The 2007 National Research Council Decadal Survey for Earth Science identified needed measurements to improve understanding of the Earth’s climate system, recommending acquiring Earth spectral radiances with an unprecedented 0.2% absolute radiometric accuracy to track long-term climate change and to improve climate models and predictions. Current space-based imagers have radiometric uncertainties of ~2% or higher limited by the high degradation uncertainties of onboard solar diffusers or calibration lamps or by vicarious ground scenes viewed through the Earth’s atmosphere. The HyperSpectral Imager for Climate Science (HySICS) is a spatial/spectral imaging spectrometer with an emphasis on radiometric accuracy for such long-term climate studies based on Earth-reflected visible and near-infrared radiances. The HySICS’s accuracy is provided by direct views of the Sun, which is more stable and better characterized than traditional flight calibration sources. Two high-altitude balloon flights provided by NASA's Wallops Flight Facility and NASA’s Columbia Scientific Balloon Facility are intended to demonstrate the instrument’s 10× improvement in radiometric accuracy over existing instruments. We present the results of the first of these flights, during which measurements of the Sun, Earth, and lunar crescent were acquired from 37 km altitude. Covering the entire 350-2300 nm spectral region needed for shortwave Earth remote sensing with the HySICS’s single, flight-heritage detector array promises mass, cost, and size advantages for eventual space- and air-borne missions. A 6 nm spectral resolution with a 0.5 km spatial resolution from low Earth orbit helps in determinations of atmospheric composition, land usage, vegetation, and ocean color.