If my math is correct LOL A rough calculation shows that the moon would have about a 0.0003% effect on the weight of an object. F=(G*Moon mass*Subject mass)/D^2 G=6.67x10^-11 Moon mass=7.3x10^22kg Subject mass=100kg (good for theory) D=distance between moon and subject. The mean distance between the earth and moon is taken to be 3.84x10^8 meters, although the distance varies a lot, change as appropriate. This distance is between the centers, so to get the effect on someone standing on the earth's surface, subtract 6.4x10^6meters from that, leaving 3.78x10^8 meters between the center of the moon and a person on earth. (6.67x10^-11 x 7.3x10^22 x 100)/ ( (3.78x10^8)^2 ) = 0.0034Newtons, which would be the equivalent of 0.35 grams This means a 100kg person would see about 0.3 to 0.4 grams of change due to the moon. Which is why the sensitivity of the scale needs to be very high. Measuring over a full rotation of earth should give a variability in the weight of about double, meaning close to 0.7 or 0.8g. The scale would have to measure 100.0000kg to show these changes. It would go from 100.0003kg to 99.9997kg over the max and min readings. The moon gets closer and farther than the mean distance, so additional calculations can be done to get more accurate readings.