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    Re: Lunars-lite
    From: Roger W. Sinnott
    Date: 2018 Jan 29, 21:34 -0500

    Peter,

     

    For more detail on the subject of crater timings at a lunar eclipse, you should consult the paper I co-wrote with Dave Herald in the Journal of the British Astronomical Association, vol. 124, issue 5, 2014, pages 247-253.  We analyzed 22,539 timings made at 94 different lunar eclipses during 1842-2011, all done visually with small telescopes.  We looked at and discuss a whole host of possibilities – such as whether the shadow size correlates with the season of the year (it does not), whether it is different for a bright eclipse compared to a very dark one (it isn’t), whether the shadow’s shape differs from its expected shape taking into account the known oblateness of the Earth (it doesn’t), and on and on.

     

    The purpose of these observations has nothing to do with navigation – I only thought of that connection this morning!  Rather it is to evaluate the size and shape of the Earth’s shadow and look for long-term changes.  We did not see evidence in the timings of any irregularity around the edge of the shadow, or changes during the course of a given eclipse, although for most eclipses there aren’t enough timings to investigate such effects properly.  So there are still unanswered questions, such as why the obvious difference in the umbra’s deduced size at the two well-observed eclipses cited in my website article.

     

    In any event, the 22,539 timings are readily available for anyone who may wish to study them further.  They are archived on VizieR, hosted by the Strasbourg Astronomical Data Center at http://vizier.u-strasbg.fr/viz-bin/VizieR as catalogue VI/140, and it even includes the name of the observer who made each timing.

     

    But I need to stress one point:  No one should try to come up with a clever new way to make crater timings, such as analyzing a video, taking a series of timed digital images, or using some kind of photometer.  The response of an electronic device will surely differ from that of the human eye, meaning you can’t compare such results with the gigantic collection of visual timings over the last couple of centuries.  (Have you ever noticed that the umbra’s edge appears much sharper to the human eye than it does on most photographs?)  In effect, adopting some new technique means tossing out all those prior visual observations and starting over.  And who’s to say that some exotic, “even better” timing method won’t come along a decade or two from now?

     

    Roger

          

     

    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Peter Monta
    Sent: Monday, January 29, 2018 7:39 PM
    To: rsinnott@post.harvard.edu
    Subject: [NavList] Re: Lunars-lite

     

     

    That's interesting.  If both shadow-ingress and shadow-egress observations are available, then taking the mean should remove this bias.

     

    On second thought, these two shadows are coming from opposite sides of the Earth.  What's the mechanism behind the shadow-variation?  If it becomes uncorrelated over times of a few hours, or across every 1000-km patch of troposphere, then there's not as much benefit.

    Cheers,

    Peter

     

    Hi Roger,

    But these can't be PERFECTLY accurate because the shadow's size varies slightly from one eclipse to another, for unknown reasons.

     

    That's interesting.  If both shadow-ingress and shadow-egress observations are available, then taking the mean should remove this bias.

     

    Visual photometry is one thing, but I've wondered how well eclipses of Jupiter's moons could be timed with, say, webcam photometry.  The same question would apply to lunar craters I guess.

    Cheers,

    Peter

     

       
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