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    Re: Lunar longitudes, not by lunar distance.
    From: Hanno Ix
    Date: 2009 Aug 9, 11:40 -0700
    Gentlemen:

    I certainly will study your suggestions in all detail. I thank you again for the mental energy and time you have invested. The instruments you show in the responses are fascinating and incredibly ingenious. I envy you. I hope to hear more from you about them and their use.

    I think you all agree the concept is sound, and a trial, particularly with a theodolith, would be worthwhile. Now, I have to confide in you that I promised the Vienna Observatory to make an attempt to recreate their Chronodike in return for their digging up info about it.  They don't know if one still exists. Which makes me honor bound to do build one! I do have the tools - about my skills I will find out.

    So, with your permission, I will forgo the purchase of a theodolith and spend my money and time on building a Chronodike. I will report in time. Please be patient  - I am also in a construction project.

    At this point, may I expand, though, on our discussion? Assume, you have on board an almanach, a stopwatch - and a well stuffed woodworker's tool chest. You are skilled in woodworking.  You arrive at our lonely island which has lots of trees and great views of the skies. ( Food and water, too! )

    You don't know where you are, nor do you know GMT. Now, you start wondering if you could apply our concept by using a camara obscura - the camera with nothing but a hole in one side.

    My questions:

    1. Is it astronomically a sound idea? Is it optically realistic?

    2. If the answer is yes, and given an the best, but realistic, camera obscura you can think of  how would you plan your observations and sight reduction?

    2. How would you build your camera obscura, what featuers would it have? Are there any improvements that you could apply to the known camera obscura to accomplish your goal? These improvements should be commensurable to your tools.

    3. Can you think of any other set-up you could build on the island that works equally good even better than your camera obscura?


    Actually, I personally do like the idea of spending time on a nice tropical island  pursueing an avocation of mine. Actually, San Diego is not bad either.

    Best regards

    H




    --- On Sun, 8/9/09, Nicolàs de Hilster <groups@dehilster.info> wrote:

    From: Nicolàs de Hilster <groups@dehilster.info>
    Subject: [NavList 9433] Re: Lunar longitudes, not by lunar distance.
    To: NavList@fer3.com
    Date: Sunday, August 9, 2009, 7:17 AM


    Geoffrey Kolbe wrote:
    > First is that for altitudes greater than about 50
    > degrees, it is usually not possible to get your
    > eyeball behind the telescope using a standard
    > transit theodolite. I think Hanno says he lives
    > in San Diego, whose latitude is low enough that
    > he will be able to see Polaris without difficulty
    > using a standard transit theodolite. Here in
    > Northern Europe, it would not be possible to do so.
    >
    > There are so called 'broken' theodolites, where
    > the viewing is done along the axis about which
    > the telescope and vertical scales rotate. There
    > is a 45 degree mirror which redirects the view up
    > the telescope.
    I measure sun azimuth's quite regularly professionally using either a
    Wild T2 theodolite or a Leica TCRA1101 total station, both being
    instruments with standard telescopes. In order to overcome the problem
    mentioned Wild (and most other theodolite manufacturers as well)
    supplied a rectangular eye piece that can be mounted to the telescope
    (it fits the Leica as well).
    > There is no limit to the altitude
    > one can view using such a theodolite, the most
    > famous example of which is the Wild T4 - but you
    > won't find one of those on ebay!
    Oh yes, they do appear on e-bay as well, twice even quite recently, but
    at prices fitting their use: astronomical...
    > Third and final point. As George mentioned, when
    > doing measurements on the sun or moon, it is
    > usual to take a measurement as the edge of the
    > body (top of bottom for altitudes, sides for
    > azimuth) touches the appropriate cross hair and
    > then make a correction to get the centre of the
    > body. For altitudes, the correction (to first
    > order) is simply the semi-diameter of the body.
    > For azimuths, the correction is the semi-diameter
    > divided by the cosine of the altitude.
    >   
    When I measure azimuth's I use a solar prism, also known as a Roelofs
    prism. This prism was invented by the Dutch professor Roelofs in 1947
    (patent requested 19 May 1947 and granted on 16 May 1949) and produces
    four overlapping images of the sun, the centre of which is used to aim
    at (see http://www.dehilster.info/instrumenten/theodolite4/index.html
    where you need to scroll down to picture 8 and further). Apart from a
    small centring error (which will be corrected for by transiting the
    telescope) the filter will give the sun's centre.
    > With both these last two points, it can be seen
    > that it is advantageous to observe bodies of low
    > altitude to avoid significant errors in azimuth.
    >
    >   
    The best time to observe the sun's azimuth is in indeed the early
    morning or late evening, especially when in the tropics. The azimutal
    motion is then quite slow, with forthcoming better results (it is all
    about timing).

    Nicolàs





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