← Encyclopedia of Astrology · lesson 153/235
Moon's age and the lunar day
What it is
The Moon's age is the number of days that have passed since the last New Moon. It starts at 0 when the Moon and the Sun meet, reaches about 14¾ at the Full Moon, and runs on to about 29½, when the next New Moon begins the count again. A Moon "five days old" is a thin evening crescent; a Moon "twenty-two days old" is near the last quarter, rising around midnight.
Printed almanacs gave it a line of its own for centuries, often as the lunar day or "day of the Moon". For a farmer, a fisherman or a barber-surgeon without a chart, the age was the simplest way to know where the Moon stood in her round, and therefore which jobs the tradition thought she favoured.
How it is worked out
The age is measured from the exact moment of the last New Moon, so it is usually given with a decimal — 3.6 days, 17.2 days. Because the lunation cycle averages about 29.53 days but varies by several hours from month to month, the Full Moon does not always fall at exactly the same age: anywhere from about 14 to 15½ days.
The age is closely tied to the angle between the Sun and the Moon. Each day the Moon gains about 12° on the Sun, so:
| Age (days) | Angle from the Sun | Phase |
|---|---|---|
| 0 | 0° | New Moon |
| about 7½ | 90° | First quarter |
| about 14¾ | 180° | Full Moon |
| about 22 | 270° | Last quarter |
That angle, not the count of days, is what astrologers use to name the eight phases, which is why a reading may call a Moon "crescent" or "gibbous" rather than giving her age.
The tithi
Indian astrology keeps the same idea in a more exact form. The month is divided into 30 tithis, each the time the Moon takes to gain 12° on the Sun. Fifteen belong to the bright half (shukla paksha, the waxing Moon) and fifteen to the dark half (krishna paksha, the waning Moon). Because the Moon's speed varies, a tithi lasts anywhere from about 19 to 26 hours, so it does not line up with calendar days. The tithi is still central to choosing auspicious moments in the Indian tradition, beside the nakshatra the Moon occupies.
How it was used
The age told the old almanac users whether the Moon was increasing or decreasing in light — the one fact most of their rules turned on. William Ramesey's seventeenth-century elections are full of it: hair cut to grow back quickly "with the Moon increasing in light", peas sown with her decreasing. Electional writers such as deVore narrowed the good window for beginnings to the span from about the third day of the Moon to about three days before the Full — when she is clear of the Sun's beams and still growing.
Some medieval and later traditions went further and gave each of the thirty days of the Moon its own character — lucky or unlucky, good for travelling, for bloodletting or for dreams. These lunaries circulated widely in manuscripts and chapbooks. They vary greatly from one copy to another and are best read as folklore.
How to read it
Today the age is mainly a quick way to place the Moon in her cycle. A low number means a young, growing Moon, read as favouring beginnings and growth; a number past about fifteen means the light is lessening, read as favouring completion, harvest and letting go. It inclines the reading of a day; it does not decide it.
In the almanac
The Almanac's Moon panel gives the Moon's age in days beside the picture of her current phase, with the dates of the coming New, first-quarter, Full and last-quarter Moons.