What a lunation is

The definition

A lunation is one complete run of the lunar phases, measured from one new Moon to the next. Astronomers call that interval the synodic month, and its mean length is 29 days, 12 hours and 44 minutes. It is longer than the 27.3 days the Moon takes to go once round the sky against the stars, because while the Moon orbits, the Earth carries on round the Sun; the Moon has to travel a little further to line up with the Sun again in the same way.

The individual lunation is never exactly the mean. The Moon's orbit is an ellipse, so it moves faster near perigee and slower near apogee, and successive lunations run anywhere from about 29 days 6 hours to about 29 days 20 hours. That is why the gaps quoted on the dated pages are 29 or 30 days for a full cycle and 14 or 15 days between one phase and the next, rather than a fixed figure.

The four quarters

At the new Moon the Sun and the Moon occupy the same degree of the zodiac. Nothing on the near face is lit, and the Moon rises and sets with the Sun, which is why it cannot be seen. A fortnight later they stand at opposite degrees: the near face is fully lit, the Moon rises as the Sun sets, and it is at its brightest. The two quarters fall between them, at ninety degrees either side, when half the visible disc is lit.

The waxing half of the cycle runs new to full, the waning half full to new. Traditional astrology reads the waxing half as the part of the cycle where something builds and the waning half as the part where it is used up or cleared, and treats the new Moon as an opening rather than a conclusion. That is a reading convention with a long history behind it, not a mechanism; the sky supplies the geometry and nothing else.

Why the dates drift against the calendar

The synodic month is shorter than every calendar month except February, so lunations creep backwards through the month, arriving roughly eleven days earlier each year. Two consequences follow. The first is that a given sign hosts its new Moon on a different date each year, and occasionally in a different calendar month altogether. The second is that a calendar month long enough to hold two full Moons occasionally does so — the popular blue moon — and the same arithmetic produces a second new Moon, the black moon. Neither is an astronomical event. Both are artefacts of counting the sky with a Roman calendar.

The folk names — Wolf, Snow, Worm, Harvest and the rest — follow the calendar month too, since they come from farming and hunting practice in northern latitudes rather than from astronomy. That is why the same name lands in a different zodiac sign from one year to the next, and why a name is a poor guide to what a chart will do with the position.

Distance, and what a supermoon is

The Moon averages 384,400 km from Earth, but the ellipse takes it as close as roughly 356,500 km and as far as roughly 406,700 km. When a full Moon falls near the close end of that range the disc is about seven per cent wider and appreciably brighter than an average one, and the term supermoon is applied; near the far end the term micromoon is used. The difference is real and measurable in a photograph, and almost impossible to judge by eye without one to compare against. Astrologically the distance carries no standard meaning at all: no traditional technique uses it, and the dated pages quote the figure because it is a fact about the event rather than because it changes a reading.

What a chart does with it

The useful part of a lunation for chart work is the degree. A lunation at 5 degrees of Libra contacts natal placements near 5 degrees of Libra, Aries, Cancer and Capricorn, and leaves the rest of the chart alone. The sign name on its own is too coarse to place it, since a sign is thirty degrees wide and a working orb is a degree or two.

The second useful part is the house. Counted from your own rising sign, the lunation falls in one of the twelve houses, and that is the area of life the tradition reads it against. The dated pages give the house for each sign as a first approximation, which is accurate if your rising sign is the one you look up and approximate otherwise. Neither the degree nor the house makes a lunation produce an event; what they give is a position and a place to read it.

Questions people ask about this