Understanding the relationships between rainfall and lunar phase- a case study in Kandy region in Sri Lanka

There is a common belief among Sri Lankans that rainfall always occurs around “Poya day” (the day of full moon). The background for this belief is that the rainfall is influenced by the phase of the moon and this hypothesis was tested using data from Kandy region.

Statistical analyses on this hypothesis were conducted using ephemeris and official rainfall data covering a period of 38 years, from January 1964 to December 2001, for three locations in Kandy region. These analyses were based on four types of comparisons namely, yearly average possibility of rainfall on a moon phase day with that of a control day, yearly average rainfall on a moon phase day with that of a control day, yearly average possibility of rainfall on a moon phase day with that of the opposite moon phase day and yearly average rainfall on a moon phase day with that of the opposite moon phase day. These comparisons were repeated for ‘around 3 day period of moon phase day’, ‘around 5 day period of moon phase day’, ‘one day before moon phase day + moon phase day’, ‘two day before moon phase day + moon phase day’, ‘one day after moon phase day + moon phase day’, and ‘two day after moon phase day + moon phase day’.

As a control sample for the first two types of comparisons, the third day of each month was chosen randomly. Statistical analyses were done using the sign test and the t-test. Using results of the 24 types of comparisons, it was found that no significant correlation between rainfall and the occurrence of the four moon phases. Hence, it can be concluded that the rainfall is not influenced by the phase of the moon. The comment or the belief that ‘rainfall always occurs around the “Poya day” may have arisen from people only enriching the myth when raining on “Poya days” during rainy periods, as it is well known, it’s probably the heads-I-win, tails-doesn’t count approach.