The Moon (Tai Yin, 太阴) star

What the classic Zi Wei Dou Shu Quan Shu (紫微斗数全书) says it is

The essence of water, ruler of the Property palace, transformed to wealth, and the consort of the sun. In the heavenly instrument it has its waxing and waning use, and the division of light and dark is what settles how well it is placed. In a person it is clever and fine-featured, and in nature upright, refined, pure and auspicious. Waxing, it is the hinge of what matters; waning, its force is judged less. What it falls on need not be seen to do harm. If it sits in production with the Sun (太阳) — the sun at Rabbit (卯) and the moon at Rooster (酉), both strong — that is the ground of wealth and rank. The Self seated in the silver-lit palace with the benefics gathered is read as blessing enjoyed. Badly placed, it takes the name of the weak and fallen. Waxing or waning, it is best not meeting the Voice (巨门). The Body sitting there means being raised by a second mother, or leaving home and being adopted. If the Self meets harsh malefics clashing across, it must be read as injury; unless it is a religious life, which turns it to good fortune instead. Deciding fortune here matters most and must not be muddled. And sharing the Self with the Artist (文曲), it is certainly one of the nine schools of practitioner. For a man it is the wife star, and also the mother. Master Xiyi (希夷先生) says: The Moon (太阴) with the Fortune transformation (化禄) is the sun's consort. Rabbit, Dragon (辰), Snake (巳), Horse (午) and Goat (未) are its badly-placed ground and Rooster, Dog (戌), Pig (亥), Rat (子) and Ox (丑) its own wall. Rooster is the gate of the western hill and the place of sinking in the east. It dislikes the Voice winding round it and fears the Blade (擎羊) and the Millstone (陀罗) sharing its degree; with the incorruptible (廉) trapped (囚) offending it and the General (七杀) clashing, the wall is not one it is content in and injury must be read. This star is Water and rules the Property palace: with light it is blessing, and out of light it is certainly ill. For men and women alike it is the mother star and also the wife. Well placed in the Self with benefics gathered, wealth and rank. In Health meeting the Millstone or darkness, eye trouble; meeting the Flare (火星) or the Ember (铃星), calamity; falling on the Wolf (贪狼) or a malefic, injury to the eyes. In Parents, badly placed and out of light, meeting the year's White Tiger (白虎) or the year’s ruling star (Tai Sui, 太岁), the mother has trouble. Pure and mild as this star is, out of light and held down it is not favourable. Meeting the White Tiger, the Mourning Gate (丧门) or the wife should take care as well (弔客).

答曰：太陰乃水之精，為田宅主，化富，與日為配。天儀表有上弦下弦之用，黃到黑到分勢尚好，虧數定廟樂。其為人也聰明俊秀，其稟性也端雅純祥。上弦為要之機，下弦減威之論。所值不以所見無妨，若相生坐於太陽，日在卯，月在酉，俱為旺地，為富貴之基。命坐銀輝之宮，諸吉咸集，為享福之論。若居陷地，則落弱之名。若上弦下弦，仍以不逢巨門為佳。身若居之，則有隨娘繼拜，或離祖過房。身命若見惡殺交沖，必作傷殘之論。除非僧道，反獲禎祥。決禍福最為要緊，不可參差。又或與文曲同居身命，定是九流術士。男為妻宿，又作母星。希夷先生曰：太陰化祿與日為配，以卯辰巳午未為陷地，以酉戌亥子丑為得垣。酉為西山之門，為東潛之所。嫌巨曜以來纏，怕羊陀以同度，廉囚相犯，七殺相沖，恐非得意之垣，定作傷殘之論。此星屬水，為田宅宮主，有輝為福，失陷必凶。男女得之，皆為母星，又作妻宿。若在身命廟樂，吉集主富貴。在疾厄遇陀暗為目疾，遇火鈴為災，值貪殺損目。在父母，如陷地失輝，遇流年白虎太歲，主母有災。此雖純和之星，但失輝受制則不吉。若逢白虎、喪門、弔客，妻亦慎之。