It's not wrong, and makes the maths easier ... as long as we're happy to shift our reference point when appropriate, it seems valid to me. The CoM of the local

It's not wrong, and makes the maths easier ... as long as we're happy to shift our reference point when appropriate, it seems valid to me. The CoM of the local galactic cluster isn't particular useful as a reference and is no more valid a default. One of my personal /bête noires/ is people who say "the Earth isn't the centre of the Universe" ... it's a valid choice. /fight-me