The need to understand and monitor climate change has led to proposed radiometric accuracy requirements for space-based remote-sensing instruments that are very stringent. Many of these requirements are unmet by the current fleet of Earth-orbiting instruments. A major problem is quantifying the changes that instruments undergo during the launch and throughout the mission. While on-orbit calibrators and monitors have been developed, they too can suffer changes from the launch and harsh space environment. One potential solution is to use the moon as a calibration reference source. Already the stability of the moon has been used to remove drift and to cross-calibrate different instruments. But, at present, the uncertainty of the absolute lunar spectral irradiance is too high for absolute on-orbit calibration of climate monitoring instruments. To enable use of the moon as an absolute calibration standard, we present in this paper an Earth-based instrument to measure the lunar spectral irradiance to an uncertainty of 1 % (k=1) over the spectral range from 320 nm to 2500 nm with a spectral resolution of approximately 0.3 %. The instrument would be flown on high altitude balloons and deployed at high elevation astronomical observatories in order to mitigate the effects of the Earth's atmosphere on the lunar observations. Periodic calibrations using advanced instrumentation and techniques available from NIST would ensure SI traceability and low radiometric uncertainties for the lunar irradiance measurements.