Question for the Astro-Navvies

[Roberto]

plotting sheets and a nice big chart table. I still have those plotting sheets on my boat but I doubt many do.

I have a home made one (laminated and used with non-permanent markers), seldom used and not for astro, though nice to have sometimes.
This was an approach to Horta, the sheet covered a few degrees lat/long, there was some very serious weather coming and I had the option to try and arrive before the worst, or if too late divert to the south to seek shelter, I had drawn some sort of iso-distance and iso-chrone lines to monitor the advancement of the boat vs forecast during the final days. Dimensions on the large scale chart were too small, draw all that into a plotter really not practical.

 

[GHA]

for practicing there's a good opencpn plugin as well, shows what data it uses to do the calcs & gives LOP on the chart.


more practice needed here, that's for sure But well worth it, to get the hang of which mirrors work best etc etc.

 

[GHA]

I'm afraid that I'm polar-opposite and don't find that "graphing by hand is boring", I rather enjoy the whole process.

For practicing just the physical taking sights this is actually proving to be very useful. Horizon/height of eye/corrections are irrelevant as long as there's something flat enough to be repeatable. Spreadsheet open on a tablet, take a load of sights and put the numbers straight into the spreadsheet, save and upload, look. Try standing different or whatever, add the sights & look again, repeat and repeat and repeat with instant visual info of how they look. Very quick & keeps our big monkey brain focused on just the physical process.

 

[Humblebee]

I was recommended Celestial Navigation: A Complete Home Study Course by a friend of mine.

It takes you in baby steps through the process and you can download David Burch's proformas from his web site.

I'm not a fan of Cunliffe as he waffles.

Another good site is The Nautical Almanac. I've downloaded the stuff I need and have created a folder with those pages. If I am going off on a trip I print the daily pages from the NA and check that I have the data for the latitudes I am sailing in.

Another vote for David Burch's book. Clearly written with plenty examples to follow and exercises to work on. Practice plotting sheets too.

 

[GHA]

Another vote for David Burch's book. Clearly written with plenty examples to follow and exercises to work on. Practice plotting sheets too.

Agree, also found the barometer and modern marine weather books very interesting & useful
Lots of astro posts on his blog
Article Categories

 

[Skylark]

Agree, also found the barometer and modern marine weather books very interesting & useful
Lots of astro posts on his blog
Article Categories

Re his weather book. I think I'm probably a statistical outlier. Highly recommended by many, I bought the third edition and found its progression to be disjointed and illogical. I couldn't get on with it.

 

[GHA]

Re his weather book. I think I'm probably a statistical outlier. Highly recommended by many, I bought the third edition and found its progression to be disjointed and illogical. I couldn't get on with it.

That's being human for you There is no objective universe out there, we make our own.

Back on the tools, opencpn plugin lets you plot a whole load of sights & compare. Today's graph didn't seem too bad but is 4Nm out from the last one. Wonder what's going on there, taken from exactly same spot, IE nearly zero both times, time taken from exact on the web...

Brown LOP is today.

 

[mattonthesea]

Following GHA's post (only just got that Doh!) I had a go with CoPilot. It takes a while to drill down but here is the course I took. (note that I queried with a 'please'; can't quite get used to machines!





Sign in

You said​

how do i explain to a novice how to calculate where i am from a sun sight

Copilot said​

Alright, let's break it down step by step, nice and simple!

Calculating Your Position from a Sun Sight​

1. Preparation:
   • Tools Needed: You'll need a sextant, a chronometer (accurate clock), and the Nautical Almanac.
   • Sextant: This is used to measure the angle between the sun and the horizon.
   • Chronometer: You'll need the exact time of your measurement, ideally set to GMT/UTC.
   • Nautical Almanac: This gives the sun's position at any given time of day.
2. Taking the Sight:
   • Measure the Altitude: Use the sextant to measure the altitude (angle) of the sun above the horizon.
   • Record the Time: Note the exact time of your measurement using the chronometer.
3. Determine the Sun's Position:
   • Consult the Nautical Almanac: Find the sun's exact position (latitude and longitude) at the time of your measurement.
4. Calculate Your Line of Position (LOP):
   • Plot on Chart: Using your measured altitude and the sun's position from the Almanac, you can plot a line on a navigational chart. This line is called your Line of Position.
5. Find Your Position:
   • Intersection of Lines: For a precise location, you'll need a second sighting (usually of the sun at a different time or another celestial body). The intersection of the two LOPs will give you your exact position.

