Studying Celestial "d" corrections

[Mike1956]

I thought I understood "d" corrections until I read this:

"the d correction for the sun will be added from 21 march to 22 june while the declination increases from 0 to n23 deg and again from 21 sept to 22 december 0 to 23 deg s. but it must be subtracted from 22 june to 21 september n23 to 0 deg and from 22 december to 21 march s23 to 0 degrees".

NOW I AM NOT SO SURE!?

Forgive lack of punctuation & niceties etc.

Look forward to replies

Mike

[vascodagama1]

Hi,
The "d" Cor that you ask about is the hourly rate of declination of both Sun. When the Sun's declination is at its greatest, ie when the Sun is either on the Tropic of Capricorn or the Tropic of Cancer it will decrease at the hourly rate stated until the Sun is above the equator after which it will increase again.
Hope this helps

[vascodagama1]

Originally Posted by vascodagama1

Hi,
The "d" Cor that you ask about is the hourly rate of declination of both Sun. When the Sun's declination is at its greatest, ie when the Sun is either on the Tropic of Capricorn or the Tropic of Cancer it will decrease at the hourly rate stated until the Sun is above the equator after which it will increase again.
Hope this helps

Sorry, I meant to say that it's the hourly rate of either increase or decrease of the Sun's declination.
It takes roughly 185.5 days to go from 23.30N to 23.30S so it's pretty easy to work out

Cheers

[Mike1956]

Originally Posted by vascodagama1

Sorry, I meant to say that it's the hourly rate of either increase or decrease of the Sun's declination.
It takes roughly 185.5 days to go from 23.30N to 23.30S so it's pretty easy to work out

Cheers

Thanks vascodagama,

Your interpretations introduce a third and fourth explanation of the "d" correction. Neither of which are remotely close to my previous/existing understanding.

I am not wishing to muddy the waters further at this stage and hope someone can offer a comprehensible (and correct) explanation in due course.

The wording in my original post which I believe to be nonsense was copied from a well respected book on celestial navigation.



Mike

[El Pinguino]

I've never seen it expressed like that before... I always just determined whether to add or subtract by inspection and left it at that.


You may find some help in here http://www.cruisersforum.com/forums/...ff-144799.html

[Mike1956]

Here's my shot at an explanation of "d" corrections

The declination of a HB is equal to the latitude of its GP in degrees N/S of the equator, tabulated in the daily pages of the almanac hourly.

"d" corrections are generally small corrections which accommodate any ACCELERATION or DECELERATION of the HB during the hour.

Do not be surprised to see the "d" correction is contrary to the general trend in declination. ie. During the 3 month period 22nd March to 22nd June (N hemisphere) when declination is definitely increasing from 0 to 23 degrees the "d" corrections will sometimes be found to be negative.

Perfectly happy if someone is able to offer a better explanation.

Mike

[John_Trusty]

First a caveat: I am not a celestial instructor. In actually performing sight reductions, I am like El Pinguino's mechanical approach - whatever way the table trends, I add and subtract.

For understanding the "why", which is useful for remembering stuff, I try to picture the calculations from the outside like I was looking down at a basketball. I have not seen the text you refer to where it gives dates for positive and negative declination interpretation factor. As vascodagama1 noted (the poster above, not the original navigator!), this factor is the minute adjustment for the partial hour that you observed - sun tables are hourly. In the northern hemisphere, the sun is climbing in the sky (increasing declination) each minute from the spring equinox until the solstice, then declining until the fall equinox. The pattern is roughly a sine-wave dependent on the rotation of the slanted earth in an elliptical solar orbit. I didn't look the tables up, but I'll bet the declination interpretation factor comes to zero at each solstice.

[Mike1956]

Need to say although I do not yet fully understand the "d" correction the book I am reading and all of you are in fact correct & I am confused!

I have been encouraged not only to learn to apply the corrections but to also understand the reasoning behind them.

At this stage the application seems much more straightforward than the understanding!

Thanks

Mike

[gjordan]

Dont worry too much about the understanding. My first lessons in celestial was a class given by an astronomer that I found so confusing that I gave up after a huge amount of theory. I later learned from a book TEN EASY STEPS TO CELESTIAL NAVIGATION by Ketteridge. I got a little help from an ex navy quartermaster, but the book was a no nonsense cook book style of teaching. Too much theory just confuses the learning process. From those ten easy steps I learned sun, stars, planets and moon. Another little word of advice is DONT learn to do a noon sight. Too many people that first learn to do a noon sight, pat themselves on the back and think they are a navigator and go no further. Learn a proper sun shot/ line of position and the rest is just small additions to that. Just my 2 cents worth. _____Grant.

[El Pinguino]

What I had to say on the subject over 20 years ago.... be warned ..it may give you a brain bleed...

'Note that you may have to either add or subtract this correction for declination and that no sign is given on the daily pages.

