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Let me add that the longitude of Point Venus
(Cooks observatory has a memorial sign) which I determined from
Terraserver is 149d 29'6.

Alex.

On Mon, 7 Jan 2013, Henry Halboth wrote:

>
> It's rather refreshing to see the subject of Lunars again being discussed.
> For those who may be interested, I post, without comment, the following
> excerpt from Captain (later Admiral) Bligh's narrative concerning his
> second voyage to Tahiti, as first published in 1792.
>
> "The result of the mean of 50 sets of lunar observations taken by me on
> shore, gives for the Longitude of Point Venus                 210 33 57 E
> Captain Cook, in 1769, places it in    210 27 30
> In 1777, his last voyage                    210 22 28"
>
> Regards,
>
> Henry
>
> On Mon, Jan 7, 2013 at 3:30 PM, Frank Reed wrote:
>
>> Hanno Ix, you wrote:
>> "I wonder if the ephemeris especially of the moon as used by Cook can
>> still be studied today. Naturally, Cooks errors cannot have been any
>> smaller than the ones of those."
>>
>> Even better. Although we can certainly study the tables (Mayer) that were
>> used to generate the early Nautical Almanacs, we may as well jump straight
>> to the product itself. Very nearly all Nautical Almanac editions from the
>> first in 1767 through the early 20th century are available in full on
>> Google Books. To compare these with correct values (as nearly as can be
>> determined given some continuing small uncertainties in delta-T for that
>> era), we need lunar distance tables given, preferably in degrees, minutes,
>> and seconds, as was normal back then, and we need those tables for the
>> rather unusual argument of GAT, Greenwich Apparent Time, which was standard
>> before the switch to GMT which came in 1834 (and was considered seriously
>> late by commentators at the time). Back in 2004 I made available online
>> software which does just that. I am attaching an image of a portion of the
>> tables for Sun-Moon lunar distances from August 1767 compared against data
>> from my online app for the same dates, lined up to match the tabular format
>> of the old Nautical Almanac. As you can see, there are cases where the
>> difference is as large as 50 seconds of arc, but there are others where the
>> difference is only 1 second of arc. For an "average" lunar this will add an
>> uncertainty equivalent to an error of something like 0.25' in the distance.
>> Whatever the observational error may be, this error would sometimes
>> increase it and other times cancel it out. On average two sources of error
>> add as the square root of the sum of the squares, so if they had, let's
>> say, 0.25' error from observations, then the net error would be
>> sqrt(2)*0.25 or about 0.35' (equivalent to 42 seconds in time or 10' in
>> longitude). By 1805 the errors in the lunar distance tables had been
>> significantly reduced. By 1875, they were nearly perfect for navigational
>> use (though lunars were essentially obsolete at sea by that date, there
>> were still a handful of practitioners, and there were definitely lunarians
>> on land, exploring and mapping Africa, for example).
>>
>> -FER
>>
>>
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