AMS Citation: Egeland, R., W. Soon, S. Baliunas, J. C. Hall, A. A. Pevtsov, and L. Bertello, 2017: The Mount Wilson Observatory S-index of the Sun. The Astrophysical Journal, 835, 25, doi:10.3847/1538-4357/835/1/25. 2017-01-16 article The Mount Wilson Observatory S-index of the Sun The most commonly used index of stellar magnetic activity is the instrumental flux scale of singly ionized calcium H & K line core emission, S, developed by the Mount Wilson Observatory (MWO) HK Project, or the derivative index ${R}_{\mathrm{HK}}^{\prime }$. Accurately placing the Sun on the S scale is important for comparing solar activity to that of the Sun-like stars. We present previously unpublished measurements of the reflected sunlight from the Moon using the second-generation MWO HK photometer during solar cycle 23 and determine cycle minimum ${S}_{23,\min }=0.1634\pm 0.0008$, amplitude ${\rm{\Delta }}{S}_{23}=0.0143\pm 0.0012$, and mean $\langle {S}_{23}\rangle =0.1701\pm 0.0005$. By establishing a proxy relationship with the closely related National Solar Observatory Sacramento Peak calcium K emission index, itself well correlated with the Kodaikanal Observatory plage index, we extend the MWO S time series to cover cycles 15–24 and find on average $\langle {S}_{\min }\rangle =0.1621\pm 0.0008$, $\langle {\rm{\Delta }}{S}_{\mathrm{cyc}}\rangle =0.0145\pm 0.0012$, $\langle {S}_{\mathrm{cyc}}\rangle =0.1694\pm 0.0005$. Our measurements represent an improvement over previous estimates that relied on stellar measurements or solar proxies with non-overlapping time series. We find good agreement from these results with measurements by the Solar-Stellar Spectrograph at Lowell Observatory, an independently calibrated instrument, which gives us additional confidence that we have accurately placed the Sun on the S-index flux scale. Refereed Copyright 2017 the American Astronomical Society. ark:/85065/d7m90bfs 10.3847/1538-4357/835/1/25 Ricky Egeland - NCAR/UCARWillie SoonSallie BaliunasJeffrey C. HallAlexei A. PevtsovLuca Bertello