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The Earth's magnetic field induces Zeeman splitting of the magnetic dipoletransitions of molecular oxygen in the atmosphere, which produces polarizedemission in the millimeter-wave regime. This polarized emission is primarilycircularly polarized and manifests as a foreground with a dipole-shaped skypattern for polarization-sensitive ground-based cosmic microwave backgroundexperiments, such as the Cosmology Large Angular Scale Surveyor (CLASS), whichis capable of measuring large angular scale circular polarization. Usingatmospheric emission theory and radiative transfer formalisms, we model theexpected amplitude and spatial distribution of this signal and evaluate themodel for the CLASS observing site in the Atacama Desert of northern Chile.Then, using two years of observations at 32.3 GHz to 43.7 GHz from the CLASSQ-band telescope, we present a detection of this signal and compare theobserved signal to that predicted by the model. We recover an angle betweenmagnetic north and true north of $(-5.5 \pm 0.6)^\circ$, which is consistentwith the expectation of $-5.9^\circ$ for the CLASS observing site. Whencomparing dipole sky patterns fit to both simulated and data-derived sky maps,the dipole directions match to within a degree, and the measured amplitudesmatch to within ${\sim}20\%$.

Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A First Detection of Atmospheric Circular Polarization at Q Band