LAUROESCH, Edinder & Lauroesch may sound like a firm of Swiss solicitors, but in fact they are the American authors of a recent scientific paper. It appears in the International Journal of Climatology, and addresses the well known piseog which has it that "The full moon eats clouds", or "The moon grows fat on clouds".
The implication is that the sky is less cloudy when the moon is full than other times of the lunar cycle.
The belief has been held even by many men of science over the years. It can be deduced, for example, from the words of the Arctic explorer Sir George Back, quoted by the authors from 1836:
"As it was now about full moon, we looked for a favourable change in the weather; and not without some anxiety, for I was apprehensive that with the constant drenchings and fatigue two or three of the weaker hands might be laid up.
LE&L, as we shall call them, have taken the weather records for Rochester, New York, for a 44 year period from 1950 to 1993, and examined the full moon cloudiness statistically to see if it does indeed differ from that at any other time of month.
First they had to decide how the night of the full moon might be defined - not as simple as it seems, since full moon is an instantaneous astronomical event which may well occur in the middle of the day. They decided that the appropriate night was that on which the moon was full after 12 noon local time and before 12 noon the following day.
The associated cloud data was taken as the average of the amounts of cloud reported at each of the hourly observations during the hours of darkness, and this full moon cloudiness was then compared to the average cloud reported on a control night" each month - taken as the first of the month in question. For obvious reasons, dates when the full moon occurred on the control date - on the first of the month - were eliminated from consideration.
LE&L reported their results as follows: "With a confidence of 95 per cent the null hypothesis should be accepted; the difference between the mean cloud cover at the full moon and that on a control date has not been shown to be statistically significant." Translated into English, this might read:
"Rubbish! It is just as cloudy when the moon is full as when it's not."
LE&L sensibly conclude that the impression that the sky is always clear on the night of a full moon arises from people noticing a full moon on clear nights, and not noticing a full moon on nights when the sky is overcast - simply because the sky is overcast!