that is needed to take sunsights and work out your position. The author takes some liberties with the sextant corrections, but it's the simplest introduction to astro I've found anywhere on the web.

You will need to buy an almanac every year which will give you all the ephemeris you need for Sun, stars and planets, or you can download my Sun only perpetual almanac here: http://www.4shared.com/dir/31216587/ad3b16e1/sharing.html

Sight reduction tables for your chosen latitude can be downloaded here: http://www.nga.mil/portal/site/maritime/?epi_menuItemID=8755b5dbee96c4327b2a7fbd3227a759

A sun-run-sun sight is the most common means of fixing your position. You take a sight in the morning, say a couple of hours before noon, and calculate a Line of Position (LOP). Take another sight a couple of hours after noon and calculate a second LOP. Transfer your morning's LOP by the distance and bearing run, and where the two LOPs intercept that is your position, give or take a few miles.

 

[sarabande]

"How do you configure the sextant to take a web sight? "

Have you never heard of a mirror site ? :0

 

[Ex-SolentBoy]

Download this sextant user's guide: http://www.minitransat650.com/Sextant_Users_GuideV6.pdf

This will guide you through all that is needed to take sunsights and work out your position. The author takes some liberties with the sextant corrections, but it's the simplest introduction to astro I've found anywhere on the web.

You will need to buy an almanac every year which will give you all the ephemeris you need for Sun, stars and planets, or you can download my Sun only perpetual almanac here: http://www.4shared.com/dir/31216587/ad3b16e1/sharing.html

Sight reduction tables for your chosen latitude can be downloaded here: http://www.nga.mil/portal/site/maritime/?epi_menuItemID=8755b5dbee96c4327b2a7fbd3227a759

A sun-run-sun sight is the most common means of fixing your position. You take a sight in the morning, say a couple of hours before noon, and calculate a Line of Position (LOP). Take another sight a couple of hours after noon and calculate a second LOP. Transfer your morning's LOP by the distance and bearing run, and where the two LOPs intercept that is your position, give or take a few miles.

Thanks for all that. Great links. Something for me to do this weekend.

 

[stephenh]

"You take a sight in the morning, say a couple of hours before noon, and calculate a Line of Position (LOP). Take another sight a couple of hours after noon and calculate a second LOP."


And for good measure you might take another one at noon to get your latitude !!!

 

[30boat]

I'm trying to brush up on my astro navigation.

1. Do I need to take both a sun sight and a sun meridian to get a proper fix?
2. Is there a web sight that might remind the how to plot the position lines on a plotting sheet? There seems lots out there on the calculations but not what to do with them afterwards!!

Many thanks

I can email you a plotting sheet if you need one.

 

[jdc]

plotting sheet

I agree with Porthandbouy that Sun Run Sun is all you need to do, now we have accurate wrist watches I find that noon fixes are an irrelevance. Frequently you can't see the sun at noon anyway!

I paste below an extract (it was in pretty Word document, but that hasn't come across here) of instructions I wrote on using an air tables equivalent I constructed, including making a plotting sheet. Since it's a snippet from a document it's a little disjointed, but maybe it's some use.
_____________________________________________________________

1. Measure the True Altitude
Using the sextant measure the altitude, applying all the normal corrections for parallax, refraction, height of eye and semi-diameter. This is the True Altitude, TA. Record to the nearest second the exact time at which the altitude was measured.

2. Look up Declination and GHA
At any given moment the sun is due overhead at one spot, and with a nautical almanac one can look up the Latitude (which is called Declination) and Longitude (which is called ‘Greenwich Hour Angle’) of this spot. These are usually tabulated every 2 hours and one has to interpolate for the exact time when the altitude was measured. One can use the tabulated Declination directly since it changes slowly, but must interpolate the GHA using the fact that the earth rotates 360 in 24 hours, thus 15 per hour. Use either a calculator or the GHA correction table in Reeds to work it out to the nearest second.

3. Choose a position to plot (CP)
For the purposes of a sun sight choose a position close to where you think you are but at a whole degree of Latitude, and at a Longitude such that the LHA (= GHA – Longitude) is a whole number of degrees.

4. Look up the Altitude and Azimuth
Use the tables! Now it’s obvious why the CP has been chosen in the arcane way described above; since Longitude and LHA are whole numbers of degrees one has to interpolate only for Declination, which I find I can do in my head. The altitude taken from the table is known as Calculated Altitude, CA. If the LHA is positive, one can use the tabulated Azimuth directly, but if LHA is negative Azimuth = 360 – tabulated Azimuth.

5. Difference the TA and the CA
This difference, in minutes, is the distance in nautical miles between the CP and the position line on which you actually are according to the sun sight; when TA is greater than CA your position must be closer to the sun than the CP – pretty obvious really, the closer one is to the sun the higher it is in the sky.

