the other datasets described above. Despite this, a useful comparison is possible by converting the $B$-band magnitudes into fluxes and then simply adjusting the fluxes to make the mean and standard deviation to be at the same scales as those of the other datasets. [*We do not include this dataset for subsequent analysis*]{}, but take it as an auxiliary dataset to illustrate the long-term variations of Ark 120. Figure \[fig\_miller\] plots the light curve data of [@Miller1979] together with the merged light curve of [the]{} $V-$band and 5100 [Å]{} continuum.

![(Top) Power spectral density $P(f)$ and (bottom) $f\times P(f)$ of Ark 120 in the optical and X-ray band (0.3-10 keV). The normalization of the X-ray PSD is adjusted to match the optical PSD. Blue points represent the binned PSD and [the]{} grey line represents the raw PSD. [Slate blue]{} points represent the X-ray PSD from [@Lobban2018]. Vertical dashed lines correspond to the period of 20.5 years.[]{data-label="fig_psd"}](fig_psdall.pdf){width="48.00000%"}

Long-term Optical Variations {#sec_var}
----------------------------

As shown in Figure \[fig\_lc\], all the light curves of [the]{} $V-$band, 5100 [Å]{} continuum and H$\beta$ integrated fluxes oscillate with a wave-like pattern. In the merged light curve plotted in Figure \[fig\_miller\], there [are clearly]{} two major crests 