Simplified Example:​

Imagine you're at sea, you measure the sun's altitude at noon with a sextant and it's 45 degrees. The Nautical Almanac tells you that at that time, the sun should be at 90 degrees (directly overhead) at a specific latitude. The difference (45 degrees) is your distance from that latitude. You plot this on a chart, then repeat the process at a different time or with another celestial body for a second LOP, and where those lines intersect is where you are.

Does that make sense?

It's a fascinating mix of astronomy and navigation! If you need more details or have any questions, I'm here to help.

You said​

how do i explain to a novice how to calculate where i am from a sun sight. Ineed to include how to use the nautical almanac and how the calculations are made

Copilot said​

Absolutely! Let's break it down in an easy-to-understand way.

Explaining Sun Sight Navigation​

Tools You'll Need:​

1. Sextant: For measuring the angle between the sun and the horizon.
2. Chronometer: A precise clock to record the exact time of your sighting.
3. Nautical Almanac: A book that tells you the position of celestial bodies at any given time.

Step-by-Step Guide:​

1. Taking a Sun Sight:

• Measure the Altitude: Hold the sextant and adjust it until the sun is on the horizon. The angle you measure is the sun's altitude.
• Record the Time: As soon as you've measured the altitude, note down the exact time using your chronometer.

2. Using the Nautical Almanac:

• Find the Date and Time: Open the Almanac to the date and time of your sighting.
• Locate the Sun’s Position: The Almanac will give you the sun's Greenwich Hour Angle (GHA) and Declination (Dec).
  • GHA: This tells you how far west the sun is from the Prime Meridian.
  • Dec: This tells you the sun’s position north or south of the equator.

3. Doing the Calculations:

• Calculate Local Hour Angle (LHA):
  • Find your longitude and combine it with the GHA to get the LHA.
• Determine Latitude:
  • Using the sextant reading (the altitude), subtract the measured altitude from 90 degrees to find your zenith distance.
  • Correct this zenith distance for any observational errors (like atmospheric refraction) using the Almanac's correction tables.
  • Add or subtract this corrected distance to/from the Declination to find your latitude.

4. Plotting Your Position:

• Line of Position (LOP): The altitude gives you a circle of equal altitude. You are somewhere on this circle. When you plot this on a chart, it becomes a line.
• Intersection of Lines: Take a second sighting (or more) and plot this. The intersection of the lines from different sightings gives your exact position.

Simplified Example:​

1. Measure: You measure the sun at 45 degrees altitude at exactly noon.
2. Almanac: The Almanac tells you at noon, the sun is at a GHA of 123 degrees and a Declination of 23 degrees north.
3. Calculate:
   • If your local time zone is GMT+0, then LHA = GHA (123 degrees).
   • Zenith distance is 90∘−45∘=45∘.
   • Latitude is 23∘N+45∘ (depending on whether you're north or south of the sun's declination).
4. Plot: Repeat the process later or with another celestial body to cross-check. The intersection of these lines gives your position.

Does that make sense? If you need any more details, I'm here to help!

You said​

please include an explanation of termns used and how to use the correction tables

Copilot said​

Sure thing! Let's dive deeper and clarify the terms and how to use the correction tables.

Terms Explained​

1. Sextant: A navigational instrument used to measure the angle between the horizon and a celestial body (like the sun). It helps determine the altitude.
2. Altitude: The angle between the horizon and the sun (or other celestial body), measured using the sextant.
3. Chronometer: A highly accurate clock used to record the exact time of your sighting, ideally set to GMT (Greenwich Mean Time).
4. Nautical Almanac: A publication that lists the positions of celestial bodies (sun, moon, planets, stars) at various times throughout the year.
5. Greenwich Hour Angle (GHA): The angular distance measured westward along the celestial equator from the Greenwich Meridian to the hour circle of the celestial body.
6. Declination (Dec): The angular distance of a celestial body north or south of the celestial equator, similar to latitude on Earth.
7. Local Hour Angle (LHA): The angular distance between your meridian (longitude) and the meridian of the celestial body.
8. Zenith Distance: The angle between the zenith (the point directly above you) and the sun. It is calculated as 90 degrees minus the altitude.
9. Line of Position (LOP): A line on a navigational chart along which you are located, based on your celestial sighting.

