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Lunar Phase and the Sun–Moon Pair
By the end you will know that a moon phase is simply the angle between the Sun and Moon, be able to see from your own chart which stretch of the phases you were born in, and recognise a new and a full moon.
Approved original lessons, free on the web · 2026-10-12
THE IDEA
A moon phase is how far apart the Sun and Moon are: full moon is an opposition, new moon a conjunction.
Diagram: four circles shading from light to dark for new moon, first quarter, full and last quarter, with a note that the text calls new to first quarter moist, first quarter to full hot, full to last quarter dry and last quarter to occultation cold. The circles show order only.
A phase is how many degrees the Moon is from the Sun.
A phase is an angle
A phase is the aspect idea from earlier lessons, applied to the Sun and Moon: new moon is a conjunction (around 0 degrees), full moon an opposition (around 180), and the quarters are close to a square (90). The further the Moon moves on from the Sun, the more of it is lit, until it is fully lit when opposite.
Derived calculationBy NASA's figures, one full round of phases (the synodic month) takes about 29.53 days, and one orbit of the Moon about 27.3217 days.
Cited sourceThe round of phases is a little longer than one orbit because the Sun has moved on as well: when the Moon finishes its orbit the Sun has moved ahead, and the Moon needs a little over two more days to catch up.
Derived calculationWhat the old texts say: Ptolemy's Tetrabiblos, Book I ch. 8, splits the month into four by phase — moist from new moon to first quarter, hot from first quarter to full, dry from full to last quarter, cold from last quarter until the Moon disappears. The text uses phase as a relation between where the Sun and Moon are. Book III ch. 2 also uses the word syzygy for a new or full moon.
Cited sourceA new moon is the Sun and Moon meeting, a full moon them facing each other; the old text used them as reference points for correcting a birth time.
New moon and full moon
A new moon is often read as a time for beginnings, and a full moon as a time for seeing results and letting go. Many people write down what they want to start at a new moon, then look back at the full moon to see how far it has come.
Later interpretationSpotting them is easy: Sun and Moon in or near the same sign is the new-moon stretch; the Moon in the sign opposite the Sun is the full-moon stretch. With the Sun in Libra, for example, the Moon reaching Aries is close to full.
Derived calculationThe chart page shows the Sun and Moon at your moment of birth and the aspect between them; from that aspect you can tell whether you were born at a new moon, a full moon, or somewhere in between.
How this tool worksWhat the old texts say: Book III ch. 2 first notes that birth times taken from water clocks and sundials often err. To fix the degree of the birth moment, take the syzygy — new or full moon — just before the birth, work out its degree exactly (both lights at a new moon, the one above the horizon at a full moon), then see which stars rule that place at the birth. So in the old texts new and full moons are reference points for a calculation.
Cited sourceWhat you look at is how far apart your Sun and Moon are.
Reading your own Sun and Moon
The birth phase is often read as “how far your inner needs sit from the way you come across”: near a new moon the two point the same way, so what you want and what you show are close; near a full moon they face each other, and you often look for a balance between the two.
Later interpretationYou can check it yourself: how many signs apart are your Sun and Moon? Zero or one is roughly the new-moon stretch; six is the full-moon stretch; about three is near a quarter. Sun in Leo and Moon in Aquarius, for example, are six apart: the full-moon stretch.
Reflection promptThe Moon moves about 12° a day. So without a birth time the number of signs apart is usually still clear, but the exact angle needs the time.
Derived calculationWithout a birth time, this app shows the range of the angle across your whole birth day instead: the two ends of the range are the earliest and latest angles possible that day.
How this tool worksWhat the old texts say: Book I ch. 8 uses phase to split up qualities, in order moist, hot, dry and cold.
Cited sourceOBSERVE YOUR OWN CHART
Look at my chart
Your turn: you can do this without a birth time; the exact angle just becomes a range.
- Find your Sun and Moon signs and count how many signs apart they are.
- Zero or one is the new-moon stretch, six the full-moon stretch, about three is close to a square.
- Write one line: do your inner needs and the way you come across point the same way, or pull against each other?
Without a birth time, rising sign and houses stay blank.
LOOK CLOSER
A common mix-up
“Wish on a new moon and it will come true.”
A new moon is an angle between the Sun and Moon. Writing down what you want to start at a new moon and looking back at the full moon is a good habit for sorting your thoughts; whether it happens depends on what you do.
“Phases are a separate system.”
A phase is just the angle between Sun and Moon: new moon a conjunction, full moon an opposition.
“One orbit of the Moon is one round of phases.”
One orbit is about 27.3217 days and one round of phases about 29.53, because the Sun has moved on too.
CHECK YOURSELF
Now, try it yourself.
Sources and rights
- Ptolemy, Tetrabiblos, trans. F. E. Robbins (Loeb Classical Library, 1940)
Book I ch. 8 divides the month by phase — moist from new moon to first quarter, hot to full, dry to last quarter, cold to occultation. Book III ch. 2 first notes that birth times taken from water clocks and sundials often err, then requires taking the syzygy most recently preceding the birth, establishing its degree accurately — both luminaries at a new moon, the one above the earth at a full one — and seeing which stars rule that place. Reading the birth phase as the distance between inner and outer is later usage, and new-moon wishing has no classical source, which the lesson states.
The Robbins translation is in the public domain, as the University of Chicago LacusCurtius copyright page records. The transcriber's own notes and layout are not cited. · 2026-10-10 - NASA NSSDCA, Moon Fact Sheet (updated 2024-01-11)
Synodic period 29.53 days, revolution period 27.3217 days. Explaining the difference by the Sun's own motion is this lesson's wording from those two figures, not a statement on the sheet.
Under NASA's images and media guidelines, NASA-produced content is generally not copyrighted and may be used for educational purposes, while the NASA insignia and third-party material carry separate restrictions. Only the figures are used here — no insignia, no imagery. · 2026-10-10
Limits of this lesson
Without a birth time, the angle between the Sun and Moon is shown as a range across your whole birth day, not a single number.
Reading the phases is a language for describing yourself, not a prediction, and promises no wish will come true.
YOUR STUDY
Write what you learned in your own words.
Write it down: how many signs apart are your Sun and Moon, and does that distance match how you really feel?
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