Merge pull request #7352 from nabobalis/example

Add text from an email from Albert - Lazy edition

changelog/7352.doc.rst

@@ -0,0 +1 @@
+Added explanation text to :ref:`sphx_glr_generated_gallery_plotting_plotting_blank_map.py` about the offset between "(0, 0)" in helioprojective coordinates and the heliographic equator.

examples/plotting/plotting_blank_map.py

@@ -54,11 +54,17 @@
 yc = [0, 100, 400] * u.arcsec
 
 ################################################################################
-# Place and mark coordinates on the plot.
+# Plot the blank map with the specified coordinates.  Note that the marker for
+# (0, 0) in helioprojective coordinates is at the center of the solar disk, yet
+# the heliographic equator (zero degrees latitude) does not go through the disk
+# center and instead curves below it.  The reason for that is the observer,
+# specified as Earth in this example, is almost always at non-zero heliographic
+# latitude, and disk center as seen by such an observer will have that same
+# heliographic latitude.  The :func:`~sunpy.coordinates.sun.B0` function returns
+# Earth's heliographic latitude at a specified time.
 
 coords = SkyCoord(xc, yc, frame=blank_map.coordinate_frame)
-p = ax.plot_coord(coords, 'o')
-# Set title.
+ax.plot_coord(coords, 'o')
 ax.set_title('Plotting fixed points on a blank map')
 
 plt.show()