Abstract
<title>Abstract</title> <p>Galaxies' peculiar velocities are a premier cosmological probe, yet only their line-of-sight component has been measured; motion across the sky has stayed out of reach. When a massive object moves across the line of sight, its changing gravitational potential blueshifts cosmic microwave background photons on the trailing side and redshifts them on the leading side, a small ``moving-lens effect'' predicted by Birkinshaw and Gull in 1983. Here we report a $4.8\sigma$ detection of this signal ($3.7\sigma$ in a more conservative galaxy sample) by cross-correlating temperature maps from the Atacama Cosmology Telescope with galaxies from the Dark Energy Spectroscopic Instrument Legacy Imaging Surveys, using a harmonic-space estimator that separates scales to suppress foregrounds. Parity and frequency-difference null tests are consistent with noise, and empirical bounds on foreground contamination are small compared with the signal. This is the first statistical detection of galaxies' transverse motion at cosmological distances; combined with line-of-sight velocities from the kinetic Sunyaev--Zel'dovich effect, it opens a path to mapping the three-dimensional velocity field of the Universe.</p>