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    Re: Point Venus, May 1774
    From: Gary LaPook
    Date: 2007 Apr 25, 11:19 -0700

    Gary LaPook wrote:
    I don't know what happened there in May, 1774 but I do know what
    happened there on July 11, 2005, I sat there with my sailing crew
    drinking Hinano beer! The area is a park now with a lighthouse and a
    commemorative plaque to the 1769 expedition.
    On Apr 24, 8:24 am, alex  wrote:
    > What exactly happened in Point Venus in Thaiti
    > in the first week of May, 1774?
    > Point Venus, S 17d29'8, W 149d29.6 (from Terraserver)
    > In the records we find the following data:
    > date, time-by-the-clock,
    > zenith distance of the Sun,
    > altitude of the Moon,
    > distance from the Sun to the Moon,
    > Barometer, thermometer,
    > derived longitude. Errors of the Quadrants (all three).
    > No dip or alt above sea level is recorded.
    > The question is what can we do with these data.
    > Let me try to assess the reliability of each piece.
    > Time-by-the-clock seems pretty useless;
    > my preliminary investigation shows that this was a really
    > lousy clock (see below).
    > Zenith distance of the Sun seems to be the most reliable
    > number. It was taken by an "astronomical quadrant"
    > A stationary one, of the type of transit/theodolite,
    > I suppose. And it took the zenith distance rather
    > than altitude, so I suppose it is dip-independent.
    > The Error of this quadrant is recorded as 21",
    > a quantity one can neglect for the first approximation.
    > (See below why I prefer to neglect their recorded IC,
    > at least in the first step of this investigation).
    > The altitudes and the distances were recorded
    > "as read on the sextant scales", no corrections added,
    > The main problem is to determine the GMT of each observation
    > as precisely as we can.
    > I used the following method. For the given date,
    > I determine GMT from their presumably
    > most precise observation, the Sun zenith distance.
    > I use Frank's calculator to do this.
    > The procedure is exactly the following.
    > I take their Sun ZD, convert it to altitude,
    > subtract refraction and add/subtract Sun SD,
    > according to the limb observed.
    > Then I find by trial-and-error, using the Frank
    > calculator, the GMT of the moment when the Sun was
    > on this altitude on that day, in their position
    > (Terraserver position).
    > Then I use this GMT, and this position, to compute
    > the rest of the quantities: Moon alt, and Sun-Moon dist.
    > Then I reduce their Moon alt using parallax, refraction
    > and Moon SD.
    > Then I record the errors:
    > a) of their watch, as compared with GMT
    > b) of their Moon altitude and
    > c) of their Lunar distance.
    > 1. The clock was terrible. Going 1-2 MINUTES PER DAY!!!
    > Irregularly. They should had better used a sandglass:-)
    > I conclude from the citation below that the clock was
    > regulated to sideral time, rather than mean solar time,
    > but in any case, their time records do not help in
    > determining the GMT with reasonable precision.
    > It is very interesting, what Wales (the astronomer
    > in this expedition) says about this clock:
    > "...it may not amiss to take notice of some
    > very extraordinary irregularities, which happened to
    > the going of the Clocks..."
    > "The Clock B lost 1m22s a day on syderal time at Otaheite,
    > lat 17d29'1/4 S and long 210d25' E from April 23d
    > to May 9th, 1774; but I here reject its loss
    > between April 30-th and May 1st,
    > as it appears to have lost exactly 1m more on that day
    > than on any other;
    > a circumstance I cannot account for PROPERLY,
    > as I never, that I know of, left the case or face of
    > the Clock unlocked." And  he continues:
    > "There is however little doubt but that some WITTY
    > Gentleman or other found means to open it, and put the
    > Clock a minute back,
    > I suppose to try whether or no the
    > ASTRONOMER could find it out."
    > (Astronomical Observations... p. xvi. Emphasized words
    > are in his text). I suppose when he says 1m22s a day
    > he means that the AVERAGE LOSS was 1m22s EVERY DAY.
    > I only investigated 3 days so far (May 1-3).
    > And I have to say that they were negligent in recording
    > the date, but this is easy to fix.
    > (For example, they never make clear whether they record
    > the local date or Greenwich date, and how do they count the
    > local date. Given that their longitude was
    > "210 East" as they say:-) this makes the thing puzzling
    > in the beginning. But I was able to fix all GMT dates
    > beyond reasonable doubt:-)
    > This message is too long. I will continue in the next one.
    > Alex.
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