Affiliation
American Association of Variable Star Observers (AAVSO)
Mon, 05/09/2022 - 16:51
Does VStar use differential dates from an epoch in the middle of the data set for all date calculations or only when modeling.
Brad
For phase plots, the epoch is user or VSX supplied and shown in a dialog box.
For the AoV period search plugin, the epoch is arbitrarily the average of the first and last JD.
For polynomial fit in raw mode, the mean JD is used as the model function's zero point (and 0 in phased mode). Not epoch though, just a value subtracted from the model's time parameter
Other than that, do you have a particular scenario in mind where epoch is used?
David
Thanks for the response, David
I did not have a specific scenario in mind. I was simply trying to refresh my memory about reference date selection to avoid error build up in date calculations.
I am curious, however, why you single out the JD subtracted in polynomial model fitting as not being an Epoch. It is a date that is used as the reference point which fits the definition of "epoch." Is there something I am missing?
Brad
Hi Brad
The motivation for use of a zero point in the polynomial fit function is really just that it's better for floating point computation since the values being multiplied and raised to powers of 2, 3, ...N will be closer in magnitude, than if a zero point had not been used.
David