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Determining anisotropies in the arrival directions of cosmic rays at thehighest energy is an important task in astrophysics. It is common and useful topartition the sky into spherical harmonics as a measure of anisotropy. The twolowest nontrivial spherical harmonics, the dipole and the quadrupole, are ofparticular interest, since these distributions encapsulate a dominant singlesource and a plane of sources, as well as offering relatively high statistics.The best experiments for the detection of ultra high energy cosmic rayscurrently are all ground-based, with highly nonuniform exposures on the skyresulting from the fixed experimental locations on the earth. This nonuniformexposure increases the complexity and error in inferring anisotropies. It turnsout that there is an optimal latitude for an experiment at which nonuniformexposure does not diminish the inference of the quadrupole moment. We derivethe optimal latitude, and find that (presumably by a fortuitous coincidence)this optimal latitude runs through the largest cosmic ray experiment, PierreAuger Observatory (PAO) in the Southern Hemisphere, and close to the largestcosmic ray experiment in the Northern Hemisphere, Telescope Array (TA).Consequently, assuming a quadrupole distribution, PAO and TA can reconstructthe cosmic ray quadrupole distribution to a high precision, without concern fortheir partial sky exposure.

The Fortuitous Latitude of the Pierre Auger Observatory and Telescope Array for Reconstructing the Quadrupole Moment