Covid shots

[pabrides]

Hello fellow CN freaks,

Its been 2 years since I logged in last.  I hope some of the old shipmates are still breathing.

I was wondering if any of you guys have become more active in celestial navigation since the covid hoax began. 

I started taking more shots with an artificial horizon and with no little difficulty.  For some reason my lops are always 4 or 5 nautical miles short of my gps fix.

I've gone over the math a hundred times, checked my time, checked the sextant, checked everything. 

Recently I began to wonder if my sun shots are wrong.  My habit is to bring the sun down to the AH then touch the two suns with the AH sun on the bottom.  Do I have this backwards?

I add index error, divide by 2, then add altitude correction.  Am I missing something?  should I be using SD, semi diameter,  instead of altc, or is there a step of which I'm unaware?  I dont add dip to AH shots.

Anyway,  I hope to hear from some of the old gang; including admin.

Merry Christmas

[Fred_B]

pabrides,

Yes, have been doing more cn than usual.

The times since March haven't been good at all for most of us. A lot of unemployment and the destruction throughout the cities and the economy have been horrific. "Hoax" is the perfect word. And we put up with it! Awful.

But to answer your question....

I'm in a similar situation and have never exactly known why my LOPs are consistently off assuming I don't make an error in the initial observation getting Hs.

The following may be helpful;

1- If your AH has any kind of glass "tent" over it that could be causing refraction. That's the first thing that comes to my mind and what I often blame for these errors.

2- Your method of bring the Sun down is correct- the sun "in the sky" sits on the reflected Sun in the AH (LL- Lower Limb). If you had the them inverted you'd be off a huge amount and it would be obvious.

3- The Sun Altitude Correction table takes into consideration SD, parallax and refraction. Yes, it's true that CelNav's Altitude Correction table is dated 2003 but the figures in it for the planets Venus and Mars are the only ones that significantly change from year to year. The Altitude Corrections for the Sun and stars only vary in tenths of a minute of arc or at most 1 minute of arc from year to year.

4- Index Error. That might be what's causing the trouble. I'm sure you know the rule- "if it's on the arc- take it off" or "if it's off the arc- put it on". I use the Sun for determining Index Error. My III-B stays consistent with 2 minutes off the arc. Those two minutes will indeed amount to a good error that should be accounted for.

5- Side error. My sextant does have side error which I zero out once in a long while but it always sneaks back in.

6- Time- make sure your time is correct or you know what your chronometer error is.

7- When in doubt- suspect your GPS. There are a number of online services that provide maps that'll help you determine the longitude and latitude of your location.

8- Lastly, post some of your Hs figures and let's see if we can reduce them to an Ho that's in agreement with yours.

Merry Christmas,

Fred

[pabrides]

Boy, do I feel silly... I complained about being 5 miles off when its my brain that was 5 miles off... I was using the 2019 almanac for the reduction.  I downloaded the 2020 one and my lop gets much closer.  however the zn azimuth now runs a bit further away.  ill have to take some more shots tomorrow and be super careful.  I got in a rush this afternoon when after I discovered the almanac wrong year I took a hurried sun shot as it was diving in and out of the clouds.

Ill update all this again.  below you can see this mornings shot and the reduction with the wrong almanac (2019).


ust did a sun shot this morning - 5 nm off again.  Before I took the shot I checked for index error using the sun... spot on... no error.

Dec 22 23:48:23
Clock error +2
Corrected time gmt 23:48:21
Hs 46 24.4
/2 = 23 12.2
Altc +14.0
Ho 23 26.2

Gha 165 20.5
Inc +12 05.3
Tgha 177 25.8
Ap long +124 34.2
Lha 302
Dec -23 26   d=0
Tdec -23 26
Ap lat 10 contrary

Hc 24 22.0     d= -24    Z 121
Diff = 26/60 × -24 = -10.4
Thc 24 11.6 - ho 23 26.2
Intercept 45.4 away
Zn 121

These are my figures.  Ive been over them and over them.  My AH is open so no glass. Time checked with wwv. Checked gps again... n10 17.5  e123 57

The lop is 5 nm past the gps fix.  Always 4 or 5 nm long or short.

Merry Christmas to you as well and thanks.

[EdCa]

pabrides,

All of your final figures are correct.

As we all know, mostly by experience, an accuracy close to 4 or 5 nm is excellent....while at sea.

Here's an idea to try- remove your sextant's telescope and make an observation without it. Then do the reduction and see if there has been any change.

Another idea is to bring the Sun down in the AH so that it overlaps the reflected Sun. You'd only need corrections for parallax and refraction and not Semi-Diameter.

Concerning time- your chronometer would have to be off consistently 20 seconds to get the error you're finding.

GPS can truly and really be in error too.

I can't think of anything else, at the moment.

Merry Christmas and a Happy New Year!

Ed

[CarlosPindle]

Here's an idea to try;

Make several observations for longitude- a few before MP and the same amount after MP. Carry out "the meridian by equal altitudes" method.

My amateur guess is the error will be reduced to zero.

Happy New Year, all.

Carlos

[pabrides]

I used the wrong year almanac.... I'm such a dunce.  

Nevertheless,  thanks for all the great ideas...

Merry Christmas to all,  and to all good sailing.

[Rdutton]

So you figured it out! Good.

Here's another idea I've wondered about;

When using an AH, what effect does any glass have on the Hs?

Example- you're using a dish of water for an AH on a window sill. Any window glass must bend the light somewhat...doesn't it? It might be that any refractive effect is insignificant.

Any thoughts?

Roland

[pabrides]

Good question,

I assume the reason why most liquid AH's you see today have canted glass elements is to reduce the possibility of refraction. Ideally both the observer and the angle of the sun will be close to perpendicular to these plastic or glass elements. However, there is such a thing as optically true transparent materials such as camera filters and such. These might offer the least chance of distortion.

I'm currently seeking a source of mercury. Easier to keep than oil or syrup and better able to withstand rippling from a breeze, mercury dates back to at least lewis and clark and probably further. I'm also investigating the use of a mirror as an AH. silvering the top like a reflector telescope rather than from behind the glass like a shaving mirror illiminates glass distortion. How to level would be the big question.

I'm sure others have more concrete thoughts than my own.