Instruments on satellites for Earth observation on polar orbits usually employ a two-point calibration technique,in which deep space and an onboard calibration targetprovide two reference flux levels. As the direction of thedeep-space view is in general close to the celestial equator,the Moon sometimes moves through the field of view andintroduces an unwelcome additional signal. One can take advantageof this intrusion, however, by using the Moon as athird flux standard, and this has actually been done for checkingthe lifetime stability of sensors operating at visible wavelengths.As the disk-integrated thermal emission of the Moonis less well known than its reflected sunlight, this conceptcan in the microwave range only be used for stability checksand intercalibration. An estimate of the frequency of appearancesof the Moon in the deep-space view, a description ofthe limiting factors of the measurement accuracy and modelsof the Moon’s brightness, and a discussion of the benefitsfrom complementing the naturally occurring appearances ofthe Moon with dedicated spacecraft maneuvers show that itwould be possible to detect photometric lifetime drifts of afew percent with just two measurements. The pointing accuracyis the most crucial factor for the value of this method.Planning such observations in advance would be particularlybeneficial, because it allows observing the Moon at welldefinedphase angles and putting it at the center of the field ofview. A constant phase angle eliminates the need for a modelof the Moon’s brightness when checking the stability of aninstrument. With increasing spatial resolution of future microwavesensors another question arises, viz. to what extentforeground emission from objects other than the Moon willcontaminate the flux entering the deep-space view, which issupposed to originate exclusively in the cosmic microwavebackground. We conclude that even the brightest discreetsources have flux densities below the detection limit of microwavesensors in a single scan.