[Branch]: differential_rotation

This is the feature branch that contains some diff rot code, I think it needs some love so I am opening this so people don't forget about it!!
Both B0 and the semi-diameter are dependent on the position of the
instrument in space relative to the Sun.  SOHO EIT files encode the radius
of the Sun as seen from EIT.  B0 is in their too.

As for solar P (and the rest of them)...  from
http://gong.nso.edu/data/DMAC_documentation/Vmbical/Solar_coord_defined.html

*P-angle (or P):* *The position angle between the geocentric north pole and
the solar rotational north pole measured eastward from geocentric north.
The range in P is +/- 26.3l degrees. * *B0:* *Heliographic latitude of the
central point of the solar disk; also called the B-angle. The range of B0
is +/- 7.23 degrees, correcting for the tilt of the ecliptic with respect
to the solar equatorial plane. * *Example:* *If (P,B0) = (-26.21 degrees,
-6.54 degrees), the heliographic latitude of the central point on the solar
disk is -6.54 degrees (the north rotational pole is not visible), and the
angle between the projection onto the disk of the geocentric north pole and
the solar north rotational pole is 26.21 degrees to the west. * *L0:*
*Heliographic
longitude of the central point of the solar disk. The longitude value is
determined with reference to a system of fixed longitudes rotating on the
sun at a rate of 13.2 degrees/day (the mean rate of rotation observed from
central meridian transits of sunspots). The standard meridian on the sun is
defined to be the meridian that passed through the ascending node of the
sun's equator on 1 January 1854 at 1200 UTC and is calculated for the
present day by assuming a uniform sidereal period of rotation of 25.38
days. *

*Once P, B0, and L0 are known, the latitude, central meridian distance, and
longitude of a specific solar feature can be determined as follows:*
*Latitude:* *The angular distance from the solar equator, measured north or
south along the meridian. * * Central meridian distance (CMD):* *The
angular distance in solar longitude measured from the central meridian.
This position is relative to the view from earth and will change as the sun
rotates; therefore, this coordinate should not be confused with
heliographic positions that are fixed with respect to the solar surface. *
*Longitude:* *The angular distance from a standard meridian (0 degrees
heliographic longitude), measured from east to west (0 degrees to 360
degrees) along the sun's equator. It is computed by combining CMD with the
longitude of the central meridian at the time of the observation,
interpolating between ephemeris values (for 0000 UT) by using the synodic
rate of solar rotation (27.2753 days, 13.2 degrees per day).*



On Wed, Aug 27, 2014 at 10:02 AM, Stuart Mumford <notifications@github.com>
wrote:

> In sunpy/physics/transforms/differential_rotation.py:
>
> > +    # It appears that there is a difference in how the SSWIDL function
> > +    # hel2arcmin and the sunpy function below performs this co-ordinate
> > +    # transform.
> > +
> > +    newx, newy = convert_hg_hpc(longitude.to(u.deg).value + drot.to(u.deg).value,
> > +                                latitude.to(u.deg).value,
> > +                                b0_deg=vend["b0"].to(u.deg).value,
> > +                                l0_deg=vend["l0"].to(u.deg).value,
> > +                                dsun_meters=(constants.au * sun.sunearth_distance(t=dend)).value,
> > +                                occultation=False)
> > +    newx = Angle(newx, u.arcsec)
> > +    newy = Angle(newy, u.arcsec)
> > +    return newx.to(u.arcsec), newy.to(u.arcsec)
> > +
> > +
> > +def _calc_P_B0_SD(date, spacecraft=None, arcsec=False):
>
> what's P again? but B0 and semi-diameter should be encoded, or computeable
> from a good header right?
>
> --
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> <https://github.com/sunpy/sunpy/pull/902/files#r16776394>.
>