6. Plot the position line
On a chart or plotting sheet draw a line through the CP at an angle equal to the Azimuth. Put a point on this line at the distance just calculated in step 5, towards or away from the sun as appropriate. Then draw a line through this point at right angles (to the Azimuth). This line is the position line, PL. You are on it somewhere, but not necessarily at the point closest to the CP.


Constucting a Plotting Sheet

The PL just obtained should be accurate to 1 or 2 nautical miles, so it’s silly to throw away this accuracy by plotting on too small scale a chart. To preserve accuracy one needs a scale of a millimetre or two per mile, ie about 1:500,000.

Most ocean charts are 1:3,000,000 or 1:10,000,000, and besides, the chart would get covered in lines very rapidly if the chart itself was used to plot all position lines. So it is customary to create a plotting sheet showing latitude and longitude scales, and nothing else, and plot on this. I use graph paper ruled in mm and cm, as follows:

1. Draw parallel lines of Latitude, with degrees at 12 cm intervals. I mark in the margin each 10 of latitude (ie at 2cm intervals) so one minute is 2mm. On an A4 sheet one can fit a little more than 2.

2. Construct the Longitude scale. Somewhere near the middle of the chart I draw a vertical line on a whole cm grid line (11 in the example below). Then draw a line, going through a point at the crossing of this Longitude line with a whole degree of Latitude, at an angle to the horizontal equal to the Latitude.

3. Along this line mark off each cm, up 12cm. Make a small mark on the top margin at the point directly above each of the 1cm spaced marks on the angled line. These little marks on the top margin are at 5 intervals of Longitude. Draw a heavy line at the 60 mark to signify a whole degree. The example below should make this a little clearer.


Although not strictly Mercator’s projection, unless in polar latitudes the error may be ignored (the maximum possible error does not exceed 1 mile for latitudes less than 63 and is only 1.5 miles at 70).

Transferring a position line to make a fix

Mark on the plotting sheet the CP and then plot the PL. As noted before you are somewhere along this line, but this isn’t a fix, only a line; to get a fix one needs at least one other line which crosses the first at a reasonable angle. This can be from a simultaneous sight taken of a second heavenly body, such as the moon, but more frequently it is also from the sun, but taken at a different time so that the position lines cross. It’s really only a ‘running fix’ as used in conventional coastal navigation.

I find it useful to take three sights in a day: one in mid morning when the Azimuth is about ESE, one near midday when it’s nearly due south and another later in the afternoon when it’s about WSW. These then all cross at angles near 60 which is perfect. Naturally one has to allow for movement of the boat between the sights by moving the earlier sight(s) by the course and distance travelled by Dead Reckoning. The sight thus moved is called a Transferred Position Line, TPL.

 

[Independence]

Thanks everyone

 

[Skylark]

Lat vs Long Scale

With a little extra work, the plotting sheet described eloquently by jdc can be improved to give more accuracy to the Long scale.

Use the simple formula:

Long scale = Cos (Lat degree) x Lat scale.

So, using his example of 12cm = 1 deg Lat,

Say at 45 degree Lat.

Long scale = Cos 45 x 12. = 0.717 x 12 = 8.48 cm.

So, at 45 degree Lat, 1 deg Long is 8.48 cm.

A calculator helps but is not essential (given that you already have the knowledge to reduce a sight to a position line).

David

 

[timbartlett]

The easiest way to "make" a plotting sheet is to use any coastal chart of the right latitude (north or south -- turn it upside down if you've got the wrong one) and ignore all the cartography, (just use the lat long grids and scales)

 

[JayBee]

The easiest way to "make" a plotting sheet is to use any coastal chart of the right latitude (north or south -- turn it upside down if you've got the wrong one) and ignore all the cartography, (just use the lat long grids and scales)

Yes, but not very practical when working in a large latitude range and the cartography tends to clutter up the plot.

Easier to make a plotting sheet on the back of an admiralty chart (or any large sheet of tough paper). One horizontal line representing a parallel of latitude, crossed by two vertical lines representing meridians a degree apart, plus two simple scales are all it takes. This 3 line grid is good for any latitude, but becomes a little unwieldy close to the poles. It can be permanently inked in and labelled temporarily in pencil, as necessary. The chosen longitude scale can also be inked in, along the parallel, between the meridians.

The latitude scale, for plotting and measuring distances, can be obtained from a small diagram, drawn in one corner of the plotting sheet, using the relationship: lat scale = long scale/cos lat, as davidjackson points out. In use, one measures along a simple pencil line crossing, at an angle, a small permanent longitude grid, doing away with the need for any calculation. The angle of this pencil line, from the horizontal, is made equal to the latitude.

A plotting sheet like this can be constructed in about 10 minutes and can be used repeatedly, almost anywhere, until it falls apart. JDC's version is fine, but I think this one is probably a bit simpler to make and use, can cover a larger area at a useful scale and is doubtless more robust than A4 graph paper.

