The moon is universally believed to exert a most powerful influence upon terrestrial phenomena, and during the Middle Ages Christians and Jews rarely entered upon an important activity without having first observed the lunar auspices. The waxing moon advances growth and development, the waning moon promotes decay and death. Elazar of Worms diagnosed a mental ailment as due to the contraction of the brain during the last quarters of the moon, and its expansion during the first. Again he warned that clothing soaked in water, trees that have been cut down, fruits and grains harvested while the moon is diminishing rot away very rapidly. Marriages were celebrated during a waxing moon, "not for any superstitious reason, but only as a good omen"! A conception that occurred during this phase of the moon was considered especially auspicious for the child; people moved into new homes in the first half of the lunar month. The day of the new moon was the most favorable for new enterprises; children were brought to school; and courses of study begun on that day. A rational explanation of this last usage was offered, namely, "to give out-of-town students time to arrive," but even its author realized that it was not very convincing, and admitted "and also because it is a good omen." Cutting the hair or fingernails on the day of the new moon was frowned upon "because of danger," for growth should not be checked on the day which is most auspicious for it. Such beliefs were very prevalent among German Christians, and though the rabbis forbade these practices, their disapproval had little effect on their flock. The pagan veneration of the new moon, which had by no means disappeared in Biblical times, has no direct connection with the ceremony of blessing the new moon which was outlined in the Talmud and is observed to this day. But certain superstitious practices have been associated with this rite, pointing to its continued occult importance in human affairs. Some of these are first mentioned in a work composed during the post-Talmudic period, the Masekhet Soferim, others are medieval accretions. In the first group are the practices of skipping three times at the close of the blessing, and addressing the moon three times: "As I skip before you and do not reach you, so, if others jump before me may they not strike me, " and of then thrice bidding one's neighbor "Peace be unto you." The ceremony, as well as the threefold repetitions, are typical of magical acts. In the latter group are the practice of shaking one's clothes "to cast off the spirits," and the belief that one who has performed the full rite need not fear death during the ensuing month. In the sixteenth century the Safed school of Kabbalists instituted the custom of fasting on the eve of the new moon (the day of the new moon was a feast day). The practice was probably in vogue at an earlier time and may be connected with a Christian usage, deplored by a fifteenth-century German writer, but observed by "many people, both laity and clergy, even including masters [who] bend the knee or bow the head on the New Moon or fast on that day, even though it be Sunday or Christmas when the church forbids fasting." A halting recognition that the fast was observed because of "the shrinking of the moon" is evidence of the persistence of primitive apprehensions.   From Joshua Trachtenberg, Jewish Magic and Superstition:A Study in Folk Religion. JPS/Behrman, 1939; Atheneum, 1974