TL;DR: The Sun sign and the Moon sign are the zodiac signs the two luminaries occupied at birth, calculable from the date and time of birth. Reading them together is a convention systematised by Alan Leo in Astrology for All (London, 1910): the Sun corresponds to the enduring Individual part, the Moon to the Personal part expressed day to day, and 12 signs times 12 signs gives 144 soli-lunar combinations. This is the interpretation of one astrological school; the blind tests surveyed by Gerhard A. Mayer in 2020 have not shown astrology to be effective.
What are the Sun sign and the Moon sign?
The Sun sign is the zodiac sign the Sun occupied at birth, and the Moon sign is the sign the Moon occupied at birth. Both can be calculated from the date, time and place of birth and checked against an astronomical ephemeris. The Sun stays in each sign for about a month, while the Moon covers a full 30-degree sign in roughly two and a half days.
Alan Leo, in Astrology for All (fourth edition, revised and enlarged, London 1910), writes that the Sun represents "the positive centre, from which the true life is flowing", whereas the Moon "is representative of the receiver, or the negative and magnetic point"[1]. He asks students to impress on their minds that "the Sun represents the life side, governing the Individual, and the Moon the form side, governing the whole of the Personal (and physical) characteristics"[1].
Why did Alan Leo read the two signs together instead of separately?
Because, according to Leo, the Sun sign alone does not match how a person actually appears day to day. In a lecture printed in the book's appendix, he recalls that some years earlier many major American newspapers published character readings by birth month, and when he published his own interpretations in Modern Astrology magazine, many readers wrote in to say "how marvellously true the descriptions were"; yet if you ask a person which month they were born in and then read them a character portrait by Sun sign, they may not recognise themselves[1]. His explanation: the Sun indicates "the character that is at the root of us, behind our personality", while "in ordinary everyday life, the Moon or lunar characteristics only are apparent"[1].
From this Leo sets out soli-lunar combinations, which he also calls "polarities": the Sun is the positive, primary, life-giving factor; the Moon is the negative, secondary, formative factor; understanding the nature of each sign shows how far the life side affects character and how far the form side affects personality[1]. Twelve signs on each side give 144 combinations; in the lecture he says there is not enough time to go through all 144 pairs, and prints them in chapter XVIII[1]. He also likens the Sun to a telescope, seeing everything on a large scale, and the Moon to a microscope, magnifying the personal side of life: "The Sun represents the ideal, the Moon the details"[1].
How do the Sun and the Moon differ in Leo's reading?
Leo arranges the two luminaries into a positive and a negative pair, each governing one layer of the person. The table below sets out the contrasting pairs he uses in the book[1].
| Item | Sun | Moon |
|---|---|---|
| Polarity | positive, central, life-giving | negative, recipient, magnetic |
| Layer of the person | Individual: enduring character, morals | Personal and physical: the part of consciousness expressed through the brain |
| Analogy | telescope, ideal | microscope, detail |
| Period of rule | "rules by day" | "rules by night" |
| Rank in the chart | first | second |
How is a specific soli-lunar combination read?
Leo writes a short passage for each pair. For the Sun in Gemini and the Moon in Leo, he holds that the personality "suffers somewhat" because the background is not solid enough to support the Leo tendency, so the person is easily impulsive and emotionally unsteady; at times very self-confident and capable of much, at other times feeling they have greater ability than they manage to express; strong imagination, with a gift for poetry and drama[1]. For the Sun in Aquarius and the Moon in Aries, he describes a very firm character, reserved, little given to display, with an eye for judging people, a strong will, and unwilling to be controlled or obstructed[1].
The general principle he states in the lecture: "the personality expresses the individuality in accordance with the sign the Moon is in at birth"[1]. The Sun sign supplies the material, the Moon sign supplies the way that material appears.
Why is the birth time needed to know the Moon sign?
Because the Moon changes sign quickly. Leo notes that the Moon moves about 1 degree in two hours[1], so for someone born near a Moon sign change, knowing only the date does not settle the Moon sign. At the end of the book, Leo prints tables of the Moon's position at Greenwich noon for each day from 1850 to 1905 and explains how to interpolate: on 16 January 1850, at noon the Moon is at 29 degrees of Aquarius, so by 2 p.m. it has already moved into Pisces[1]. He writes that these tables are for people who are unsure of their birth time and cannot cast a chart, so that they may "gain some general and fairly accurate idea" of their character through the soli-lunar combination[1].
Is the soli-lunar combination the whole chart?
No, and Leo himself says so. He writes that the combination descriptions apply generally to everyone born on the same day regardless of the positions of the other planets, but are still influenced by them, and "to a much greater extent" by the rising sign[1].
According to Leo, a chart has four factors ranked by importance: the position of the Sun, the position of the Moon, the rising sign, and the aspects between the planets; the first three are likened to a pound, a shilling and a penny[1]. Adding the rising sign as a third factor brings the number of combinations to 12 x 12 x 12 = 1,728[1]. He likens the rising sign to a window through which the soul looks out on the world[1].
How does Ptolemy use the Sun and the Moon differently from Leo?
Ptolemy, in Tetrabiblos (English translation by J. M. Ashmand, 1822), uses the two luminaries as reference points for calculation rather than building character descriptions by sign pairs. The part of Fortune is calculated from the number of degrees between the Sun and the Moon, placed that many degrees from the rising sign, so that the Moon bears the same relation to it as the Sun bears to the rising sign, and it thus becomes "as it were, a lunar horoscope or ascendant"[2]. When choosing the prorogator, by day the Sun is preferred if it is suitably placed, otherwise the Moon; by night the Moon is considered first[2]. The power to determine profession belongs to two factors: the Sun and the sign at the midheaven[2].
What do scientific tests say about this kind of reading?
Blind tests so far have not shown astrology to be effective. Gerhard A. Mayer, a researcher in Freiburg (Germany) who does not reject astrology as a whole, surveyed the experimental studies in the Journal of Scientific Exploration in 2020. He quotes Geoffrey Dean (1977) on the blind trials that did give significant results: "the significant blind trials have not demonstrated that astrology works but only that astrologers work"[3]. He also describes a matching test (Narlikar and colleagues, 2009), in which astrologers distinguished the charts of 100 high-achieving students from those of 100 students with intellectual disabilities slightly less well than a roll of the dice[3].
Mayer also cites a 2015 YouGov survey in Germany, the UK and the US, asking whether people believe the zodiac sign can say something about themselves or others; the proportion agreeing was markedly higher among women than among men[3]. That figure measures how widespread the belief is, not how accurate astrology is.
Nhật Nguyệt's suggestions for reading the two signs together
Nhật Nguyệt's suggestion: for the part that can be calculated, namely which sign the Sun and Moon occupied, calculate it with an accurate birth time, especially if the birth was near a Moon sign change. The interpretive part, namely what those two signs say about a person, is the voice of Leo and his school; read it as a document of its time, not as a verdict about yourself, and do not use it to decide whether you are compatible with a particular person.
