I would like to get some perspective on what people usually mean when they don't specify any reference and discuss things like the size of the observable universe or the distance between Andromeda and the Milky Way. The thread I would have posted this on is now defunct, so starting a new one. There is no universal baseline 'now' in SR / GR. It seems to me that the intuitive human 'now' is the instant that we are receiving information and from here we need to reason out how and why our intuitive 'now' may not be universal. I claim there is a very basic frame shift that almost anyone talking about astronomical observations does often without thinking about it, and I offer the below example in an attempt to illustrate it. I don't think I am talking about SR / GR in the below, except that I am talking about c, but I am talking about c in what is to me a classical context more than a relativistic context. If I am going to launch a satellite with the intent that it end up in orbit around Proxima Centauri, everyone with a basic knowledge of what 'light year' means can understand why I cannot "aim" the satellite to where I see PC "right now", I must account for the 4 light years it took for the signal to reach me, and I must model where I believe PC to be "right now" to calculate the engine accelerations I need my satellite to execute, or I will send the satellite to where the image I receive of PC is going to be by the time the satellite gets to its destination, and PC will be 4 years along its path relative to earth, ahead of the satellite. SO - if I ask someone to do the exercise of plotting the course, they will AUTOMATICALLY make this frame shift as they get started and not even think about it. They will ask themselves "Where is PC right now" and the the "now" they mean is relative to photons PC is emitting, not relative the photons they receiving. Not all observers in the universe will agree that PC is 4 LY from earth, I suppose, but that is not my question. Finally, my question ... when discussing the locations of distant bodies, is it normal to state those distances as directly measured, or is it normal to state those distances with the above conversion applied? I think the former, and usually it won't make much difference in the numbers discussed. When we discuss the size of the observable universe, does the above thinking matter in what numbers are tossed out?