I'm studying the variable stars (periodic variables with brightness changes repeating over time), and to examine the shape of the light curve it is useful to fold the data points at this period and study the folded light curve.

I couldn't understand how one actually folds a light curve. What is the process to do it?

Is it possible to give a simple explanation of the process, and give an example to clarify?

  • 1
    $\begingroup$ We have a lot of Q&A on the topic of light curve folding ((astronomy.stackexchange.com/search?q=folding) and discussions of advanced algorithms, but I am not sure if there are any basic answers that actually describe the underlying basic concept. So +1 for a good question. $\endgroup$
    – uhoh
    Jan 13, 2023 at 22:47

1 Answer 1


Assume I have a bunch of measurements of something like brightness or flux versus time, and I know or guess a period for these data. Pick a zeropoint in time - say the time of the first observation - and subtract this from all the times. Then, divide the result by the period and keep the remainder - i.e. you get a result like 10.352, then keep the 0.352.

Plot your brightness or flux vs that remainder. You have a phase-folded light curve.

Mathematically, you are plotting flux versus $(t - t_0)\ \%\ p$, where $t$ are your times of measurement, $t_0$ is your chosen zeropoint time, $p$ is your period (in the same units) and the % represents the modulo function that returns the remainder.


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