"""
========================================
Getting the observer location from a Map
========================================

How to access the observer location from a `~sunpy.map.Map` and interpret it.
"""
import matplotlib.pyplot as plt
import numpy as np

from astropy.constants import R_earth

import sunpy.map
from sunpy.coordinates import get_body_heliographic_stonyhurst
from sunpy.data.sample import AIA_171_IMAGE

###############################################################################
# We use the sunpy sample data.

aiamap = sunpy.map.Map(AIA_171_IMAGE)

###############################################################################
# You can access the observer coordinate with:

print(aiamap.observer_coordinate)

###############################################################################
# This provides the location of the SDO as defined in the header and is
# necessary to fully define the helioprojective coordinate system which
# depends on where the observer is. Let's see where this is with respect
# to Earth. SDO is a geosynchronous orbit with a semi-major axis of
# 42,164.71 km and an inclination of 28.05 deg.
# We will convert it to Geocentric Celestial Reference System (GCRS)
# whose center is at the Earth's center-of-mass.

sdo_gcrs = aiamap.observer_coordinate.gcrs
sun = get_body_heliographic_stonyhurst('sun', aiamap.date)

##############################################################################
# Let's plot the results. The green circle represents the Earth.
# This looks like the Earth is in the way of SDO's
# field of view but remember that it is also above the plane of this plot
# by its declination.

fig = plt.figure()
ax = fig.add_subplot(projection='polar')
circle = plt.Circle((0.0, 0.0), 1.0, transform=ax.transProjectionAffine + ax.transAxes, color="green",
                    alpha=0.4, label="Earth")
ax.add_artist(circle)
ax.text(0.48, 0.5, "Earth", transform=ax.transAxes)
ax.plot(sdo_gcrs.ra.to('rad'), sdo_gcrs.distance / R_earth, 'o', label=f'SDO {sdo_gcrs.dec:.2f}')
ax.plot(sun.lon.to('rad').value * np.ones(2), [1, 10], '-', label='to Sun', color='black')
ax.legend()

plt.show()