The phases of the Moon are eight: four that happen at an exact instant and four that are the stretches between them. It is a distinction almost no calendar makes, and it changes how you use the information.

The instantaneous ones are New Moon, First Quarter, Full Moon and Last Quarter. Each has an hour and a minute, because it is a crossing: the distance between the Moon and the Sun, measured along the ecliptic, passes through 0, 90, 180 and 270 degrees. Outside those four seconds, the Moon is in one of the four stretches, waxing crescent, waxing gibbous, waning gibbous and waning crescent, and each stretch lasts about a week.

Counted by the distance between the Moon and the Sun, the New Moon sits at 0°, at an instant, and marks the beginning, the sowing, what cannot be seen yet; the waxing crescent runs from 0° to 90° and marks the first effort, what needs a push; the First Quarter sits at 90°, at an instant, and marks the first resistance, the decision to carry on; the waxing gibbous runs from 90° to 180° and marks adjustment, refinement, the almost ready; the Full Moon sits at 180°, at an instant, and marks what was hidden and comes out; the waning gibbous runs from 180° to 270° and marks sharing, what gets told to others; the Last Quarter sits at 270°, at an instant, and marks review, what does not get carried forward; and the waning crescent runs from 270° to 360° and marks rest, what is let go of before the new.

Notice that the Full Moon is an opposition, Moon and Sun on opposite sides of the sky. It is no accident that the tradition links it to revealing and to confronting: it is the same thing an opposition does anywhere in the chart, and the subject is in what an aspect is.

A lunar cycle lasts on average 29 days, 12 hours and 44 minutes, and the average misleads. Measuring the 865 lunations between 1966 and 2035 with the same ephemeris this site uses to build charts, the shortest lasted 29 days and 6 hours and the longest 29 days and 20 hours.

The shortest lunation lasted 29 days, 6 hours and 39 minutes; the average of the 865 was 29 days, 12 hours and 44 minutes; the longest lasted 29 days, 19 hours and 55 minutes; and the difference between the extremes is 13 hours and 18 minutes.

Thirteen hours is more than half a day of slack in a cycle of twenty-nine. The cause is that the Moon's orbit is elongated and so is the Earth's: when the Moon is in the stretch where it moves fastest, and the Earth in its own, the next meeting arrives sooner. None of that is irregularity, it is the geometry of two ellipses that do not match.

The practical consequence is direct: a lunar calendar has to be calculated, not counted. Adding 29 and a half days to the last New Moon is off by hours, and off in different directions each month. Whoever publishes the date without calculating gets it right some of the time.

The Full Moon happens at the same instant all over the world, but not at the same clock time. The phase is the Moon's position relative to the Sun, and that position does not depend on where the person is, it is a fact about the solar system, not about anyone's balcony.

What changes is the reading of the local clock. The same Full Moon can fall at 11pm on a Tuesday in Brazil and at 11am on Wednesday in Japan. Two calendars can mark different days without either being wrong, they are counting the same second in different time zones.

That is why the date of a lunation always comes with an hour on this site, and why it is calculated from the clock of whoever is reading.

There is one more detail that confuses a lot of people: the Moon looks full for three nights, because the eye cannot tell 97% illumination from 100%. The instant is a single one; the look of it lasts a weekend. When someone says “the Moon was full on Saturday”, they are right about what they saw and may be wrong about the date of the phase.

Moon phase and Moon sign are not the same thing, and confusing the two is the most common mistake in this subject. The phase is the distance between the Moon and the Sun; the sign is where the Moon stands on the zodiac wheel. They are two different numbers running at different rates.

The Moon changes sign every two and a half days and goes all the way round in 27 days and 8 hours. The phase takes 29 and a half days to close. Since the two periods do not coincide, the same phase happens in a different sign each month, and that is why “New Moon in Aries” is not the same thing as “New Moon”.

In practice the two pieces of information add up: the phase says at what moment of the cycle, and the sign says with what temperament. A Full Moon in Scorpio and a Full Moon in Gemini reveal with the same force and reveal things of opposite natures. What each sign does with the Moon is in the series on the Moon in the twelve signs.

A Blue Moon is the second Full Moon within the same calendar month. It has no colour at all, the name comes from the English expression for “hardly ever”, and the translation stuck.

The cause is arithmetic. The lunar cycle lasts 29 and a half days, and almost every calendar month has 30 or 31, nearly a day is left over each month, and from time to time that leftover piles up enough to fit one more Full Moon. February is the only month that never has two, and sometimes has none.

I measured the Full Moons from 1966 to 2035: there were 28 months with two, out of 840 months, one every two and a half years. That is frequent enough not to be an event, and the effort the internet makes to turn each one into a landmark says more about websites' publishing schedules than about the sky.

Today's phase is not the same as your birth phase, and they are two things that share only a name. Today's phase is a transit: it changes every week and is the same for everyone alive right now. The phase you were born under is a fixed trait of your chart, and never changes.

The difference changes the question. Today's phase answers what the sky is doing; the natal phase answers at what rhythm you begin things. Whoever was born under the New Moon tends to start in the dark, not yet knowing where they are heading; whoever was born under the Full Moon tends to need the whole picture before taking the first step.

The text that describes the eight natal phases, with the proportion of each, is the Moon phase at birth. It answers something else, and the two pages do not replace each other.

The Moon phase does not say what will happen to you, and the reason is one of scale: the phase is the same for eight billion people. Anything it explained on its own would have to hold for all of them at once.

Three things individualise it, and none of them is in a calendar:

• Which house of your chart the lunation falls in. A New Moon at 12 degrees of Libra falls in one person's house 2 and another's house 7, money in one case, relationship in the other.
• What it touches in your chart. A Full Moon on top of your natal Sun is a different week from a Full Moon in an empty patch of your sky.
• Who you are. The same New Moon that asks for a beginning finds one person in a position to begin and another in the middle of a change that has not finished.

And the limit said out loud holds: the Moon phase does not affect anyone's sleep, birth or mood in a way that measurement confirms, the studies that looked for that did not find it. Astrology reads the phase as rhythm and timing, and not as a physical cause. Saying where it does not reach is what makes what it does say trustworthy.

And the questions that come up most on this subject, with the short answer: