NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
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? gl --- On Fri, 1/29/10, FrankReed@HistoricalAtlas.com <FrankReed@HistoricalAtlas.com> wrote:
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