How to fit a curve with an equation

I don't know how I can copying and pasting my datas (I'm sorry but I'm verry new in this field). So I exported my dataframe in a csv and I copy and past 64 row of my 2 columns "pupil_timestamp_secondes" and "diameter_3d_corrige". Is it works like that ?

pupil_timestamp_secondes
870.485044999979
870.491905000003
870.499957999971
870.507964999997
870.515828999982
870.524032999994
870.531897999987
870.539931999985
870.547885000007
870.559999999998
870.567870999977
870.575948999962
870.583884999971
870.591989999986
870.59990500001
870.608161000011
870.615955999994
870.62399599998
870.631705999956
870.640095999988
870.647958999965
870.659928000008
870.667883999995
870.67597099999
870.683879999968
870.692008999991
870.700002999976
870.70792700001
870.715874999994
870.724040000001
870.731809999968
870.739983999985
870.747930000012
870.759995999979
870.767835999955
870.775861000002
870.783901999996
870.791981999995
870.799994000001
870.808032999979
870.816004999971
870.824086999986
870.831732999999
870.840151999961
870.847921999986
870.860103999963
870.86785499996
870.875912999967
870.883927999996
870.892128999985
870.899872999988
870.908027999976
870.915923999972
870.924155999965
870.932074000011
870.939974999987
870.947936999961
870.960197999957
870.968147999956
870.975954999973
870.983956000011
870.991959999956
870.999833000009
871.007992999977

diameter_3d_corrige
1.48164707707831
1.49456095678936
1.48414707057656
1.48726408870283
1.48205378247385
1.47781943384314
1.47231104221379
1.46745207409162
1.45597322337438
1.43931190797885
1.42335791963431
1.42252471948163
1.39743728364405
1.39648141401239
1.38202497575066
1.36839658349799
1.3533448389161
1.34086410176018
1.32458607292278
1.32177503309484
1.31662963954374
1.30383311815029
1.28886017800971
1.28617193356825
1.2679510253103
1.26079463527277
1.25409525242261
1.2433266606438
1.23210515620124
1.21748213073997
1.21730916830565
1.20922005151322
1.19459113838058
1.18862563974446
1.18891287365467
1.1827331916104
1.16622961119455
1.16675354050288
1.16004219806803
1.15531594345702
1.15201180284821
1.14496753650957
1.14352465118899
1.12535131292713
1.12543821423565
1.12961750763861
1.11209925382821
1.11601875932558
1.11436788570255
1.1097258568122
1.10941190041669
1.10137899634844
1.10550057001415
1.09148442833974
1.10311317989005
1.08919209487414
1.09063657884501
1.08955663942749
1.09088134950307
1.09373823583395
1.09270374068362
1.07977138687109
1.08426291442292
1.08552464145148