Norm Hecht Oct 11 8:50 PM
> On Oct 11, 2016, at 10:13 AM, Bill Gray pluto@... [find_orb] <find_orb@yahoogroups.com> wrote:
>
> On 2016-10-07 16:48, norm_hecht@... [find_orb] wrote:
> >
> > I'd like to determine the orbit of Earth's moon. Is Find_Orb a good
> > tool for this project? I'll actually be doing observations via an
> > iPhone; the camera is good enough to observe the moon (at least as
> > a blob on screen), and the device can capture its orientation, GPS
> > location, and the time of the observation.
>
> Hi Norm,
>
> I rather like this idea. I could imagine people getting phone camera
> "astrometry" of one sort or another for the Moon, and/or perhaps Venus
> (dunno just how faint various breeds of smartphone will go), and computing
> an orbit from the observations. The idea of doing it with the alt/az data
> measured from a smartphone is appealing, though I'd wonder about the
> accuracy of that data; if it's good to within a degree, I'd be rather
> impressed.
>
> But even if that method didn't work, I could imagine some reasonably
> simple methods for measuring lunar or planetary positions well enough
> to get good orbits (*). It strikes me as an excellent educational project.
> You should, without too much trouble, be able to get the length of
> the lunar month, and the fact that the lunar orbit is elliptical,
> and so forth.
>
> I thought I'd have to make some small modifications to Find_Orb before
> it could do something like this. However, I fed it some totally artificial
> astrometry "taken" by Peter Birtwhistle at (J95) Great Shefford and by
> (703) Catalina Sky Survey (actually computed by asking Guide for lunar
> RA/decs at various semi-random times with the Moon up and the Sun down),
> and got the orbit right away :
>
> http://www.projectpluto.com/temp/mpec.htm
>
> (If you load the astrometry at the above URL into Find_Orb, it will
> first decide that this is an object orbiting the Moon a few hundred
> meters from its center. You'll have to go into the Settings dialog and
> tell Find_Orb that you want a geocentric orbit. Do that, and you should
> get the result shown in the pseudo-MPEC.)
>
> > Right now I'm using some functions from Find_Orb to convert azimuth
> > and elevation angles to right ascension and declination, although
> > it looks like I'm not using them correctly: October 7, 2016,
> > 20:14 UTC become Julian day 2457669.36330594, which day_to_dmy
> > turns into 2016, 9, 24.
>
> Hmmm... 13 days is the difference (during our lifetimes) between
> the Julian and Gregorian calendars. That JD does match 2016 Oct 7
> (Gregorian) and 2016 Sep 24 (Julian). When you call day_to_dmy,
> the last argument should be either CALENDAR_GREGORIAN (if you want
> to use Gregorian for all dates) or CALENDAR_JULIAN_GREGORIAN (if
> you want 1582 October 4 to be followed by 1582 October 15).
>
> -- Bill
>
> (*) The first and simplest measurement method I thought of was rise/set
> times. In theory, time the rising and/or setting of an object six
> times, and you've got the data needed to compute its orbit. The
> accuracy will be better if those times are spread out a little bit,
> and it would probably help to measure them from different points
> on the earth (should be easy enough to collaborate with others on
> this).
>
> The big drawback I see with this is that rise/set times vary a bit
> due to atmospheric conditions, and a difference of one minute would
> correspond to a quarter of a degree. Probably a lot better than a
> smartphone camera could do, but still not great.
>
> Second method would be to measure the apparent angular distance
> between the limb of the moon and a bright star: "The Moon's limb
> was 6.3 degrees from Regulus at thus-and-such time/date from this
> place." Again, six observations would get you an orbit. This
> might work somewhat better; it shouldn't be too hard to build
> a pseudo-sextant to get that level of accuracy.
>
> Either of these would require some modifications to Find_Orb
> to allow "observations" to be, not just RA/decs, but alt/azzes
> or angular distances from a known RA/dec.
>
> Third method would be to just take pictures with a smartphone
> and run them through astrometry.net to get for-real RA/decs.
>
>