I was trying to use the SPP RR Lyrae Ephemeris of plan an imaging session. I can't see how the predicted "time of maximum" lines up with the ephemeris you get from VSX.
For example, I was considering RR Gem for tonight. Here is what the SPP ephemeris says for the days around tonight (Feb 5th in local time)
I would think I would use the 5-6 line and in my time zone (-7 UTC), that would be 7 PM on the 5th.
However, if I look at the VSX ephemeris, I see:
2459980.345 04 Feb 2023 20:16
2459980.742 05 Feb 2023 05:48
2459981.139 05 Feb 2023 15:20
2459981.537 06 Feb 2023 00:52
2459981.934 06 Feb 2023 10:24
2459982.331 06 Feb 2023 19:57
2459982.729 07 Feb 2023 05:29
The closest thing to 2.0 on the 6th would be 06 Feb 2023 00:52, but in general thins don't seem to align.
I must be thinking about this incorrectly. Can someone explain?
I believe that Gerry Samolyk updates the RR Lyrae ephemerides to take account of recent period changes of the RR Lyrae stars. It looks to me as though the ephemeris in the VSX is now rather old. Gerry may be able to say more, but I suspect that the VSX ephemeris is no longer accurately predicting times of maximum for RR Gem.
The period of RR Gem had been decreasing for decades however recently there has been an increase in the period. As Horace mentioned, I make updates to the light elements each year in an attempt to keep the ephemeris as accurate as possible.
Keep in mind that RR Lyr stars do not repeat from cycle to cycle. The ephemeris is only a guideline as the time of maximum can occur earlier or later than the prediction.