<p>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 <em>the remainder</em> - i.e. you get a result like 10.352, then keep the 0.352.</p>
<p>Plot your brightness or flux vs that remainder. You have a phase-folded light curve.</p>
<p>Mathematically, you are plotting flux versus <span class="math-container">$(t - t_0)\ \%\ p$</span>, where <span class="math-container">$t$</span> are your times of measurement, <span class="math-container">$t_0$</span> is your chosen zeropoint time, <span class="math-container">$p$</span> is your period (in the same units) and the % represents the <a href="https://en.m.wikipedia.org/wiki/Modulo_operation" rel="noreferrer">modulo</a> function that returns the remainder.</p>

CC BY-SA 4.0
2023-01-14T06:36:02.230Z
2023-01-14T06:42:21.100Z
2023-01-14T06:42:21.100Z
ProfRob
<p>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.</p>
<p>I couldn't understand how one actually folds a light curve. What is the process to do it?</p>
<p>Is it possible to give a simple explanation of the process, and give an example to clarify?</p>

CC BY-SA 4.0
2023-01-13T22:47:47.803Z
We have a lot of Q&A on the topic of light curve folding ((https://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.
uhoh
<observational-astronomy><data-analysis><photometry><light-curve><variable-star>
How does one actually fold a light curve?
Saturn.
# How does one actually fold a light curve?

(Asked by: username_0 on 2023-01-13)

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?

**Tags:** observational-astronomy, data-analysis, photometry, light-curve, variable-star

> **Comments:**
> **username_1**: We have a lot of Q&A on the topic of light curve folding ((https://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.  

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## 1 answers

### Answer by username_2 on 2023-01-14. Score: 5

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](https://en.m.wikipedia.org/wiki/Modulo_operation) function that returns the remainder.


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