<p>&quot;Phase folding&quot; is just a procedure whereby you replace the time-axis values by t % P, where P is the period and % is the modulo operator that returns the remainder of t/P.</p>
<p>There are are a number of proceduures/algorithms to find the period. The most commonly used in astronomy is probably the Lomb-Scargle periodogram for unevenly spaced data. I highly recommend reading <a href="https://arxiv.org/abs/1703.09824" rel="nofollow noreferrer">VanderPlas 2018</a> as a way of understanding the technique. There are commonly used python implementations in <a href="https://docs.scipy.org/doc/scipy/reference/generated/scipy.signal.lombscargle.html" rel="nofollow noreferrer">scipy</a> and <a href="https://docs.astropy.org/en/stable/timeseries/lombscargle.html" rel="nofollow noreferrer">astropy</a>.</p>

CC BY-SA 4.0
2022-06-15T09:53:45.547Z
2022-06-15T09:53:45.547Z
ProfRob
<p>I have some observational data for a star where I've done aperture photometry to get a partial period. I understand that you need to use other techniques to estimate a period for stars whose period is longer than one night of observing. I know phase folding your data is quite popular. How does one go about phase folding data? Right now, I have multiple days of data plotted that look similar to the picture of the plot I've attached below.</p>
<p>The X-axis is time (Julian date). T1 is the target star, and the c2, c3, and c4 are check stars</p>
<p><a href="https://i.sstatic.net/1G4CN.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/1G4CN.png" alt="Image of my data" /></a></p>

CC BY-SA 4.0
2022-06-15T07:31:38.580Z
Do you mean how do you estimate the period, or literally, that you have the period and just want to plot phase-folded data using that period?
ProfRob
CC BY-SA 4.0
2022-06-15T08:56:37.457Z
@ProfRob I want to estimate the period. I have several observations spread out across multiple days, but because the period is longer than one night of observing I don't have a full period. I would like to use phase-folding (at least I think that's what I want) to estimate a period from all of my independent observations.
Zachary Kennedy
<star><observational-astronomy><data-analysis><photometry><light-curve>
How do I phase fold the light curve for a variable star?
Zachary Kennedy
# How do I phase fold the light curve for a variable star?

(Asked by: username_0 on 2022-06-15)

I have some observational data for a star where I've done aperture photometry to get a partial period. I understand that you need to use other techniques to estimate a period for stars whose period is longer than one night of observing. I know phase folding your data is quite popular. How does one go about phase folding data? Right now, I have multiple days of data plotted that look similar to the picture of the plot I've attached below.

The X-axis is time (Julian date). T1 is the target star, and the c2, c3, and c4 are check stars

[![Image of my data](https://i.sstatic.net/1G4CN.png)](https://i.sstatic.net/1G4CN.png)

**Tags:** star, observational-astronomy, data-analysis, photometry, light-curve
> **Comments:**
> **username_1**: Do you mean how do you estimate the period, or literally, that you have the period and just want to plot phase-folded data using that period?  
> **username_0**: @ProfRob I want to estimate the period. I have several observations spread out across multiple days, but because the period is longer than one night of observing I don't have a full period. I would like to use phase-folding (at least I think that's what I want) to estimate a period from all of my independent observations.  

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## 1 answers

### Answer by username_1 on 2022-06-15. Score: 3 | Accepted answer

"Phase folding" is just a procedure whereby you replace the time-axis values by t % P, where P is the period and % is the modulo operator that returns the remainder of t/P.

There are are a number of proceduures/algorithms to find the period. The most commonly used in astronomy is probably the Lomb-Scargle periodogram for unevenly spaced data. I highly recommend reading [VanderPlas 2018](https://arxiv.org/abs/1703.09824) as a way of understanding the technique. There are commonly used python implementations in [scipy](https://docs.scipy.org/doc/scipy/reference/generated/scipy.signal.lombscargle.html) and [astropy](https://docs.astropy.org/en/stable/timeseries/lombscargle.html).


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