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tokens: yielding mutually consistent rotation speeds seem to have all but invalidated those suggestions; comprehensive and up-to-date Galactic rotation curves no longer show any dip near the Sun’s Galactocentric distance, at least not those based on the main mass tracers (see, e.g., Bland-Hawthorn Gerhard 2016 ). It is still possible that the appearance of such a dip could be limited to the young(est) populations located in the Perseus spiral arm (e.g., Sofue 2013 ; Genovali et al. 2014 ). Genovali et al. ( 2014 ) also suggested that the clumpy distribution of classical Cepheids across the Galactic thin disk and their possible association with giant molecular clouds may hamper their use for Galactic rotation measurements. Note that the H i , CO, and field population tracers also imply larger values for both (at kpc) and the ratio than the canonical IAU recommendation, although the widths of their full distributions encompass the IAU values within their 1 σ ranges. In view of these distributions, and considering that both the field populations and the H i or CO gas kinematics reflect the underlying Galactic mass distribution most closely, we contend that the post-1985 combination of these measurements may indeed yield the most appropriate value for the Galactic rotation speed for . The 40 post-1985 measurements of based on field star samples, H i or CO observations, and Galactic mass modeling yield . Note that, while dominated by a statistical component, the uncertainty does include a contribution from a systematic component, given that the contributing values include such uncertainties. Similarly, the resulting ratio based on these values is Θ 0 / R 0 = 27.12 ± 0.39 km s −1 kpc −1 ( σ = 2.45 kms −1 kpc −1 ). At this point, it would be wise to perform a final check of the Galactic rotation speed at the solar circle by considering the Galaxy’s rotation curve with respect to the external reference frame provided by the Milky Way’s satellite galaxies and other Local Group members, as well as the Milky Way’s GD-1 giant stream. Fortunately, a number of studies have attempted to do just this. Lynden-Bell Lin’s ( 1977 ) and Einasto et al.'s ( 1979 ) original attempts yielded rotation speeds that bracketed the expected range. Lynden-Bell Lin ( 1977 ) determined km s −1 for kpc (with a reduced uncertainty of 13 km s −1 resulting from the application of more a priori constraints), while Einasto et al. ( 1979 ) found km s −1 , again for kpc. More recent derivations of in external reference frames, all recalibrated for , have resulted in (30) km s −1 (Kochanek et al. 1996 ) and (216 ± 18) km s −1 (Koposov et al. 2010 ). This thus implies that there is no compelling evidence suggesting that the IAU-recommended km s −1 is no longer valid, although it would be more appropriate to increase the recommendation to km s −1 , provided that the recommended Galactocentric distance is reduced from to kpc (Paper IV ). A comparison of the Galactic rotation speeds implied by