[AstroPy] Use of separation

Fri Mar 26 05:48:37 EDT 2021

It's your "pointing" coordinate that's not Earth-centred. FK5 has an origin
at the barycenter IIRC.

On Fri, 26 Mar 2021 at 09:43, Emil Lenc <elenc at me.com> wrote:

> Hi Stuart,
>
> So that’s the bit I don’t get, I did provide a location in get_moon so
> shouldn’t both be as viewed from the Earth?
>
> Cheers,
>
> Emil.
>
> On 26 Mar 2021, at 20:00, Stuart P Littlefair <
> s.littlefair at sheffield.ac.uk> wrote:
>
> It is counterintuitive. The point is that the separation of two objects
> doesn’t make sense without a coordinate frame in which that separation is
> measured.
>
> A celestial object may be in the same direction as the moon when viewed
> from the earth (a GCRS frame), but the two will not line up for an observer
> at the centre of the solar system (an ICRS frame).
>
> So when you calculate the separation between two coordinates that are
> measured in different frames, you *have* to choose one or the other frames
> to measure the separation in.
>
> The unintuitive nature of this code comes from the assumption of the user
> that “get_sun” or “get_moon” will return a coordinate in the same frame as
> “pointing”, which is not true.
>
> Stuart Littlefair
> Dept. of Physics & Astronomy
> Univ. of Sheffield, Sheffield, S3 7RH
>
> email: s.littlefair at shef.ac.uk <s.littlefair at shef.ac.uk>
> Phone: +44 114 2224525
> Sent from my iPhone
>
> On 26 Mar 2021, at 08:35, Emil Lenc <elenc at me.com> wrote:
>
> Hi Stuart,
>
> Oh, thanks, I had completely overlooked this in the documentation. I’d
> have to admit that this is indeed unintuitive behaviour! So much so that
> I’m actually not sure how to reliably determine the separation in this
> instance? It also seems to be at odds with their example where they state
>
> Also note that the two input coordinates were not in the same frame — one
> is automatically converted to match the other, ensuring that even though
> they are in different frames, the separation is determined consistently.
>
>
> Is it just that because the frame used to determine the moon’s coordinates
> is a bit odd? It seems a bit odd to convert the RA/Dec of the moon to
> another SkyCoord (with the same frame as my pointing coordinate) to ensure
> that the result is consistent … though perhaps I am just doing this wrong
> (sorry, coordinates and frames are not my strong point)?
>
> Cheers,
>
> Emil.
>
> On 26 Mar 2021, at 18:50, Stuart P Littlefair <
> s.littlefair at sheffield.ac.uk> wrote:
>
> Hi Emil
>
> The documentation for separation has an explanation of this.
>
>
> https://docs.astropy.org/en/stable/api/astropy.coordinates.SkyCoord.html#astropy.coordinates.SkyCoord.separation
>
> “ If the other coordinate object is in a different frame, it is first
> transformed to the frame of this object. This can lead to unintuitive
> behavior if not accounted for. Particularly of note is that
> self.separation(other) and other.separation(self) may not give the same
> answer in this case.“
>
> Stuart Littlefair
> Dept. of Physics & Astronomy
> Univ. of Sheffield, Sheffield, S3 7RH
>
> email: s.littlefair at shef.ac.uk <s.littlefair at shef.ac.uk>
> Phone: +44 114 2224525
> Sent from my iPhone
>
> On 26 Mar 2021, at 07:45, Emil Lenc <elenc at me.com> wrote:
>
> Hi All,
>
> I’ve been trying to determine the separation between a point in the sky
> and the moon and I’ve come across a strange situation that I don’t quite
> understand. Depending on how I determine the separation I get two very
> different results i.e. if I determine the separation between the moon and
> the sky location as opposed to the separation between the sky location and
> the moon. I expected that these should give the same result but they don't.
> Here is a simple script to demonstrate what I mean:
>
> from astropy import units as u
> from astropy.time import Time
> from astropy.coordinates import EarthLocation, Angle, SkyCoord
> from astropy.coordinates import get_moon
>
> # Where are we observing from?
> latitude = Angle("-26:41:46.0", unit=u.deg)
> longitude = Angle("116:38:13.0", unit=u.deg)
> observing_location = EarthLocation(lat=latitude, lon=longitude)
>
> # When are we observing?
> t = Time("2021-03-04 21:51:58", format='iso', scale='utc')
>
> # Where is the moon?
> moon = get_moon(t, observing_location)
> pointing = SkyCoord("15h33m45.000s -18d40m3.793s", frame='fk5’)
>
> # Print out the location of the moon
> dir_str = moon.to_string(style='hmsdms')
> print("Moon (at specified location) is at: %s" %(dir_str))
>
> # Print out the location of the pointing
> dir_str = pointing.to_string(style='hmsdms')
> print("Pointing is at: %s" %(dir_str))
>
> # Determine the separation between the moon and the pointing
> sep = moon.separation(pointing).deg
> print("Moon separation from pointing = %0.1f deg" %(sep))
>
> # Determine the separation between the pointing and the moon
> sep = pointing.separation(moon).deg
> print("Pointing separation from Moon = %0.1f deg\n" %(sep))
>
> The result of running this script producing the following result:
>
> Moon (at specified location) is at: 15h48m35.4296s -18d29m19.3018s
> Pointing is at: 15h33m45s -18d40m03.793s
> Moon separation from pointing = 3.5 deg
> Pointing separation from Moon = 71.5 deg
>
> Does anyone understand why the second case giving a result of 71.5
> degrees? Any help would be greatly appreciated.
>
> Best regards,
>
> Emil.
>
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-- 
Stuart Littlefair

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Dept. of Physics & Astronomy,
Univ. of Sheffield, Sheffield, S3 7RH.

email: S.Littlefair at sheffield.ac.uk
phone: +44 114 2224525
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