What we'll do in this section is create an Excel spreadsheet that will enable you to determine the period of a lightcurve. If you created your differential photometry with TheSky, then you can export your data into Excel and use this spreadsheet to "wrap" the data from several sessions into a single lightcurve. This spreadsheet approach may be useful for your first few projects. It is a bit time-consuming, and it demands that you have some experience with spreadsheet formulas and graphs, but with it you can create a nice lightcurve. After you've used it a few times, and assuming that you get the "asteroid lightcurve'' bug, you'll recognize the value of special-purpose data analysis software such as MPO Canopus or Peranso.
The general idea is that your data points form a series of samples of the lightcurve function f (t). Since we assume that this is a periodic function, with period = P, it should obey the rule that f (t)=f (t — NP)
where t = time of observation (in the same units as P—either hours or days);
P = period of rotation (in hours or days, whichever is most convenient for your analysis);
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