Lunar Calendar

Although there have been numerous calendars over the millennia of civilizations, they fall into two basic types, solar and lunar calendars. Today, while practically everyone uses the solar calendar with 365.2422 days per tropical year, rice farmers in many parts of the world continue to use the lunar calendar based on a 29.53-day lunar month. Can you figure out a scientific reason why?

Answer

Modern farming methods tend to plant crops at approximately the same time year after year, with minor adjustments for quirks in the weather. Hence, a particular crop is usually planted about 365 days after its planting the previous year, plus or minus about 10 days. One example is spring wheat, usually planted on about April 15 in the northern Plains states of the United States. Rice is a different kind of organism than wheat as far as its environmental needs. Rice planted at about the same date every year will sometimes permit two good rice crops per year, but in most years the farmer will get only one good rice crop. The cause is the sometimes detrimental appearance at night of the full Moon, which can interfere with the growth cycles of the rice plant.

By planting rice according to the same date on the lunar calendar instead of the solar calendar, farmers can often harvest two good rice crops every year. The young rice shoots are very sensitive to the light intensity at night during their photoperiod-sensitive stage, so the timing of the Moon’s brightness is essential for a good crop. Because the lunar calendar shifts with respect to the solar calendar dates each year, the solar calendar provides bad timing for planting rice.

The photoperiod-sensitive stage occurs before panicle initiation (where the seed parts develop) and may vary extremely from one variety to the next, from days to months. Photoperiod sensitivity is a natural mechanism based on the plant’s ability to distinguish precise differences in the ratio of day length to night length. The biological mechanism causing photoperiod sensitivity is quite complex and involves several genes. Essentially, some varieties should be planted only during certain times of year to ensure that prevailing day length/night length conditions will trigger panicle initiation when desired.

Many varieties of rice are sensitive to bright moonlight, which can interrupt their growth sequence. However, newer varieties have been bred and others will be genetically modified to decrease their light sensitivity during critical photosensitivity times so that the moonlight will have a minimal effect.

The use of the Moon for timing of events is not restricted to rice farming and its related festivals worldwide. For Christians, Easter Sunday is the first Sunday after the first full Moon after the vernal equinox!