"v" corrections GHA

[Mike1956]

I am studying Celestial nav and am currently looking at GHA/SHA/LHA. I do not feel I fully understand just 1 element. The application of the "v" correction for the Sun & Planets. In the Ephemeris there is a "v" correction at the bottom of the column which covers 3 days. This figure for the Sun is typically 3. I do feel I understand the "v" correction in general. For example for the Moon a correction is given daily. It is a simple matter to go to the increments/corrections section and read off the correction opposite the figure given in the appropriate minutes table. Just not sure what to do when the correction is given for 3 days? Look forward to replies. Mike

[seaward 42]

There is no "v" for Sun (and Stars), only for Moon & Planets.

[jstevens]

Mike,
I assume your question concerns the "SD" and "d" values for the Sun and/or the "v" and "d" values for the planets. Just use the single value for each of the 3 days on that page of the Almanac.
I assume this arrangement was done to save space on the Almanac page and provides sufficient accuracy for navigators, if not astronomers.


John

[seaward 42]

Tabulated GHA and v Correction Factor:
For the Sun, the Moon, or a planet, extract the value for the whole hour of GHA corresponding to the sight. For example, if the sight was obtained at 13-50-45 GMT, extract the GHA value for 1300. For a star sight reduction, extract the value of the GHA of Aries, again using the value corresponding to the whole hour of the time of the sight. For a planet or Moon sight reduction, enter the v correction value. This quantity is not applicable to a Sun or star sight. The v correction for a planet sight is found at the bottom of the column for each particular planet. The v correction factor for the Moon is located directly beside the tabulated hourly GHA values. The v correction factor for the Moon is always positive. If a planet’s v correction factor is listed without sign, it is positive. If listed with a negative sign, the planet’s v correction factor is negative. This v correction factor is not the magnitude of the v correction; it is used later to enter the Increments and Correction table to determine the magnitude of the correction.

------------------------

Tabulated Declination and d Correction factor:
Obtain the tabulated declination for the Sun, the Moon, the stars, or the planets from the daily pages of the Almanac. As you allready mentioned declination values for the stars are given for the entire three day period covered by the daily page of the Almanac. The values for the Sun, Moon, and planets are listed in hourly increments. For these bodies, enter the declination value for the whole hour of the sight. For example, if the
sight is at 12-58-40, enter the tabulated declination for 1200. There is no d correction factor for a star sight. There are d correction factors for Sun, Moon, and planet sights. Similar to the v correction factor above, the d correction factor does not equal the magnitude of the d correction; it provides the argument to enter the Increments and Corrections tables in the Almanac. The sign of the d correction factor, which determines the sign of the d correction, is determined by the trend of declination values, not the trend of d values. The d correction factor is simply an interpolation factor; therefore, to determine its sign, look at the declination values for the hours that frame the time of the sight. For example, suppose the sight was taken on a certain date at 12-30-00. Compare the declination value for 1200 and 1300 and determine if the declination has increased or decreased. If it has increased, the d correction factor is positive. If it has decreased, the d correction factor is negative.


d correction: Enter the Increments and Corrections
table with the d correction factor discussed above. Extract
the proper correction, being careful to retain the proper
sign.

[Mike1956]

Thank you.

I need to explain.

I have no internet connection at home and can only communicate from the library!

Reason for mentioning this is all my study material is at home but I can say I have 3 sources of information and one of them does give v corrections for the Sun. From memory it is a 1978 Macmillan Almanac.

Mike

[Mike1956]

Just to make clear.

The other 2 sources of information/tables I have do not give v corrections for the Sun. I assumed therefore if the tables I am using do give the v corrections I should use them?

Again just to make clear. The only element I am referring to is the v correction and only for the Sun (if appropriate) and planets. I have no problem understanding and application of the v corr for the Moon.

Additionally I feel I have no problem with the theory but need to understand the application. I need to see some examples which sadly are lacking from the material I am studying at the moment.

For example.

If the v for Mars at the bottom of the column for 3 days is 2 minutes 14 seconds. Is this 2 minutes and 14 seconds for each and every hour of the 3 days? as for the Moon. OR is the 2 minutes 14 seconds for the whole of the 3 days? OR other?

I look forward to more replies.