Inspection of the daily pages of the almanac will show that the GHA of the sun increases by 15 degrees per hour this being the rate at which the sun orbits the earth.
There is also a small but important change in the sun’s declination. This hourly change is at a maximum at the equinoxes and is negligible at the time of the solstices.

If you ignore it then the maximum error produced will be in the order of one mile in latitude, if you apply it the wrong way a maximum error of two miles may result but remember - all your little short cuts and omissions are cumulative.
On most occasions you can allow for it simply by mental interpolation. If the sun’s declination at 0500 is 5° 40.6’ and at 0600 it is 5° 41.6’ then at 0430 it will be 5° 41.1’. You may, however, prefer to use the ‘d’ correction. This is listed at the bottom of each of the daily pages .....Simply note the value of ‘d’ and whether the declination is increasing or decreasing. When you enter the ‘Increments and Corrections’ pages to get the increment of your hour angle extract the correction for ‘d’ from the right hand side of the page.'

[Mike1956]

Thank you for your further advice,

I did have a light bulb moment in the middle of last night regarding the d corrections.

My issue was and the reason I posted initially was the book I am studying advised d corrections are always applied in accordance with the general trend in the direction of declination. I think I realised why last night.....................

Whereas GHA calculations for Moon & Planets require HOUR, INCREMENTS & v corrections

Declination calculations require only HOUR & d corrections. Where are the incremental corrections?

My supposition is both the incrementals and the d corrections are wrapped up in a neat package labelled d corrections. If the d correction element of this package is ever negative (which I believe will be the case) it will be outweighed by the much larger incremental. This explains why the d corrections are always applied in accordance with the general trend in declination (be that additive or negative).?????????????????

Mike

[Reggy]

Originally Posted by El Pinguino

What I had to say on the subject over 20 years ago.... be warned ..it may give you a brain bleed...

'Note that you may have to either add or subtract this correction for declination and that no sign is given on the daily pages.

Inspection of the daily pages of the almanac will show that the GHA of the sun increases by 15 degrees per hour this being the rate at which the sun orbits the earth.
There is also a small but important change in the sun’s declination. This hourly change is at a maximum at the equinoxes and is negligible at the time of the solstices.

If you ignore it then the maximum error produced will be in the order of one mile in latitude, if you apply it the wrong way a maximum error of two miles may result but remember - all your little short cuts and omissions are cumulative.
On most occasions you can allow for it simply by mental interpolation. If the sun’s declination at 0500 is 5° 40.6’ and at 0600 it is 5° 41.6’ then at 0430 it will be 5° 41.1’. You may, however, prefer to use the ‘d’ correction. This is listed at the bottom of each of the daily pages .....Simply note the value of ‘d’ and whether the declination is increasing or decreasing. When you enter the ‘Increments and Corrections’ pages to get the increment of your hour angle extract the correction for ‘d’ from the right hand side of the page.'

Hi there, isn't your result;
05.30 = 5 degrees 41.2'.
Mental interpolation?
I am trying to understand the conversations.
Cheers.

[El Pinguino]

Originally Posted by Reggy

Hi there, isn't your result;
05.30 = 5 degrees 41.2'.
Mental interpolation?
I am trying to understand the conversations.
Cheers.

Declination is changing at the rate of 1 minute of arc every hour. ( 'd' corrn = 1.0 )
Therefore in 30 minutes it will change by 0.5 minutes of arc.
By observation it is increasing.....
So it shall increase from 40.6 ( 0500 ) to 41.1 ( 0530)....

40.6 + 0.5 = 41.1

or if you prefer 41.6 - 0.5 = 41.1

Ping

[El Pinguino]

Originally Posted by El Pinguino

.....
On most occasions you can allow for it simply by mental interpolation. If the sun’s declination at 0500 is 5° 40.6’ and at 0600 it is 5° 41.6’ then at 0430 0530 it will be 5° 41.1’. .......

Oooops...
Ping the Inumerate

[El Pinguino]

Originally Posted by Mike1956

Thank you for your further advice,

I did have a light bulb moment in the middle of last night regarding the d corrections.

My issue was and the reason I posted initially was the book I am studying advised d corrections are always applied in accordance with the general trend in the direction of declination. I think I realised why last night.....................

Whereas GHA calculations for Moon & Planets require HOUR, INCREMENTS & v corrections

Declination calculations require only HOUR & d corrections. Where are the incremental corrections?

My supposition is both the incrementals and the d corrections are wrapped up in a neat package labelled d corrections. If the d correction element of this package is ever negative (which I believe will be the case) it will be outweighed by the much larger incremental. This explains why the d corrections are always applied in accordance with the general trend in declination (be that additive or negative).?????????????????

Mike

I would suggest leaving planets alone until you are entirely comfortable with the sun.
I would suggest avoiding the moon forever......