Now we will look at Chichester's illustration of the old method on page 234 to compare it with the short method already discussed.
What he is doing with this example is using the traditional Haversine- Cosine method of calculating Hc and azimuth. The formulas used for this were derived from the standard Sine - Cosine formulas and, in fact, uses the same method and formula for calculating azimuth.
The formula for calculating Hc is:
hav ZD = hav LHA cos Lat cos Dec + hav (Lat ~ Dec)
(Lat ~ Dec means the difference between latitude and declination, subtracting the smaller from the larger if of the same name and adding if of different names)
(ZD is zenith distance)
so Hc = 90º - ZD
For calculating azimuth we use
sin Z = (sin LHA cos dec ) / cos Hc
usually rearranged into the more convenient form of
sin Z = sin LHA cos dec sec Hc
Since csec ZD is the same as sec Hc
we can rearrange this formula to
sin Z = sin LHA cos dec csec ZD
Chichester used these formulas and solved them using logarithms by using this format:
Hc Az
LHA ___________ log hav LHA ___________ log sin LHA ______________
Lat ___________ log cos Lat ____________
Dec __________ log cos Dec + ___________ log cos Dec ______________
______________
inv hav _______________
(L ~ D) ___________ hav (L~D) + ____________
89-60
ZD - ___________<<<< inv hav _____________>>> >>>>>>log csec ZD +___________
Hc ____________
Ho-_____________
A______________ Z ______________<<<>>>>>>>> log csec ZD +_10.16166____
Hc __46-26_____
Ho-__46-23___________
A____3 away___ Z ___57_________<<<<< I am attaching a marked a up version of page 235, page 29 from Volume 3
> of H.O. 249 and the correction table from that volume. To make it easier
> to follow this explanation I have marked up page 235 with red boxes
> labeled A through G.
>
> "A" show his computation of GMT or Zulu time for entering the Almanac.
> 12:11:22 is his watch time and the watch is obviously set to GMT. 02:30
> + 1/2 is the correction for his watch error which is running that many
> minutes slow on GMT, Adding these two numbers produces the GMT of the
> observation of 12:13:52 (he dropped the extra half second.)
>
> "B" shows the computation of LHA for entry into H.O. 249. Taking the
> entry of the sun's GHA (Greenwich Hour Angle) for 12:00:00 GMT he takes
> out the GHA of 000º 53.1'. since the sight was taken 13 minutes and 52
> seconds after 12:00 o'clock you look in the increments table in the
> Nautical Almanac or in the Air Almanac and take out 3º 28' additional
> for that extra time which you add to the GHA at 12:00: to find the GHA
> of the Sun at the time of the observation to be 004º 21'. Chichester
> automatically added 360º to this and wrote down 364º21' to make it
> easier for the next step of subtraction his assumed longitude. Since the
> DR position is 43N - 25W ("C") he choses an assumed longitude of 25º
> 21' so when he subtracts this from the GHA he ends up with a whole
> number of degrees of LHA which is 339º.
>
> "C" shows the DR position.
>
> "F" shows the declination of the sun taken out of the Almanac at the
> same time that the value of GHA was obtained.
>
> "D" show the computation of Hc using H.O. 249. Looking on page 29 of
> volume 3 for latitude 43 and declinations 15-29 same name, we go down
> the column for 20º declination and come across from 339º LHA and we take
> out the tabulated Hc of 61º 02', the "d" correction value of +50 and the
> azimuth of 136. The tabulated Hc is for 20º declination exactly. Since
> the declination of the sun was actually 6 minutes more at the time of
> observation we must correct this tabulated Hc for this difference. We go
> to the correction table and under the 50 column (the "d" value) we read
> down to the 6 minutes of extra declination line and extract 5' which we
> add to the tabulated Hc of 61º 02' to determine the actual Hc of 61º 07'.
>
> "E" shows the computation of Ho. Starting with the HS (sextant altitude)
> of 60º 50' we add the correction for semi diameter (this is obviously a
> lower limb shot), subtract refraction and subtract dip which Chichester
> has combined into one correction of + 13'. (The SD alone is + 16 and
> refraction for this Hs is - 1' making a +15'. Since Chichester uses +13'
> he is including a dip correction of -2'.) On the next line he applied
> the Index correction (IC) of + 1 to arrive at the Ho of 61º 04'.
>
> "G" shows him subtracting the Ho from Hc to arrive at the intercept of 3
> away since Ho was less than Hc.
>
>
> The other examples he gives are done the same way although it is
> interesting that in three of the examples he also combines the IC with
> the other corrections to Hs writing down +14 total correction.
>
> I will get to page 234 tomorrow.
>
> gl
>
>
>
>
>
>
>
> Beverley Maxwell wrote:
>>
>> Gary,
>> Thank you. I am sending page 235, which Chichester refers to as the
>> short method, and page 234 he calls the long old method.
>>
>> Frank M.
>>
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>
>
>
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