How to Use the Correction Tables​

The Nautical Almanac includes correction tables to account for various factors that can affect your measurements. Here’s how to use them:

1. Index Error: This is a common error in the sextant, and you correct for it by adjusting the measured altitude by the known error amount. If the error is positive, subtract it from your measurement, and if negative, add it.
2. Dip Correction: This correction accounts for the height of your eye above the sea level. The higher you are, the more you need to adjust your measurement. The Nautical Almanac provides a table to correct for dip based on your height of eye in feet or meters.
3. Altitude Correction: This corrects for atmospheric refraction (the bending of light as it passes through the Earth’s atmosphere) and the sun’s semi-diameter. The Almanac includes tables to adjust the measured altitude based on the observed altitude.

Example of Using Correction Tables​

1. Measure the Altitude: Suppose you measure the sun's altitude as 45 degrees.
2. Apply Index Error: If your sextant has an index error of +2', subtract 2' from your measurement. So, your corrected altitude is 45∘−2′=44∘58′.
3. Apply Dip Correction: If your eye height is 10 feet, the Almanac table tells you to subtract 3' for dip correction. So, your corrected altitude is 44∘58′−3′=44∘55′.
4. Apply Altitude Correction: Use the table in the Almanac to find the altitude correction for an observed altitude of 44 degrees 55'. Suppose it tells you to add 10'. Your final corrected altitude is 44∘55′+10′=45∘5′.

Putting It All Together​

Once you've corrected your altitude, use it along with the sun's GHA and Declination from the Nautical Almanac to calculate your LHA and determine your position. The detailed explanation in the previous messages should help you combine these corrected values for accurate navigation.

Does that help clarify things? Navigation using a sun sight can be quite an adventure! If you have more questions or need further details, feel free to ask!

 

[mattonthesea]

Only 10 000 characters allowed so here's the next stage

You said​

please include how to read the almanac

Copilot said​

Absolutely! Let's include how to read the Nautical Almanac. This guide will help you understand the key sections and how to use them for your sun sight calculations.

How to Read the Nautical Almanac​

The Nautical Almanac is divided into several sections. Here are the main parts you need to focus on:

1. Daily Pages:
   • Greenwich Hour Angle (GHA): This tells you the sun's GHA for each hour of the day. The GHA is the angular distance west of the Greenwich Meridian.
   • Declination (Dec): This tells you the sun's declination for each hour, which is its position north or south of the celestial equator.
2. Sun's Hourly Positions:
   • Hourly Intervals: Each daily page gives the GHA and Declination for every hour of the day (GMT).
   • Interpolation Tables: These tables help you calculate the GHA and Declination for times between the listed hourly intervals. For example, if you took your sighting at 10:30 GMT, you'd use interpolation to find the values halfway between 10:00 and 11:00.

Step-by-Step Guide to Using the Nautical Almanac​

1. Find the Right Page:
   • Open the Almanac to the date of your sighting.
   • Locate the daily page for that date.
2. Record GHA and Declination:
   • Find the GHA and Declination values for the exact hour of your sighting.
   • Use the interpolation tables if your sighting was not exactly on the hour.
3. Interpolation Example:
   • Sighting Time: Let's say you took your sighting at 10:30 GMT.
   • GHA at 10:00: Suppose the GHA at 10:00 is 100°.
   • GHA at 11:00: Suppose the GHA at 11:00 is 115°.
   • Calculate GHA at 10:30: The difference is 15° over one hour, so at 10:30, the GHA would be 100°+7.5°=107.5°.
4. Using Declination:
   • Declination at 10:00: Suppose the Declination at 10:00 is 10°N.
   • Declination at 11:00: Suppose the Declination at 11:00 is 12°N.
   • Calculate Declination at 10:30: The difference is 2° over one hour, so at 10:30, the Declination would be 10°+1°=11°N.

