Error in Index Error calculation in Celestial Navigation plug-in

[MarkN]

I'm new to celestial navigation and have been using the latest version of the celestial navigation plug-in to check my calculations.

I think I have found an error in how index error is calculated for site reductions.

Normally site reductions are negative if on the scale and positive if off the scale.

But to get results that look meaningful, on the parameters tab, I have to use enter the opposite sign value in the index error field. (eg an error of 5' on the scale needs to be input as +5' rather than -5').

When I look at the calculation tab of the sight reduction.
If I entered a +5' Index error on the parameters tab, then the Index error is 0.0833 degrees (ie still positive),
but the calculation for apparent altitude = Measurement - Index error - Eyeheight correction.

So this changes my Positive Index error to a Negative Index error. Which ultimately leads to what is an incorrect Apparent Altitude.

Is my assumption that the apparent altitude calculation is incorrect right?
ie it should be apparent altitude = Measurement + Index error - Eyeheight correction.
Mark

[tkeithlu]

Let me recommend the iPhone app "Celestial Navigation" by Dr. Harald Merkel.

I'm a beginner, too. This app has you enter your assumed position, body observed, altitude, and other pertinent raw information (no calculations whatever) and returns a line of position, even drawn on a map from your assumed position.

On the one hand, this is a cheat sheet - it makes it too easy because you do no calculations and need no understanding of what you are doing. It goes against actually learning to navigate without electronics, since it needs your iPhone.

On the other hand, I've found it a wonderful tool for improving my observations. I can run numerous Sun sights in sequence, a few minutes apart, trying to reduce the error from my known (by GPS) location. It's wonderful practice at taking sights.

In your immediate case, it's a check against the procedure you are utilizing in OpenCPN. If the same sight gives different lines of position, something is wrong. Obviously, you can check against GPS, but your sight SHOULD have some error, given the lack of accuracy and precision in any sextant sight. If you get no error, you're going to get dismissed by the people out there who are not beginners.

[tkeithlu]

By the way, welcome to the forum, Mark. Good to have a compadre in learnig this stuff.

sight=visual
site=location
cite=reference

[Adelie]

The correct formula is:
Ha= Hs + IC + Dip

Technically IE (Index Error) is off or on the arc and IC (Index Correction) is positive or negative respectively.

Dip is the correction for height of eye and is always negative.

[MarkN]

Originally Posted by Adelie

The correct formula is:
Ha= Hs + IC + Dip

Technically IE (Index Error) is off or on the arc and IC (Index Correction) is positive or negative respectively.

Dip is the correction for height of eye and is always negative.

Hi Adelie,

Thanks for your reply, I agree with your answer.

The issue is that in the celestial navigation I think the formula is incorrect.

The calculation that is being used is:
ApparentAltitude= Measurement - Index Correction - EyeHeightCorrection

So when Index error is positive it is being converted to a negative and vice versa leading to an incorrect Apparent Altitude

[rooiedirk]

An index error is by definiton the opposite of the index correction.



If the reading is +2' off (error of +2'), the you need to add -2(correction) to get the correct answer.

[Adelie]

Originally Posted by rooiedirk

An index error is by definiton the opposite of the index correction.



If the reading is +2' off (error of +2'), the you need to add -2(correction) to get the correct answer.

You are introducing a lot of double negatives which is clouding the issue.

The formula in the plugin should have + operands, not - operands

[Adelie]

Or the formula could be:

Ha= Hs + IC - Dip

The problem here is that you have to remember the conventions used in deciding what the signs are for the corrections. Dip is always negative which is easy to remember but IC can be either.
All the conventions I've looked say that for errors on the arc the correction is negative and off the arc the correction is positive.