Calibration and validation (CAL/VAL) is a key technology for quantitative application of space-borne remote sensing data. However, the complex space environment can cause many uncertainties and degrade calibration accuracy. In-flight calibration is always needed. The thermal emission of the Moon is stable over hundreds of years because there is no atmosphere and no significant physical or chemical change on its surface. The deep space view of the Microwave Humidity Sounder onboard NOAA-18 has viewed the Moon many times every year. Under solar illumination, the lunar surface shows stable and periodical variation in microwave brightness temperature (TB). The Moon is a potential calibration source for thermal calibration