Every two and a half days Moon changes from one zodiacal sign to another. In her routine travels she forms aspects with other planets, but after the last aspect prior to changing signs she is considered void of course. This definition was popularized by astrologer Al H. Morrison around 1970. The definition Hellenistic astrologers used about 2000 years ago was different. The Moon was considered void if it made no further aspects for a day (within 13°). The older definition would mean that there are fewer void of course periods than modern listings