Example of Reading and Using the Almanac​

1. Measure: You measure the sun's altitude at 45 degrees at 10:30 GMT.
2. Almanac:
   • GHA at 10:00: 100°
   • GHA at 11:00: 115°
   • Interpolated GHA at 10:30: 107.5°
   • Declination at 10:00: 10°N
   • Declination at 11:00: 12°N
   • Interpolated Declination at 10:30: 11°N
3. Apply Corrections:
   • Index Error: Correct the sextant reading.
   • Dip Correction: Correct for the height of your eye above sea level.
   • Altitude Correction: Correct for atmospheric refraction and semi-diameter.
4. Calculate Position:
   • LHA: Combine your longitude with the GHA to find the LHA.
   • Latitude: Subtract the corrected altitude from 90 degrees to find the zenith distance, then adjust by the declination to find your latitude.

Putting It All Together​

By following these steps, you can accurately determine your position using a sun sight and the Nautical Almanac. It may seem complex at first, but with practice, it becomes a fascinating and reliable method of navigation.

If you have any more questions or need further clarification, feel free to ask!

You said​

please include an example for an altitude of the sun of 53 degrees 45 minutes taken at 10.35 UTC and an altitude of 52 degrees 49 minutes taken some time later

Copilot said​

Perfect, let's work through this example together step-by-step. We'll calculate your position using two sun sightings.

Example Calculation​

First Sighting​

1. Measure the Altitude:
   • Altitude: 53∘45′ at 10:35 UTC.
2. Using the Nautical Almanac:
   • Find the Date: Go to the daily page for the specific date.
   • Record GHA and Declination:
     • GHA at 10:00: 100°
     • GHA at 11:00: 115°
     • Interpolated GHA at 10:35: 100°+3560×15°=108.75°
     • Declination at 10:00: 10°N
     • Declination at 11:00: 12°N
     • Interpolated Declination at 10:35: 10°+3560×2°=11.17°
3. Apply Corrections:
   • Let's assume no index error and height of eye correction for simplicity.
   • Corrected Altitude: 53∘45′
4. Calculate Zenith Distance:
   • Zenith Distance: 90∘−53∘45′=36∘15′
5. Calculate Local Hour Angle (LHA):
   • Assume longitude is 0° for simplicity.
   • LHA: 108.75∘
6. Determine Latitude:
   • Latitude: 11.17∘N+36∘15′=47.32∘N

Second Sighting​

1. Measure the Altitude:
   • Altitude: 52∘49′ at a later time (for simplicity, let's assume 14:00 UTC).
2. Using the Nautical Almanac:
   • Record GHA and Declination:
     • GHA at 14:00: 180°
     • Declination at 14:00: 15°N
3. Apply Corrections:
   • Corrected Altitude: 52∘49′
4. Calculate Zenith Distance:
   • Zenith Distance: 90∘−52∘49′=37∘11′
5. Calculate Local Hour Angle (LHA):
   • LHA: 180∘
6. Determine Latitude:
   • Latitude: 15∘N+37∘11′=52.18∘N

Plotting Your Position​

1. First Line of Position (LOP):
   • You are somewhere on a line where the sun's altitude was 53∘45′ at 10:35 UTC.
2. Second Line of Position (LOP):
   • You are somewhere on a line where the sun's altitude was 52∘49′ at 14:00 UTC.
3. Intersection:
   • The point where these two lines intersect is your position.

This example provides a simplified overview. In practice, you'd use exact positions and correct for various factors to ensure accuracy. The key is understanding each step and applying the corrections from the Nautical Almanac.

Feel free to ask if you need more details or further clarification!

You said​

please include the pages, rows and columns, their headings from the almanac. I need to know the step by step process of finding the values

Copilot said​

Absolutely, let's dive into the details of how to use the Nautical Almanac and find the necessary values.

