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    Re: d correction
    From: Gary LaPook
    Date: 2010 Jan 29, 16:04 -0800
    The Nautical Almanac tabulates the declinations of the sun, moon and planets at hourly intervals. The declinations of these bodies are constantly changing so you need to be able to calculate the accurate declination at the intemediate time at which you took the observation. The "d" value at the bottom of the column is the amount of change that takes place between tabulated, hourly values. You can see this just by looking at consecutive declination entries, the change each hour is the "d" value at the bottom of the column. Because the rate of change of the declination of the moon changes more rapidly than that of the other bodies it has a "d" value shown for each hour instead of for the three day period given for the other bodies. You use this "d" value to enter the corrections portion of the "Increments and Corrections" tables at the back of the N.A. with the minutes and seconds of the observation and take out the "d" correction that you then combine with the tabulated declination to arrive at the body's declination at the exact time of the observation. Note that there is no sign given to the "d" value and you must determine whether to add or subtract the "d" correction by looking at consecutive tabulated declinations to determine if the numbers are getting larger or smaller.

     Note also that there is a "v" correction that is used in the same way to determine the GHA of the body. This 'v" correction accounts for the actual change in GHA per hour in excess of  the standard value tabulated in the increments table. Sometimes the "v" correction takes on a negative sign which is shown, otherwise "v" is positive.

    H.O. 249 (and other navigational tables) tabulate the computed altitude for whole degrees of latitude, LHA and declination. This works out OK for latitude and LHA because you can choose an assumed position to make these conditions true. It doesn't work for declination because you have no control over the actual declination of the body at the time of the observation. The "d" listed in H.O. 249 next to each tabulated altitude shows how much the computed altitude will change for a one degree change in declination, the difference between consecutive tabulated values. This actually varies with the azimuth of the body. You use this "d" to enter the interpolation table, Table 5, with the minutes of declination to compute the amount to adjust the tabulated computed altitude to account for the declination not being an exact whole number of degrees. This method is also used with other navigational tables, such as H.O. 214, H.O. 218, and H.O. 229.

    Is that at the 8th grade level?


    --- On Fri, 1/29/10, FrankReed@HistoricalAtlas.com <FrankReed@HistoricalAtlas.com> wrote:

    From: FrankReed@HistoricalAtlas.com <FrankReed@HistoricalAtlas.com>
    Subject: [NavList] Re: d correction
    To: NavList@fer3.com
    Date: Friday, January 29, 2010, 12:04 PM

    Unfortunately, a copy of the message copied below was sent out to some NavList email recipients with scrambled headers and the subject obscured. For those of you who did not get a scrambled version, sorry for the duplication:

    "Hello. I am living in Hot Springs, Arkansas, and am studying
    celestial on my own. Have several books, including Bowditch.

    I am unclear about the so-called "d correction." It seems the d
    number in the dailt pages of the Nautical Almanac should really
    be called a "d factor" or something similar. Mary Blewitt's
    book says to use this d factor to get into Table 5 of H.O. 249.

    My confusion stems from no one else mentioning Table 5 (except
    H.O. 249 itself).

    Can/would someone explain to me, at the 8th grade level, about
    the d factor for sun, moon and planet shots.

    Thanks in advance. E-mail is fine (dickbash[at]bashlaw.com). You
    have a fine resource here. Hope I am accepted as a list member.


    Richard Bash
    Hot Springs, AR"
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