Astro: My Disconcerting discrepancy

[Neil]

Sorry to bore the majority to tears, but........

Every now and then I like to practice the art and given that the moon and Jupiter are out at twilight at the moment, I got out my dish of oil and took sights of them both (1 Dec).

I never got round to reducing the sites until this evening and while the moon sight looked good enough, Jupiter was miles out. I used an online site reduction calculator (http://www.celnav.de/sightred.htm) to check my sums, and found that my computed altitude didn't agree with the calculator (azimuth and Ho were spot on). Now, there's not a lot to go wrong here; AP3270 (Lat 53) for the particular GHA (289) and 10 deg Same Name, gave 19 22, 48 and 99 for Hc, d and Z. With a d of 48 and 39 mins of declination, Table five gives a correction of +31 mins, therefore Hc is 19 53. But the calculator gives 19 58 (which would give a much better PL).

I've gone over the sums again and again and just can't see it........

 

[estarzinger]

It's been a long time since I did a manual reduction, but I believe you are working from two different assumed positions.

When you use the calculator you can use your fully accuracy DR position (including the minutes) as your assumed position, because its using trig calculations that can use that accuracy.

But when you use the tables, your assumed position is typically NOT the DR position, but a position near the DR that gives a whole degree LHA and whole degree latitude - so you can enter the table with whole degree numbers. Thus for your calculation from the table I believe your assumed position is 53 N and 6 26.4W. If you put this assumed position in your calculator I believe you will get your 19 53 out as Hc.

Because your assumed positions is different between the table and the calculator methods, your Hc will be different, as its the calculated angle from the assumed position.

 

[Neil]

Yes!, Phew! That explains the discrepancy, all right, thank you very much! So my bad answer must just be down to a bad sight. As it happens, the declination of Jupiter changes hardly at all (1.4 min in 24 hr) at the moment, so even a slight error could give a significantly dodgy PL.

 

[Blueboatman]

The trouble with intelligent posts like this Neil is that it inspires the rest of us to drag out the kit and my old copy of Mary Blewitt and have a practise too-how could one resist Jupiter AND the moons?

Good link too for that online calc, and v lucid prob solving response, thx.

 

[Skylark]

Not sure I'm following so may I ask a question?

I've only ever done manual sight reduction but from estarzingers extremely helpful post it seems that:-

The software option works using trig formulae so has no problem using a DR position to give Hc and Zn. Thus compare Hc with Ho from DR along Zn to give a position line.

Manual sight reduction uses Tables, simplified to whole number Lat and GHA based from an Assumed Position. As above, comparing Hc to Ho along Zn will give a PL.

Surely then, these should both give a similar PL. So why, Neil, do you think you took a bad sight.....what am I missing?

 

[Neil]

Not sure I'm following so may I ask a question?

The software option works using trig formulae so has no problem using a DR position to give Hc and Zn. Thus compare Hc with Ho from DR along Zn to give a position line.

Manual sight reduction uses Tables, simplified to whole number Lat and GHA based from an Assumed Position. As above, comparing Hc to Ho along Zn will give a PL.

Surely then, these should both give a similar PL. So why, Neil, do you think you took a bad sight.....what am I missing?

The problem was that I'd put in my DR position, not Assumed Position, into the calculator, whereas manually reducing the sight, I'd rounded down to give a whole No. for LHA, so my Assumed Long. was not my DR Long., hence the discrepancy between the calculator and my sums.

The moon and Jupiter were both up at twilight, and had nice intersecting angles on their azimuths, however, re-examining the almanac showed that Jupiter's declination changes very little over 24 hrs, so I'm guessing (??) that any sight reduction of Jupiter would not be very accurate. I had thought that the intercept that I got from my sums seemed to give a poor PL for Jupiter ('cos I know where my house is , and perhaps not a good sight. However, when I plotted it out, I got 53 14N 06 14 W (real 53 17.824N 06 14.148W), so really only about 4 miles out.

It was really just the fact that the calculator gave a different answer (you have to put the correct No.s in, stupid boy!)

 

[Skylark]

.........It was really just the fact that the calculator gave a different answer (you have to put the correct No.s in, stupid boy!)

Thanks, Neil............I think I've understood.

The lesson is clear, neverthless. You should not trust your sight reduction to electrickery. AP3270 is called "Rapid Sight Reduction Tables for Navigation" for good reason

In my post, my deliberate error was quoting GHA when, of course, I meant LHA.............but you knew that, right?

Any thread that causes me to get my astro notes out get my vote. Thanks for posting.

 

[ARO]

.... however, re-examining the almanac showed that Jupiter's declination changes very little over 24 hrs, so I'm guessing (??) that any sight reduction of Jupiter would not be very accurate.

This problem does not exist. The change in declination does not influence the precision of a PL. Otherwise star fixes would generally give poor position lines, because a star's declination never changes.

 

[Neil]

Yes, I wondered about that as I wrote it. Glad you cleared it up.

 

[nigel1]

Interesting post, similar situation a few weeks ago, teaching cadets how to take sights.
Tried the way I was taught to reduce sights using logs and haversines etc, that was met with dazed expressions, so they used a calculator, after turning the boat upside down looking for a scientific calc.'
Once that was mastered, showed them how to use the sight reduction tables, and when they plotted the PL, it was miles out. Similar error, they had used an assumed position to extract Hc and Z, but then used the actual DR to plot the result.

