[CamperBob2]

Minkowski spacetime confuses matters, though. From the point of view of the photon we observe, no time elapses between its emission at the location being observed and its arrival at the telescope image sensor. It's as if the photon were born in exactly the right place and time to be observed by us, at that very instant.

So what we see is arguably happening in real time, regardless of distance.

[andreareina]

Ehh. The measurement is happening in real time but we have well-defined ways to say when the event happened, and that's 120MY ago (given the reference frame where Earth is motionless).

[mr_mitm]

Depends on what you mean by "real time". If by "real time" you mean "the spacetime interval between the event 'supernova explodes' and 'explosion observed on earth' is lightlike (or null)", then we witnessed it in real time. I can't think of any other definition that is observer independent.

I also argue that it is not well-defined, as the time span depends on the observer, as you say. Did it happen 120MY ago or 1MY ago? Both can be true for different observers, and none is privileged over the other.

[CamperBob2]

Did it happen 120MY ago or 1MY ago? Both can be true for different observers, and none is privileged over the other.

Great way to put it.