Celestial Making Sense of Altitude Corrections

[estarzinger]

Originally Posted by NAV

Perhaps I'm wrong, but both Ho values calculated above seem not correct to me.

No, you are perfectly correct. I was not using my standard worksheet and mixed up some 'center' vs 'lower limb' corrections (the Ho values are for a 'center' sight) when I was adding the corrections up. Corrected below - all for lower limb:

For 16 degrees 10 minutes:

Sun lower limb: 12.7' (from page altitude correction tables 10-90 sun, stars, planets)
Plus if you want to get picky there should be an additional .1' correction for 'non-standard conditions' which is the relatively low altitude (this comes from the page Altitude correction tables - additional correction)
So an Ho = 16 6.8' (assuming zero dip and IC) Should be 16 22.8

Moon lower limb: 62.7' (from altitude correction tables 0-35 moon)
Plus the same additional picky .1' correction
Plus additional parallax .6' (from the bottom table on altitude correction tables 0 35 moon)
So an Ho = 16 58.7 Should be 17 13.4

Originally Posted by NAV

The non-standard conditions refer to air temperatures≠10 °C and air pressure≠1010 hPa. These meteorological values make that the given standard corrections for refraction (based on temperature=10 °C and pressure =1010) hPa are not sufficiently correct. Differences are relatively large at low altitudes of bodies.

Yes, agreed, I just showed it using my current met conditions, because it may explain some difference the OP saw between two different sources. I suspect the 'learn to' book he is using does not include this 'non standard' corrections.

[El Pinguino]

At the end of the day you just don't bother taking sights below about 20 degrees altitude. The risk of abnormal refraction , especially in polar regions and the Arabian Sea in the NE monsoon - frinstance - is just to great.
Moon + low altitude = recipe for disaster.

[Adelie]

Originally Posted by El Pinguino

At the end of the day you just don't bother taking sights below about 20 degrees altitude. The risk of abnormal refraction , especially in polar regions and the Arabian Sea in the NE monsoon - frinstance - is just to great.
Moon + low altitude = recipe for disaster.

Given that my use for celestial is as a backup in the event of complete loss of GPS for whatever reason, I want to know how to do all the corrections.

[El Pinguino]

The OP is new to all this, or so it seems. In his case its best to become very familiar with the simpler things in life.
However once the basic moon sight is mastered and 100% familiarity with the application of the standard corrections to altitude achieved it is quite a simple matter to apply the additional corrections for low altitudes as shown by Evans under 'Picky additional refraction correction'.

Where you will still come unstuck is when you are experiencing 'abnormal refraction', in which case the additional corrections aren't worth a bumper.

An emergency, where you are dealing with unfamiliar navigational methods, is exactly the time that you should be keeping things nice and simple.

[Adelie]

Originally Posted by El Pinguino

An emergency, where you are dealing with unfamiliar navigational methods, is exactly the time that you should be keeping things nice and simple.

Celestial is only an unfamiliar navigational method if you don't practice it before you get to the emergency. The OP is trying to learn celestial which is a start towards being familiar with it. Whether this attempt to learn is out of academic interest or as a backup method or for some other reason only he/she could say.

If you have an emergency where you need celestial, two things are going to be true 98% or more of the time:

A) You will be well off shore with plenty of sea room.

B) You will have plenty of time to burn so you can take your time to do things right and double check your work.

[El Pinguino]

What Nigel said over here.. http://www.cruisersforum.com/forums/...on-132807.html
"Most navigators avoid moon sights like the plague, too many corrections and too many vagaries.
However, if needs must, the following is the order of correction to be used for the most accuracy

Sextant Alt
Index Error___________
Obs Alt
Dip ___________
App Alt
Semi Dia (plus correction for augmentation)
Parallax _____________ (Horizontal parallax x Cos. App Alt)
True Alt"

Three quarters of my time in the day job was pre GPS... never sailed with anyone who had taken a moon sight in anger.. cadets took them to see what would happen and they needed the knowledge for their tickets.

I did meet a chap once who had... a WW2 submariner... who had taken one..once... when they had stuck their head up in the Med one night.

[Mike1956]

Thanks everyone,

I am in the unfortunate position of not having a computer at home due to no internet connection. In the library at the moment but only have a few minutes. Will study the above one day when I have lots of time!.

Mike

[El Pinguino]

Mike, the pdf is on its way though cyber space as I type.

Anybody else wanting a copy just pm me.

[PDA1]

Doing Moon sights isn't difficult. Granted, on a rolling boat any sight is somewhat difficult.

'Did 2 Moon sights on 10/26/2014 at GMT 22:52:25 and 22:53:42 and got within 3 nm after shooting and reducing Vega today at GMT 00:03:21 (for the fix). Both Moon LOPs were extremely close to the GPS location...a couple of miles.

We use only HO 249 and the Nautical Almanac (well, the truth is I've never even held an NA- we just get the Daily pages online for the entire year). If the body is beyond the 29 degree range of '249, Hc and Z are determined using a calculator....which is much faster than "turning pages".

As to Altitude Corrections (this isn't specific to Moon sights- just the Sun, we use the Altitude Corrections Table we found on the web. It combines Semi-diameter, Refraction and Parallax into one figure that's either added or subtracted from the Ha. When it comes down to figures of tenths of a minute of arc then we round up (or down) as necessary.

Here's another source of Corrections-

http://www.erikdeman.de/manuals/CN_corr.pdf

As to learning the art of celestial navigation it seems the biggest hindrance to learning how to do it is those that are attempting to teach it. They make it way too complex and hard to understand for those not inclined to math or have spent years turning pages that have print so small that you will go blind.

William F. Buckley- he really did a great job in his Celnav video.

For a successful sight and reduction we made a form with all of the steps in sequence. It's something we're familiar with and not somebody elses form.

You can also compare your calculations for GHA, Hc, Z, Declination at the USNO's sight-

Celestial Navigation Data for Assumed Position and Time

There are an abundance of Celestial Navigation spreadsheets from one particular website which are very difficult to figure out what form to enter degrees minutes and seconds. Thus, as far as I'm concerned, the spreadsheets need improvement before they should be used (by us).

That aside- CelNav is a lot of fun, very interesting and gives you a sharper sense of navigational skills and a connection, in a certain way, with mariners throughout the ages. This, a GPS cannot do.

[nigel1]

With regard to moon sights, the vagary I referred is that moon light creates false horizons below the moon, the horizon you see below the moon is lower than the "real" horizon, so the observed alts are bigger than what they should be, but I guess on a small boat with all the other difficulties of taking a sight, it wont make much difference.

If you dont already have this, you will find this useful, a link to HO249 Vol 1 Selected Stars. (it is not on the nga website)
http://www.waypointamsterdam.com/Han...Pub249Vol1.pdf