BTW, I've been taking sights for over 30 years, but never used the artificial horizon method
A brief description of whats involved and the corrections you apply would be interesting

 

[Sandy]

it was miles out.

First time I ever did it by my calculations I was somewhere off the coast of Africa!

 

[Neil]

BTW, I've been taking sights for over 30 years, but never used the artificial horizon method
A brief description of whats involved and the corrections you apply would be interesting

My use of an artificial horizon was forced by the desire to learn astro and not having the means to take sights at sea.

I use a dish of oil, usually perched on the garden wall. I look at the reflection of the astral body in the oil and bring it it down. I would put the reflected sextant image above and just touching the directly observed oil reflection; the equivalent of the lower limb on the horizon. Divide the declination by 2, no correction for Dip.

The challenge is in seeing the reflection of the more fainter stars and planets in the oil.

 

[nigel1]

My use of an artificial horizon was forced by the desire to learn astro and not having the means to take sights at sea.

I use a dish of oil, usually perched on the garden wall. I look at the reflection of the astral body in the oil and bring it it down. I would put the reflected sextant image above and just touching the directly observed oil reflection; the equivalent of the lower limb on the horizon. Divide the declination by 2, no correction for Dip.

The challenge is in seeing the reflection of the more fainter stars and planets in the oil.

Thanks Neil,

At home somewhere I have a 1938 edition of Volume 3 of the Admiralty Manual of Navigation which describes usings a bucket of mercury as an artificial horizon. Must have been intersting on the bridge wing of a destroyer during foul weather.

 

[dharl]

has been a few years since I had to do Astro in anger, however I was taught that while you can use the moon for azimuths, you shouldnt use it for position fixing.

Am not sure of the reason, I think its because of the relative closeness of the moon in relation to other bodies which are used such as Jupiter, Mars and of course the Sun! Am happy to be corrected..am going to go and blow the dust off my old nav books

 

[nigel1]

has been a few years since I had to do Astro in anger, however I was taught that while you can use the moon for azimuths, you shouldnt use it for position fixing.

Am not sure of the reason, I think its because of the relative closeness of the moon in relation to other bodies which are used such as Jupiter, Mars and of course the Sun! Am happy to be corrected..am going to go and blow the dust off my old nav books

In my youth I took a sight of the moon on a clear night with a good hozizon from the moon light.
The captain on the ship, who held an Extra Master's certificate and had been Examiner of Masters and Mates fior the BOT (as it was known then), explained to me that he moon should not be used as by some trickery, the moon light would cause an apparent lowering of the sea horizon. I never got the full explanation, but I still pass that on to others. maybe its one of those urban myths, I dont know.
But the moon during the day, or twlight, that's Ok, just got a few more corrections to apply

 

[Skylark]

..........I was taught that while you can use the moon for azimuths, you shouldnt use it for position fixing.

QUOTE]

I'm intrigued, hope we can get to the bottom of this one.

Where are the forum grey beards when you need 'em

 

[estarzinger]

I was taught that while you can use the moon for azimuths, you shouldnt use it for position fixing.

The moon is perfectly fine for position fixing. In fact, the most sophisticated fixing technique is called 'the lunar distance' and is obviously based on using the moon (angle between the moon and another object usually Venus). Its the only celestial technique that can give a fix without knowing accurate time. It was in the running for winning the 'longitude award' just before accurate chronometers were developed, but the sight and calculation is so fussy the longitude competition jury judged that while it worked from land it was not really practical at sea.

But, the moon is obviously the closest object, and as such has two additional corrections that must be used in the calculations. These add a bit of complexity and significant inaccuracy if they are forgotten or used incorrectly. Also, way back (pre 1810), the almanac was less accurate for the moon (there is more wobble) than for the stars and sun. This made the moon the less preferred object, if there were others available.

The Horizontal Parallax Correction (due to the fact you are standing on the earths surface and not at its center) can be almost a full degree. The semi diameter augmentation (do to the fact the moon is closer to the observer when directly overhead than when on the horizon) is smaller but can be as much as 0.2 minutes of arc.

 

[nigel1]

It was in the running for winning the 'longitude award' just before accurate chronometers were developed, but the sight and calculation is so fussy the longitude competition jury judged that while it worked from land it was not really practical at sea.

I believe that kicking a sick dog was also put forward, if what was in the book "Longitude" is a true account.
Sorry, slight thread drift, but I was fascinated that someone back then put that forward as a means of getting noon time

 

[dharl]

Thanks for the reminder Nigel, now that you mention it, it sounds simular to what I was told at the time when I was a young cadet!

I have never heard of 'Lunar Distance' way of fixing your postion without using a clock Estarzinger...I will have to do a bit of research on that as it sounds an intresting mathamatical problem.

 

[nigel1]

A bit of googling led me to the "Sextant Handbook" by Bruce Bauer, of which a small part of the book is shown on google books. A quoye from which is:
A moon illuminated horizon can be used but the navigator must be careful not to be tricked. More on this in Chapter 6"

Predictably, I cannot find Ch 6, so, if anyone out there has a copy of the book, please put us out of our misery.