[For Used-In glyphs, please see ⺼ as there are too many using 月 to display meaningfully] Most people, but not all by any means, are aware that the Moon does not generate light of its own, and that moonlight is actually sunlight reflected from the Moon's surface. Hence, it may be appropriate that the glyph for 'Sun' appears within 'Moon.' Chinese culture associates maleness with the Sun (陽, a reddish orb emanating a greenish tinge near its sunset—see any glyph meaning「green」for the reason) and femaleness with the MOON (阴, lordosis position), with a likely combination of elements here as「flashing bright glistening reflection on two orifices over-under between erection and fluid stream」a combination that, in one sense, explains the Chinese association of the sexes with these heavenly bodies.
The largest muscle group in the body is comprised of the two sets of three gluteal muscles, located on each side at the back of the hip and commonly called the buttocks or rump (i.e., the largest concentration of MEAT), an other association in many languages of MEAT with MOON and MOONING. Carnivores generally begin devouring a carcass by tearing into the rump/anus as the place with the most MEAT and easiest access.
Not too many phases of the Moon actually appear as does this glyph, and frankly, this glyph that most often is used as an element for MEAT (⺼) likely related first to MEAT PHASES of the monthly human female ovulation/menstruation cycle and knowing when ovulation enabled fertility each month (approximately 25 to 35 days, average of 29-30), rather than speculation about the dark side of the Moon (approximately 29.54 days, aligning almost exactly with the average days of the human female menstrual cycle…some coincidence?). Unlike most city folks of the modern age, the sages along with everybody else openly observed live animal behavior, especially that of Homo sapiens, and often depicted that behavior within glyphs: as for a female moon, see all the related entries describing female anatomy as various types of meat and Moon, and then tell yourself that each one is coincidental. In ancient China, each month consisted of three 'weeks,' each being ten days long, coinciding with the estrus cycle of farm animals…it is not a stretch to suggest that these same Chinese understood the cycle of the Moon and the cycle of a female human also coincided. Both systems enable easy calculations of periods.
Nobody is suggesting that this glyph does not refer to the natural satellite orbiting Planet Earth that is called the Moon—indeed it does mean the MOON—but it does not refer only to the natural satellite orbiting Planet Earth that is called the Moon—it referred first to something completely different. It is extremely important to realize that anyone today who defines the glyph only in the limited usage it has as the Moon and a month but ignores the original other meanings, as do most sources, is not explaining the whole truth. On the contrary, there is in fact some truth as to the origin of the old English statement often taught to gullible children by a snickering parent no less, that「the moon is made of green cheese」popularized in England at about the same time the opium drug lords were expelled from China, when one considers that the referent moon was a shining buttocks in lordosis position, and that the inserted contents were often the green cheese-like discharges of a client's congealed gonorrhea gleet (not intended here as a joke, but as the source of the statement: see 育 using elements of「grene moon」as gangrenous or putrid meat).
Elements as「erect and spread legs, two orifices over-under」or「erection」and「high tension for a long time with gasping」to represent the posture of a female MOONING by bowing over without bending her knees and deliberately revealing her「moon」or buttocks…note that this is exactly the posture used by a female primate still today when wanting to mate while her vulva is swollen, releasing pheromones and bright red in color, often by thrusting her mooned and exposed genitalia into the face of a sitting male in the attempt to incite his arousal. PROCEPTIVITY remains effective with most human males today as witnessed at the more respected pole and lap dancing establishments, e.g., 'shaking one's booty':「Proceptivity is defined as the behavioral gestures used by a female to induce the male to attempt to mount her. …[F]emales perform 'head bobs', 'head ducks', 'hand reaches' and genitalia presentations to males by backing their hindquarters into the male.」
[ https://academic.oup.com/humrep/article/17/9/2243/563948 2023-05-18 ]
Indeed, mooning a sitting male inches from his face. The actual MOON also appears red when near the horizon, too. In modern times, this same element with modern fonts is used to depict「meat, flesh」and references in glyphs to genitalia as MEAT are plentiful, as they are in English…not merely being receptive to mating, being PROCEPTIVE initiates actions in order to provoke the desired response from the male.

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: moon; month
Japanese EDRDG: month, moon
Unihan extended: moon; month

Used within names (nanori)
おと　･　がっ　･　す　･　ずき　･　もり