Add capability for light-travel-time correction

[ayshih]

Addresses #3039 by adding functionality to get_body_heliographic_stonyhurst(). When one specifies an observer through the keyword observer, an iterative loop determines the emission time in the past that results in photons arriving at the observation time. In the following example, Venus's apparent HGS coordinates have been corrected by the 5 Mm that the planet moves in the 144 seconds of light travel time:

>>> from sunpy.coordinates import get_body_heliographic_stonyhurst, get_earth
>>> t = '2012-06-05 22:34:48.350'

>>> without_correction = get_body_heliographic_stonyhurst('venus', t)
>>> print(without_correction)
<HeliographicStonyhurst Coordinate (obstime=2012-06-05T22:34:48.350): (lon, lat, radius) in (deg, deg, AU)
    (359.92620234, 0.02752007, 0.72602872)>

>>> with_correction = get_body_heliographic_stonyhurst('venus', t, observer=get_earth(t))
Apparent location factors in 144.063097622999 seconds of light travel time
>>> print(with_correction)
<HeliographicStonyhurst Coordinate (obstime=2012-06-05T22:34:48.350): (lon, lat, radius) in (deg, deg, AU)
    (359.92355609, 0.02734159, 0.72602853)>

>>> distance_traveled = (with_correction.cartesian - without_correction.cartesian).norm().to('km')
>>> print(distance_traveled)
<Quantity 5027.81720838 km>

Only get_body_heliographic_stonyhurst() has been modified because corrections for light travel time are usually significant only for planetary bodies, where the transverse angular motion of the body is appreciable relative to the light travel time. In principle, our coordinate transformations need to take into account the motion of the Sun in barycenter coordinates, but the Sun moves only ~5 km in the ~8 minutes that it takes light to traverse 1 AU, which is a negligible correction (i.e., <0.01 arcseconds in helioprojective coordinates).

Needs a test.

[pep8speaks]

Hello @ayshih! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file sunpy/coordinates/ephemeris.py:

Line 68:19: E127 continuation line over-indented for visual indent

Comment last updated at 2019-05-07 12:49:04 UTC

[ghost]

Thanks for the pull request @ayshih! Everything looks great!

[Cadair]

Looks good but needs a test and a changelog

[ehsteve]

Tested this and it works!

[ayshih]

I added a test.