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    Re: How close without a good Lunar?
    From: Brad Morris
    Date: 2018 Jul 25, 16:15 -0400
    The original question remains:

    if I know exactly the altitude and bearing of enough bodies simultaneously..and plot on a known latitude line...then surely I can use this set of relationships can help me zero up my clock to some proximity


    The short answer is yes, you can zero your clock to "some" proximity.  The degree of proximity you require / desire will dictate your satisfaction with the experiment.  Do you want time zero'd to within an hour? A minute? A second?  This certainly will have an effect on outcome.

    The principle reason that you can directly recover time from a lunar is that the relative proximity of the moon provides rapid motion againdt the background and therefore good angular resolution vis time.

    Once you eliminate the moon, the angular rate of the planets is generally too slow to provide a satisfactory resolution.  I say generally, because in rare circumstances (when Venus transits the sun), it is potentially possible to extract time.  Waiting 100+ years for a transit of Venus, however, is going to be a bit boring.

    Another significant impediment to resolving time will be the accuracy of the altitude and azimuth of the observation.  Perhaps sufficient angular resolution could be obtained on land with a theodolite.  Your handheld sextant simply isn't going to be up to the task, even on land.  Obtaining azimuth to even the nearest degree will be quite tricky with a handheld.  That is certainly going to limit your precision.

    You are correct, in Harrison's day, the mountain of equations to be simultaneously solved, without computational error, would have presented itself as a formidable impediment.  Not impossible, but ridiculously formidable.  Today, not so much.

    All in all, I think you will be dissatisfied with the result.  Given sufficient resolution and accuracy in the observations, and sufficient computational power, it is technically feasible to extract time in the manner you describe.  It simply isn't practical.  

    That's my two cents
    Brad

     












    On Mon, Jul 23, 2018, 10:54 PM Mark Coady <NoReply_MarkCoady@fer3.com> wrote:

    I've been mulling this one for the last few weeks without real time to play. I'm afraid I'm about to make an ass of myself...but what the heck...never stopped me before...

    Careful Lunars does a fine job of ocean navigation.  I've had success with a good sextant.

    Lets say my clock goes amuck..I sleep unaware for some crazy hours....and its just that time of month when Lunars just aren't happening.  So I wake up...make a guess and reset my clock...I should have a dead reckoning Longitude.  

    I do have cues that I am right or wrong. If I know my latitude, thats a start. Lets say I know I am North of the equator, somewhere approaching the east coast of North America. To be absurd:If it's dark out..and my assumed local time is noon...well...ok time to guess agian...

    let's say I'm lucky, and I have four planet's visible at one time...... and a good compass...

    my mind keeps saying well...I can take bearings on four planets, and altitudes, and I certainly can get latitude and some good stars.

    My mind keeps saying if I know exactly the altitude and bearing of enough bodies simultaneously..and plot on a known latitude line...then surely I can use this set of relationships can help me zero up my clock.to some proximity without a Lunar....there must be only one geographic position (longitude) and instant in time the multiple bearings and altitudes could all exist. 

    I mean, even more complex is based on the relative near proximity of planets and distant star field....the relative positon of planets to background stars ...  if looked at simultaneously...would provide a unique position plot...

    Then my mind says to me..Dummy.....If that were so very true....the Harrison wouldn't have needed to spend so much time building those lovely little timepieces of his....  except maybe he didn't have a handy little silcon brain to run the umpteen zillion iterations for a single calculation of position. 

    Are my visualization skills that bad, or is it just that the math gets so daunting nobody would attempt it without a dedicated computer?

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