NavList:

Dear George and Brad,

Brad, you just wrote :

QUOTE

Still no joy in the land of lunars!

Could it be that the altitudes given are actually in error?

Or am I still missing something!

Best Regards
Brad

UNQUOTE

***********

With the same assumptions as detailed in post [12606], I have chosen to rework with modern computation tools the example of Sep 11, 1773.

Why this one first ? Simply because it has the very same "printer dagger sign" as the one on Aug 04, 1773 that we have started studying. So, and for lack of more complete information on the exact meaning of these exotic signs, I start putting myself in exactly the same situation first.

So, here are the results :

IF WE ASSUME HEIGHT OF EYE IS 0 FT, then get the following :

Lunar Distance Observation Time UT = 19h01m07s0, and
Observed position : S 16°31'37" / E 208°23'51", and

IF WE ASSUME HEIGHT OF EYE is 12 FT, then get the following results :

Lunar Distance Observation Time UT = 19h01m03s3, and
Observed position : S 16°36'09" / E 208°22'48", and

IF WE ASSUME HEIGHT OF EYE is 24 FT, then get the following results :

Lunar Distance Observation Time UT = 19h01m01s8, and
Observed position : S 16°38'02" / E 208°22'21"

These results should be consistent with Frank's on Line Computer results. I have not checked.

As a first glance :

- The "Latitude swing" observed on Aug 04 th, has (almost) vanished - 7' vs. 29' -, simply because the relative geometry of the SUN and MOON positions have changed. Here the cutting angles of their directions are (much) more favorable, and

- Interestingly enough, the Longitude determination is also (almost) insensible to (significant) variations in HOE : less than 2' .... Good for us ! and

- These results seem "realistically reasonable", although the Latitude information derived from these examples is a bit disappointing. Possible direct consequence of "just average" heights, instead of "accurate heights" ? Computed Longitude falls within 15' of published figures. This is probably the best which could be achieved. Let us wait to compare with the current coordinates of the vessel (see my request to George down this page). And

- The 24' HOE value still looks reasonable for the time being. With both examples (Aug 04, and Sep 11) we might start thinking that HOE = 0 FT might not be our first choice assumption, but we need to work on more examples with the very same printer sign. And

- just for information, the "alternate position" lies somewhere by N45°/W138° .

A few last questions Brad :

Are my explanations on the " 29' latitude swing " sufficient ? I attempted to explain the best I could. Have I adequately understood your concern ?

Concerning the SUN declination as derived from the 1773 Nautical Almanac - I have not looked it up yet for fear of spending quite some time on it (I ... "hate !!!!" computations through Apparent Time while we now have at hand Mean Time), do you still keep observing this 3' difference on the SUN's declination ?

*******

George, you wrote :

QUOTE

The other question, that Kermit keeps mentioning, relates to delta-T, due
to slowing of the Earth's spin. Bayly's own calculations were correctly
based on the almanac of his day, and required no correction for delta-T.

UNQUOTE

Further to a recent discussion, which you almost mentioned, we have all agreed that :

- If we use Historical Almanacs, the is NO NEEED for any delta T correction on top of the published figures since delta T was already (implicitly) entered, and

- If we use modern computation tools, we ARE TO correct for some value of delta T. Since there is no absolute consensus to-day on delta T historical values, and just in case somebody might want to crosscheck my calculations through modern different computing software - which I certainly wish - it is necessary to provide this delta T value, updated each time. This is why I have kept providing it here in order to help.

*******

One very interesting data would be to get the Ohama Reno Harbour Coordinates. Could you please provide them to us all ?

Best Regards to all of you

Kermit

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