 

[snowleopard]

I agree with Porthandbouy that Sun Run Sun is all you need to do, now we have accurate wrist watches I find that noon fixes are an irrelevance. Frequently you can't see the sun at noon anyway!

Reasons for doing a noon lat rather than just a sight around the middle of the day:

The calculations are much easier

Easier to plot than azimuth & intercept

Needs no clock

Gives the historic option of running down a latitude.

The first 2 reduce the chances of an error.

 

[JayBee]

Agreed - quick'n'easy, hence time honoured obs.

 

[jdc]

Interesting twist

From JayBee: "lat scale = long scale/cos lat"

I never thought of having longitude scale fixed and latitude scale variable, this seems rather a good idea because it's universal whereas my sheet has to be made afresh every degree or two of latitude - I learn something new every day on these forums.

You and David suggest using a calculator to relate long scale, lat scale and mean latitude. The reason I prefer to construct the cos(lat) function rather than calculate it is just superstition: one voyage my calculator fell in the bilges during a knockdown and I had to construct eveything thenceforth, and I didn't enjoy that experience (the knockdown that is, the astro remained fun and I jolly well had to do it anyway, we didn't have GPS). But everyone else I know uses a calculator. (Actually we did have Geoffrey & Siddons 4-Figure log tables on board as well - remember those? - but it was faster to contruct the scale than use log(cos) tables, add up the numbers and then look up the anti-log).

However maybe I do have a useful suggestion: I record all my sights and workings in old fashioned laboratory experiment books, the green covered A4 ones for school physics, with graph paper on the left hand page and lined paper on the right. This way I never have to rub out a plot and have a permanant record. I think this is better while learning at least, and one can submit the book for the Ocean YM exam. I guess if you use astro for 100% of navigation, and make ocean passages almost full time you'd get through rather a lot, but for the once per year or two voyage it's nice to keep a momento.

 

[JayBee]

Hi JDC

"You and David suggest using a calculator to relate long scale, lat scale and mean latitude."

Not for me

In use, one measures along a simple pencil line crossing, at an angle, a small permanent longitude grid, doing away with the need for any calculation. The angle of this pencil line, from the horizontal, is made equal to the latitude.

I like your idea of having a permanent record, but on the larger plot it is often possible to have the whole "day's work", at typical yacht speeds.

 

[jdc]

to make a noon sight or not ?

Reasons for doing a noon lat rather than just a sight around the middle of the day:

The calculations are much easier

Easier to plot than azimuth & intercept

Needs no clock

Gives the historic option of running down a latitude.

The first 2 reduce the chances of an error.

Spot-on. But I think the text books over-egg the the importance of meridian passsage sights, leaving an impression that they've somehow essential and/or it's unseamanlike not to make them, whereas you've listed the actual advantages.

The only disadvantage I find is you can't make one sight at one instant, you have to observe the sun over a couple of minutes and preferably plot the altitude against time to get the exact meridian passage. This doesn't matter at all on a tradewind passage and it becomes an enjoyable part of the daily routine. However the last couple of passages I've made have been to windward with cloudy skies and with some spray coming over the deck occasionally. So I set the sextant to the calculated altitude and, preferably sheltering under the sprayhood, scan the horizon towards where the sun will be, while I or a companion holds the watch. Within a few mins one can nearly always get a faint outline of the sun through the clouds, so quickly bring it down and shout 'time'.

 

[JayBee]

When I first went to sea, a long, long time ago, only star sights were plotted.
Absolutely everything else was calculated. For example, morning sun position lines were worked up from an estimated position and the intercept terminal point on the line was then used as a starting point to estimate a new position at noon, using the traverse table. The actual intercept was calculated using Marc St Hilaire's method and 5 figure logs. Azimuths were calculated using ABC tables.

At noon, the observed latitude was compared to the previously calculated one.
Any difference was then multiplied by the morning 'C' correction and the result applied as a correction to the estimated longitude.

The day's run was then calculated using Mercator sailing.

The whole thing was a black art until the advent of AP3270 and the gradual acceptance of plotting methods!

All of this on an ordinary 7,500 ton general cargo ship - one of thousands in the UK MN.

 

[snowleopard]

When I first went to sea, a long, long time ago, only star sights were plotted.
Absolutely everything else was calculated. For example, morning sun position lines were worked up from an estimated position and the intercept terminal point on the line was then used as a starting point to estimate a new position at noon, using the traverse table. The actual intercept was calculated using Marc St Hilaire's method and 5 figure logs. Azimuths were calculated using ABC tables.

I learned astro as part of the YM Ocean course in the early 70s when it was still under the BoT. We learned the Haversine formula and used 5 figure tables but we always plotted the results and the ABC tables were used more as a curiosity than a serious method of navigation. I think in fact that the Marcq St Hilaire method refers to the CP/intercept approach rather than the Haversine formula so the air navigation tables still use the M St H method.