I'm at a loss why VSP now provides (J2015.5) for the coordinates epoch in the "new" VSP. All my planetarium programs, telescopes, etc. still use J2000.0. I presume VSP is using J2015.5 which is the Gaia DR2 epoch. Has the IAU changed the standard reference epoch? As far as I can tell and know it is J2000.0 still. I think it is a bad idea to use another epoch for the coordinates other than the IAU standard epoch, or both J2000.0 and J2015.5 could be provided.
Jim DeYoung (DEY)
I'm an outsider but there is a difference between epoch and equinox. Epoch definds the time the position was recorderd. So the star positions are taken mid 2015. Stars will move after that date only due to the proper motion which is practical nil the next 50 years for 99.999% of the stars. Epoch has nothing to do with the equinox so J2000 these days (coordinate system). This definds the coordinate system used and is based on the axis and orientation of the Earth.
So if your record stars position in the year 2000, the position have an epoch of 2000 and positions are given for equinox J2000. Star will only move due to the proper star motion and have moved 23 years due to the proper motion since recorded.
If your record stars position in the year 2015, the positions have an epoch of 2015 and positions are given for equinox J2000. Star will only move due to the proper star motion and have moved now 8 years due to the proper motion since recorded.
Gaia positions are epoch 2015.5 and equinox J2000.