The sun would be the most obvious choice for such a reference point, though it’d be amusing to make it Greenwich and therefore make everyone deal with Earth’s rotation and orbit if they want extremely precise calculations (though I expect there wouldn’t really be a practical difference on that scale).
The sun would be the most obvious choice for such a reference point, though it’d be amusing to make it Greenwich and therefore make everyone deal with Earth’s rotation and orbit if they want extremely precise calculations (though I expect there wouldn’t really be a practical difference on that scale).