The faint light of airglow, emanating from atomic oxygen in the thermosphere, means that Earth's atmosphere is never completely dark. By measuring subtle shifts in airglow's observed frequency, researchers have developed a method to track the motion of the upper atmosphere. Using High-Altitude Interferometer Wind Observation (HIWIND), a balloon-borne interferometer, Wu et al. measured the observed wavelength of airglow given off by excited atomic oxygen, which peaks in intensity in the upper thermosphere at 250 kilometers altitude. The light emitted from atomic oxygen has a consistent wavelength of 630 nanometers. By looking for deviations from this value due to the Doppler effect, the authors were able to calculate the relative motion of the source oxygen atoms and thus the speed and direction of the thermospheric wind. The properties of the thermospheric wind can have important effects on the chemical composition and structure of the ionosphere.