Find and improve all instances of defaulting the Sun-Earth distance to 1 AU

[Cadair]

If I remember correctly we do this in a few places in Map, such as in SXT as was pointed out here: #4926 (comment)

@ayshih Do we still do this anywhere in coordinates?

[ayshih]

I'm pretty sure sunpy.coordinates is all safe.

I just tried using grep to find instances anywhere else, and the only one I can find is that SXT one. (A private function in sunpy.instr.goes has it as a default if a date is not provided, but the public function that calls it should always provide a date.) However, there may be insidious implementations that I missed with my grep pattern.

[ayshih]

Here's a subtle one. If you request the alternate output of synodic from sunpy.physics.differential_rotation.diff_rot(), it calculates the synodic rotation – the apparent amount of rotation for as seen by an Earth observer – by assuming that the Earth is always at 1 AU.

sunpy/sunpy/physics/differential_rotation.py

Lines 130 to 134 in 0c1a5fd

	# Applying this correction assumes that the observer is on the Earth,
	# and that the Earth is at the same distance from the Sun at all times
	# during the year.
	if frame_time == 'synodic':
	rotation -= 0.9856 * u.deg / u.day * duration

[Cadair]

Before we close this I would like to make sure we have actually got them all.