We derive and introduce theangular reproduction number, Ω, which measures time-varying changes in epidemic transmissibility resulting from variations in both the effective reproduction number,R, and the generation time distribution,w. Predominant approaches for tracking the dynamics of pathogen spread either inferRor the epidemic growth rater. However,Ris easily biased by mismatches between the assumed and truew, whileris difficult to interpret in terms of the individual-level branching process underpinning transmission. Moreover,Randrmay disagree on the relative transmissibility of two epidemics or variants (i.e.,rA>rBdoes not implyRA>RBfor variants A and B). We find that Ω responds meaningfully to mismatches inwwhile maintaining most of the interpretability ofR. Additionally, we prove that Ω > 1 if and only ifR> 1 and that Ω agrees withron the relative transmissibility of pathogens. Estimating Ω is no harder than inferringR, uses existing software, and requires no generation time measurement. These advantages come at the expense of selecting one free parameter. We propose Ω as a useful statistic for tracking and comparing the spread of infectious diseases that may better reflect the impact of interventions when those interventions concurrently change bothRandwor alter the relative risk of co-circulating pathogens.