How to Use the Nautical Almanac​

Step-by-Step Guide​

1. Find the Daily Pages:

• Date: Open the Almanac to the specific date of your sighting. Each day has a dedicated page.
• Columns: Look for columns headed with "Hour" (usually GMT), "GHA (Sun)", and "Dec (Sun)".

2. Record the Values:

• Time: Locate the rows for the hours of your sighting. For example, if your sighting was at 10:35 UTC:
  • GHA: Locate the hour 10:00 and 11:00 rows under the "GHA (Sun)" column.
  • Declination: Locate the hour 10:00 and 11:00 rows under the "Dec (Sun)" column.

Example Calculation​

First Sighting​

1. Measure the Altitude:
   • Altitude: 53∘45′ at 10:35 UTC.
2. Using the Nautical Almanac:
   • Find the Date: Open the Almanac to the specific date.
   • Locate the Values:
     • GHA at 10:00: 100° (GHA column, row for 10:00)
     • GHA at 11:00: 115° (GHA column, row for 11:00)
     • Interpolation: GHA at 10:35=100°+(3560×15°)=108.75°
     • Declination at 10:00: 10°N (Declination column, row for 10:00)
     • Declination at 11:00: 12°N (Declination column, row for 11:00)
     • Interpolation: Dec at 10:35=10°+(3560×2°)=11.17°
3. Apply Corrections:
   • Assume no index error and height of eye correction for simplicity.
   • Corrected Altitude: 53∘45′
4. Calculate Zenith Distance:
   • Zenith Distance: 90∘−53∘45′=36∘15′
5. Calculate Local Hour Angle (LHA):
   • Assume longitude is 0° for simplicity.
   • LHA: 108.75∘
6. Determine Latitude:
   • Latitude: 11.17∘N+36∘15′=47.32∘N

Second Sighting​

1. Measure the Altitude:
   • Altitude: 52∘49′ at a later time (for simplicity, let's assume 14:00 UTC).
2. Using the Nautical Almanac:
   • Locate the Values:
     • GHA at 14:00: 180° (GHA column, row for 14:00)
     • Declination at 14:00: 15°N (Declination column, row for 14:00)
3. Apply Corrections:
   • Corrected Altitude: 52∘49′
4. Calculate Zenith Distance:
   • Zenith Distance: 90∘−52∘49′=37∘11′
5. Calculate Local Hour Angle (LHA):
   • LHA: 180∘
6. Determine Latitude:
   • Latitude: 15∘N+37∘11′=52.18∘N

Plotting Your Position​

1. First Line of Position (LOP):
   • You are somewhere on a line where the sun's altitude was 53∘45′ at 10:35 UTC.
2. Second Line of Position (LOP):
   • You are somewhere on a line where the sun's altitude was 52∘49′ at 14:00 UTC.
3. Intersection:
   • The point where these two lines intersect is your position.

If you have any more questions or need further clarification on any specific parts of the Almanac or calculations, feel free to ask!

 

[mattonthesea]

Third stage

I then asked for a challenge





Can you test me with an example question. I do not have an almanac so i will need the relevant pages to find the calculation data

Absolutely, let's run through an example together. I'll provide the necessary data as if you have the Nautical Almanac.

Example Scenario​

First Sighting​

• Time: 09:20 UTC
• Altitude of the Sun: 52° 30'

Second Sighting​

• Time: 15:10 UTC
• Altitude of the Sun: 48° 45'

Nautical Almanac Data​

For the sake of this exercise, here are the fictitious almanac values for the date you're working with:

First Sighting Almanac Data:​

• GHA at 09:00: 120°
• GHA at 10:00: 135°
• Declination at 09:00: 15°N
• Declination at 10:00: 16°N

Second Sighting Almanac Data:​

• GHA at 15:00: 210°
• GHA at 16:00: 225°
• Declination at 15:00: 18°N
• Declination at 16:00: 19°N

