Re: [find_orb] Finding the orbit of Earth's Moon

Norm Hecht Oct 11 8:50 PM

Thanks for the reply! I figured out the date issue the day after I sent
the email; I’d have figured it out sooner if I had thought to subtract
the two dates. I’m fairly confident that I’m using the alt-az to ra-dec
routine correctly now as well. I wound up having a frustrating time
creating MPC 80-column data correctly: I was having the app email the
data, and one of the mail apps was messing with the spacing (haven’t
figured out if it’s on the phone or laptop end). As a kluge, the app
uses underscores instead of spaces, and a quick search-replace fixes
the issue once it’s on my laptop.

I took several observations tonight over the course of five minutes or
so. The elevation angles were fairly consistent, within a band of about
a degree or so (not a bad guess on your part!), but the azimuth angles
were in a five or six degree range, just dreadful. So the accelerometers
are great for this purpose, but the magnetometer is lacking. Right now
the skies are cloudy, so I’ll try again tomorrow, making sure to give
the app more time to allow the device to let its filters converge.
I’m hoping that will improve the azimuth results, but I suspect they
still won’t be as good as the elevations.

I was _just_ able to get Venus to show up in the camera, which I hadn’t
even thought of trying, so I’ll try to get observations of it tomorrow
evening as well. I’ll have to rework the cross-hairs I’ve added to the
screen so there’s a gap at the center for that little dot of light to
show up.

Ideally, I’d like to do the orbit calculations on the phone itself.
My first thought was to use the non-interactive version of find_orb
as a background task, but it turns out that iOS won’t let you have
access to the file system, so I’ll have to get into the guts of the
code and try to get a basic algorithm or two running.

I’ve figured out that gauss_method will take three observations and
produce an orbit vector of doubles, which I think are the position
and velocity at the time of the middle observation. How well it will
work when I really only have one good angle is a worry for me. What
other functions should I be looking into?

Thanks again for your help!

Norm

> 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.
>
>