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v & d corrections

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  • v & d corrections

    Not sure which board to post this on........

    Trying to improve my comprehension of v & d corrections

    I note v corrections for Venus can be minus despite the GHA increasing (correction contrary to general trend in GHA)

    Please can anyone advise if d corrections are ever contrary to general trend in declination?

    Thanks

    Mike

  • #2
    In short, no.

    The V correction is to compensate for the fact that not all planets' GHA increases at the same rate, so instead of making GHA increment column for each planet in the tables, there is one for all planets and the v correction allows for any differences between a particular planet and the average rate. Sometimes the planet's GHA increases a little faster than average (positive v correction) and sometimes a little slower than average (negative v correction).

    In contrast, the d correction is simply the difference between the declination from one hour to the next, so the result in the increments and corrections table is simply a linear interpolation of that difference. Therefore the d correction is always in accordance with the general trend in declination.

    I hope that helps, just shout if you're still stuck.

    Comment


    • #3
      Thanks weasel,

      I typed a more detailed version of my situation yesterday (twice) and it failed to post (twice). Hence my brief post on this occasion.

      I am studying from a single book Celestial Navigation for Yachtsmen Mary Blewitt which uses the Nautical Almanac/AP3270/Intercept method. I mention this as I have become aware there are various systems. The book only has a page or two copied from the various publications for reference.

      According to my book the only planet that ever has negative v corrections is Venus.

      Is this correct or can Mars Jupiter or Saturn throw up the occasional negative correction?

      Thanks

      Mike

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      • #4
        Ah I have a copy of that too. I tried teaching myself from it before I started my cadetship but didn't get very far so kudos to you!

        I have a feeling that you're correct about Venus being the only planet to have negative v corrections. However, there is really no need to memorise whether this is true or not as it doesn't eliminate the need to look up the v correction value at the bottom of the daily page anyway.

        If you want to know purely for interest I'm sure there will be some astronomical forums which might explain in more detail. I suspect it's to do with Venus being so close to Earth, or the only navigational planet that's between Earth and the Sun, or possibly a combination of the two.

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        • #5
          V is positive for everything except as you noted Venus which occasionally has a negative correction down to it's orbit relative to us.

          There's a whole load of maths that goes with it that I don't understand - but it's not really relavant from a day to day calculation perspective.
          “Twenty years from now you will be more disappointed by the things you didn’t do than by the ones you did do. So throw off the bowlines, sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover.”

          – Mark Twain
          myBlog | @alistairuk | flickr | youtube Views and opinions expressed are those of myself and not representative of any employer or other associated party.

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          • #6
            Thanks Weasel & Alistair,

            I realise by making such enquiries the danger is receiving replies I do not understand. I am certain all the calculations behind all Celestial Navigation are extremely complex and way beyond my own comprehension. I just feel the need to have a general idea about how it works and be able to apply the corrections accordingly.

            I am advised Venus & Mercury have retrograde motion and this is why they have negative v corrections. However as Mercury is not a navigational planet we do not need to concern ourselves about that!

            I did have a light bulb moment in the night ref the d corrections....................

            Whereas GHA calculations require HOUR, INCREMENTAL & v corrections at least for Moon & Planets.

            Declination calculations require only HOUR & d corrections

            My latest supposition is that the incremental corrections for declination are wrapped up in a neat package called d corrections which also includes the actual d correction. This explains why the d corrections are always applied (whether additive or negative) in accordance with the general trend in declination. (any slight negative d value will always be outweighed by the larger incremental correction)????????????????????

            Mike

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