American Association of Variable Star Observers (AAVSO)
Fri, 02/02/2024 - 18:54

I've been trying to collect data on TZ Aur and capture the entire light cure myself.  I have everything except for a couple of hours around its peak. I tried to get the final section last night using the 2024 RR Lyrae Ephemeris (  Unfortunately, it appears I caught the downward section after the  peak.  I'm trying to understand where I went wrong.

There are two entries to consider in the February section for TZ Aur:

Day     Peak

1- 2     8.5 

2- 3     3.5

So one of my confusions is how to interpret the days.  The Ephemeris says all times are in UTC.  I assume that means the days as well?  In that case, it was already Feb 2 UTC when I was going to start imaging, so the second line would apply.  I'm at UTC-7 so that should imply a peak near 8:30 PM.  I imaged from 7 PM until 12:30 AM.  As I said, looking at the data this morning, I missed the peak by (maybe) a couple of hours.

If the days aren't UTC days, then the first line would apply.  So the peak would then be around 1:30 AM.  Yet the data doesn't show any upward movement even at 12:30 AM.

So where am I going wrong with this?

One other point I'll make is that the ephemeris shows Max 11.2 and Min 12.8.  This is in contrast to VSX which shows 11.08 - 12.45 V.  My previous V data is pretty spot for 12.45 being the min.  I don't have a max because that is what I'm trying to get.  Should I be concerned that these don't match up better?



American Association of Variable Star Observers (AAVSO)
TZ Aur

I believe that the statement "This ephemeris is designed for use by observers at American longitudes" means that the dates are appropriate to American observers rather than being UT dates.  So, the day 1-2 row with an 8.5 UT prediction would be the right one.  For you, 0 UT would occur at 5 pm EST.  Thus, your 1:30 am prediction is correct.  TZ Aur has a rapid climb (with a bit of a shoulder before the minimum) so you may have just missed the climb by a little.  Some RR Lyrae stars don't follow predictions because of period changes, but TZ Aur in the past has been steady in period (which doesn't mean it needs to stay that way).  The magnitudes on the ephermerides are to guide observers as to what instrument might be needed to carry out observations.  Comp stars have been updated for several of the RR Lyrae, so I would stick with comp stars on the current charts.   My guess is that the maximum just came a little too late for your observing run. A tip off that you stopped just before the rise would be the appearance of the "shoulder" before minimum.  It looks like a temporary slowing of the rate of descent before a more rapid drop to minimum occurs.  If you are unfamiliar with it, you may also want to check out the TZ Aur data at the GEOS RR Lyrae database ( Good luck!  And let me know if there are continued problems.



American Association of Variable Star Observers (AAVSO)

Thanks for the…


Thanks for the reply.  I think you are exactly right.  Looking at the LC graph again, it appears I stopped right when it was just starting an upturn.  If the graph below, the red circles are from that night.

It looks like I will have another chance to catch the peak Sunday.  Predicting clear skies and the peak is at 7:30 PM local time.

Thanks also for the link to the GEOS RR Lyrae database.  I hadn't seen that before.