Steps to Solve​

1. First Sighting Calculation:
   • Interpolation for GHA:
     • GHA at 09:20: GHA at 09:00+2060×(GHA at 10:00−GHA at 09:00)\text{GHA at 09:00} + \frac{20}{60} \times (\text{GHA at 10:00} - \text{GHA at 09:00})=120°+2060×15°=125°= 120° + \frac{20}{60} \times 15° = 125°
   • Interpolation for Declination:
     • Declination at 09:20: Dec at 09:00+2060×(Dec at 10:00−Dec at 09:00)\text{Dec at 09:00} + \frac{20}{60} \times (\text{Dec at 10:00} - \text{Dec at 09:00})=15°+2060×1°=15.33°N= 15° + \frac{20}{60} \times 1° = 15.33°N
2. Second Sighting Calculation:
   • Interpolation for GHA:
     • GHA at 15:10: GHA at 15:00+1060×(GHA at 16:00−GHA at 15:00)\text{GHA at 15:00} + \frac{10}{60} \times (\text{GHA at 16:00} - \text{GHA at 15:00})=210°+1060×15°=212.5°= 210° + \frac{10}{60} \times 15° = 212.5°
   • Interpolation for Declination:
     • Declination at 15:10: Dec at 15:00+1060×(Dec at 16:00−Dec at 15:00)\text{Dec at 15:00} + \frac{10}{60} \times (\text{Dec at 16:00} - \text{Dec at 15:00})=18°+1060×1°=18.17°N= 18° + \frac{10}{60} \times 1° = 18.17°N

Your Task​

1. Apply Corrections: Assume no index error, and height of eye correction. Correct the altitudes if necessary.
2. Calculate Zenith Distances:
   • Zenith Distance for 09:20 sighting.
   • Zenith Distance for 15:10 sighting.
3. Find Latitude:
   • Determine the latitude for each sighting.
4. Plot Lines of Position (LOPs):
   • Determine the intersection of the LOPs from both sightings to find your exact position.

Challenge​

Work through these steps to calculate and plot your position. Feel free to ask any questions or for further clarifications as you go along. Happy navigating!



I could have gone on



I've read Cunliffe';s book and 'Step-by-Step Sextant Users Guide' ;Andrew Evans (two very different approaches). I find Cunliffe annoying because he uses some data that is not included. Also he only gives examples for the stage he is at. I think the examples need to build from scratch each time as Zones of Proximal Development and Scaffolding (Vygottsky) so that familiarity is built into the development. Using this AI approach allows me to do that. I think that some background understanding is needed as I'm not sure that a complete beginner would have enough to be able to ask the right questions.



M

 

[GHA]

I could have gone on

Another convert

Bearing () in mind it's not all or nothing, you're allowed to read the books & ask an AI to explain things which don't quite make sense.
And in many ways it's just an amazing web search engine, the web isn't always right with all those chatrooms riddled with human bias & misinformation..

Perplexity seems pretty awful at any irpcs questions. Though nearly always you learn something, I never knew that port tack /starboard tack aren't mentioned once in the irpcs.

 

[S04]

That's being human for you There is no objective universe out there, we make our own.

Back on the tools, opencpn plugin lets you plot a whole load of sights & compare. Today's graph didn't seem too bad but is 4Nm out from the last one. Wonder what's going on there, taken from exactly same spot, IE nearly zero both times, time taken from exact on the web...

Brown LOP is today.

View attachment 186894

That plot looks very odd, the samples are not connected in time order, looking at points 1,2,3 and 4 which is the order of connection yet point 3 was taken earlier than point 2. Surely the correct order in terms of time is 1,3,2,4. Perhaps not significant but it wouldn't be normal to connect the data points of a time series other than in time order.

 

[capnsensible]

It's actually quite straightforward to learn conventionally.

Some though, for some reason, want to make it appear far more difficult than it actually is.

Dunno why.....

 

[GHA]

That plot looks very odd, the samples are not connected in time order, looking at points 1,2,3 and 4 which is the order of connection yet point 3 was taken earlier than point 2. Surely the correct order in terms of time is 1,3,2,4. Perhaps not significant but it wouldn't be normal to connect the data points of a time series other than in time order.

Yep, you're right. It draws the line in ascending y axis, not x. Doesn`t really matter though, it's just a tool to bang out sights & see instantly any differences. Which it's doing very well so far. So easy to focus just on the actual sight taking. I'll have a word with the AI & fix it

Fixed, well spotted

Might be nice at some point to be able to put in a lat & long and get it to plot what the curve should be.

