around 1985 and 2005 and two major troughs around 1995 and 2015. Meanwhile, [one prominent flare appears]{} around the bottom of each major trough. Both flares rise rapidly in flux by more than 50% within 200 days. The latest flare (2016-2018) then undergoes an even more sudden drop within the same timescale. All the separations between peaks/toughs and between the locations of the flares are roughly 20 years[^3]. If we extrapolate such a repeated pattern to the period of [@Miller1979]’s dataset, we find that the time (1937-1939) of the significant flare reported in [@Miller1979] remarkably coincides with the anticipated time of recurring flares with a period of $\sim20$ years (see Figure \[fig\_miller\]). Unfortunately, the data from [@Miller1979] are very noisy and scattering, therefore, we are unable to be sure whether the variations follow the wave-like pattern seen in the light curve from 1974 to 2018. On [a]{} short timescale of months, Ark 120 also undergoes strong variations, which are commonly believed to originate from local instabilities of the accretion disk (e.g., @Czerny2008 [@Kelly2009]).

[cccccccccc]{} SPL & $-2.2\pm0.2$ & $1.6\pm0.1$ & & & & & & 0 & $8\times10^{-4}$\
DRW & $\phm{+}4.3\pm0.4$ & & & $-3.6\pm0.2$ & & & & $-2.37\pm1.00$ 