Thanks

Mike

[jstevens]

Mike,
The hourly GHA values for the Sun in the NA have been adjusted for the deviations from the standard hourly increase (15 deg). An interpolation adjustment is necessary for the Aries, the Moon and Planets, so the "v" value is omitted for the Sun and included for the other bodies. The "v" correction for Aries, provides the adjustment for the Stars.
It appears you have a privately published almanac, which does not include the adjustment for the Sun in the hourly tables.
Again the adjustments for Aires and the planets is small enough that one value for a 3-day range provides adequate accuracy, but the moon requires a value for each day. In the case of your old almanac you will need to apply the "v" value when interpolating the Sun's GHA to the minutes and seconds, but it is still a small enough adjustment that a single value is good for a 3-day range.
If you use a NA published in the UK or US it will not have a "v" value for the Sun. I have an excerpt from the 1975 NA, and it does not have a "v" value for the Sun.

John

[PangurBan]

Originally Posted by Mike1956

Just to make clear.

The other 2 sources of information/tables I have do not give v corrections for the Sun. I assumed therefore if the tables I am using do give the v corrections I should use them?

Again just to make clear. The only element I am referring to is the v correction and only for the Sun (if appropriate) and planets. I have no problem understanding and application of the v corr for the Moon.

Additionally I feel I have no problem with the theory but need to understand the application. I need to see some examples which sadly are lacking from the material I am studying at the moment.

For example.

If the v for Mars at the bottom of the column for 3 days is 2 minutes 14 seconds. Is this 2 minutes and 14 seconds for each and every hour of the 3 days? as for the Moon. OR is the 2 minutes 14 seconds for the whole of the 3 days? OR other?

I look forward to more replies.

Thanks

Mike

Mike you should take another look. The v figure given at the bottom on


Sent from my iPad using Cruisers Sailing Forum

[jstevens]

Mike the daily "v" values for interpolating the GHA are for each of the hours in the 3 day range.
Sounds like you need a reference that provides examples and exercises. The ones I have are probably out of print, but they may still be available on Amazon.

Exercises in Astro Navigation by Gordon Watkins published by Stanford Maritime.
Exercises for the Ocean Yacht Navigator by Keneth Wilkes, published by Macmillan

John

[PangurBan]

Originally Posted by PangurBan

Mike you should take another look. The v figure given at the bottom on


Sent from my iPad using Cruisers Sailing Forum

Damn. Slip of the thumb and a premature send. Ok, the figure at the bottom of the planet columns is in minutes and decimal minutes. It gives the average change in GHA per hour for the three days. To get the change for minutes of time you go to the increments and corrections table for your minutes of time and extract the figure opposite your v value. For example I've just checked that if the v correction for Mars is 2.7 (I.e. the GHA of Mars changes by 2.7' per whole hour) the actual correction to be applied for 30 minutes is 1.4' makes sense as 1.4' is about half of 2.7'.

It took me a long time to get this when I was learning cel nav.


Sent from my iPad using Cruisers Sailing Forum

[Mike1956]

Thanks,

Great replies. Everything makes perfect sense! Could lecture on the subject now (only the "v" bit).

Having posted the other day I had a look in the boaty section of the library (here now) and found a book on celestial by Tom Cunliffe. There is a worked example in his book that uses the "v" correction for a planet and I could see immediately that the correction at the bottom of the column for 3 days was hourly.

From memory. The Macmillans almanac does give GHA at exactly 15 degrees/hour but also gives "v" corrections for whole days. I thought I understood the theory well enough to realise there should be "v" corrections for the Sun but I can now see clearly through the mist.

I have Celestial Navigation for Yachtsmen Mary Blewitt (tenth edition). On page 40 it does make reference to v corrections for Sun but in the tables at the back of the book there are no v corrections shown. It is not that there arent any, it is because they are included already. Got it!

Thanks again.

No doubt I will have another queerie in the next few days.

Regards

Mike

[seaward 42]

there it is, something from this summer.....

[seaward 42]

Moon & Venus....

[nigel1]

Mike

This is a link to a sample page from the Hydrographic Office Nautical Almanac, which is like the official one to use
https://upload.wikimedia.org/wikiped...almanac_01.png

For planets, the v correction at the bottom of the page is for the 3 days of data, but is applies strictly to the middle day.
If you really wanted to get super accurate, you could interpolate between the v values for the following 3 days, or the preceding 3 days.

[Mike1956]

Thanks again,

I thought I had finished with this but....

I note a few posts back "v" corrections for Aries are mentioned.

Regarding v corrections there are only 3 possibilities

1. There are no v corrections.

2. There would be very small v corrections but are disregarded for navigational purposes.

3. There are small v corrections which are averaged into the tabulated figures.

4. There are published v corrections.

Could anyone help by placing Sun, Moon, Aries, Planets & Stars in appropriate groups.

Thanks

Mike