New moon western horizon shortly after sunset and before moonset. A lunation or synodic month is the average time from one new moon to the next. In the J2000.0 epoch, the average length of a lunation is 29.530588 days (or 29 days, 12 hours, 44 minutes, and 2.8 seconds). However, the length of any one synodic month can vary from 29.26 to 29.80 days due to the perturbing effects of the Sun's gravity on the Moon's eccentric orbit. In a lunar calendar, each month corresponds to a lunation. Each lunar cycle can be assigned a unique lunation number to identify it. The
Military designation of days and hours Military designation of days and hours The military designation of days and hours within the North Atlantic Treaty Organisation (NATO), is specified in AAP-6 (STANAG 3680), "NATO Glossary of Terms and Definitions", and marked "(NATO)" in what follows. Those entries marked "(US)" are specific to the U.S., and defined only in Joint Publication JP 1-02, "Department of Defense Dictionary of Military and Associated Terms". References to days (or hours) preceding or following a designated day (or hour) use a plus or minus sign and an Arabic numeral following the letter. For example, "D−3" is 3 days prior to D-day, "C+7"