 

[S04]

Might be nice at some point to be able to put in a lat & long and get it to plot what the curve should be.

Is this any use?
Sustainable By Design :: sunposition

Input your position and it will generate the sun's altitude (and azimuth) for, say, a given day. You can email it to yourself and then put it into a spreadsheet. Oddly it doesn't let you select a year but it would be interesting to compare the data it provides with your plot.

 

[GHA]

Is this any use?
Sustainable By Design :: sunposition

Input your position and it will generate the sun's altitude (and azimuth) for, say, a given day. You can email it to yourself and then put it into a spreadsheet. Oddly it doesn't let you select a year but it would be interesting to compare the data it provides with your plot.

perplexity wrote some python in a moment but getting it in javascript for the web is a bit more involved apparently




Doing a reverse meridian passage calc to get what the actual Hs should be at solar noon & working out the local solar noon time puts the lop in opencpn within 0.15Nm so that works then And good for the brain working out how to do the calcs from scratch

 

[Skylark]

perplexity wrote some python in a moment but getting it in javascript for the web is a bit more involved apparently

View attachment 186906


Doing a reverse meridian passage calc to get what the actual Hs should be at solar noon & working out the local solar noon time puts the lop in opencpn within 0.15Nm so that works then And good for the brain working out how to do the calcs from scratch
View attachment 186908

you worked-out by hand to check the AI result I think that I'm missing something

 

[GHA]

you worked-out by hand to check the AI result I think that I'm missing something

Curiosity!

And it might be wrong, or some source of numbers might be wrong. Major part of nav, trust nothing, if the answer is the same from various sources it's probably right.

The plotting bit is just turning a load of numbers into something visual so you can see anything interesting going on. And it was on a few sites looking at local noon thinking, I should be able to figure that out from 12:00 GHA & from sun dec what angle the sun is above the horizon at noon with ever having looked at a book so go on then, and do it with no paper.
Far more educational than blindly following what's in a book & forces you to visualize what's going on. Me anyway.

So GHA is on your side or 0º at 12:00 today , GHA- 0◦33.1, dec - S23◦26.1, so with a plane extending out from 0º long the sun has already gone past that so if you are further west then solar noon will be less than your long converted into time.
Same with lat, if the sun was over the equator and you were at the north pole the sun would be 90º to a line shooting straight up.
Just doing sums by rote teaches you nothing, figure out how to do it yourself & it all makes much more sense

That was all done converting base 60 minutes into decimals and working in base 10 again, calculator came out for that, wouldn't trust doing mixed addition with integers & base 60. Better dig a book out & see how they do it now , not that there's a right & wrong way in maths or geometry if it gets the same results every time.

 

[Frank Holden]

'Simplified Example:​

1. Measure: You measure the sun at 45 degrees altitude at exactly noon.
2. Almanac: The Almanac tells you at noon, the sun is at a GHA of 123 degrees and a Declination of 23 degrees north.
3. Calculate:
   • If your local time zone is GMT+0, then LHA = GHA (123 degrees).
   • Zenith distance is 90∘−45∘=45∘.
   • Latitude is 23∘N+45∘ (depending on whether you're north or south of the sun's declination).
4. Plot: Repeat the process later or with another celestial body to cross-check. The intersection of these lines gives your position.

Does that make sense? If you need any more details, I'm here to help!'.

NO, that makes no sense, in fact it is non sense.

If your local time zone is GMT +0h ( ie 'Z' or Zulu) it can be assume your longitude is between 7º30' E and 7º30'W.
Yet the sun has a GHA/LHA of 123º so unless you are in polar latitudes in summer it has already set.
Hardly an optimum time to take a mer alt.
I found plenty of other rollocks in there but this will have to do for now.

Rule Number One - Keep it Simple.

 

[Skylark]

Curiosity!

Just doing sums by rote teaches you nothing, figure out how to do it yourself & it all makes much more sense

Agree with these. It isn’t Rocket Science. For many, rote is the only way, hence why they think that a Template is a gift from Neptune. With a little more effort and coaching by Poseidon, figure out the basics of spherical trig and it becomes straightforward.