← Encyclopedia of Astrology · lesson 154/235
The Moon's months
What it is
Ask how long the Moon takes to "go round" and there are several right answers, depending on what you measure against. Astronomers name several lunar months, and astrology uses each of them for something.
| Month | Measured from… to… | Mean length |
|---|---|---|
| Synodic | New Moon to New Moon | 29.53 days |
| Sidereal | a fixed star back to the same star | 27.32 days |
| Tropical | 0° Aries back to 0° Aries | 27.32 days |
| Anomalistic | perigee to perigee | 27.55 days |
| Draconic (nodical) | node back to the same node | 27.21 days |
Why they differ
The sidereal month is the Moon's true circuit against the background stars. The synodic month is longer by about two days because, while the Moon goes round, the Earth carries on along its own orbit and the Sun appears to move about 29° along the ecliptic. The Moon has to travel that extra stretch to catch the Sun up again before the next New Moon. That is why the phases repeat about every 29½ days although the Moon passes each sign about every 27⅓.
The tropical month, measured against the equinox rather than the stars, is shorter than the sidereal by only a few seconds, the effect of the slow precession of the equinoxes. It is the month that matters for the signs of the tropical zodiac: it is how long the Moon takes to pass through all twelve.
The anomalistic month measures the Moon's elliptical orbit, from her nearest point to the Earth (perigee) back to it again. The draconic month measures her return to the North Node, where her tilted path crosses the ecliptic; it is named after the dragon of the old eclipse myths.
What each one times
- The synodic month times the phases — the whole lunation cycle, the Moon's age, and the gardening and electional rules that depend on the Moon waxing or waning.
- The tropical month times the Moon's passage through the signs: about two and a half days in each, which is why a Moon calendar changes sign every two or three days. A lunar return comes round once a tropical month.
- The anomalistic month times her changes of speed and apparent size, and so the "supermoons" of modern language.
- The draconic month times the eclipses. An eclipse needs a New or Full Moon near a node, and the way the synodic and draconic months slowly fall in and out of step is what produces eclipse seasons about every six months and the 18-year saros cycle that repeats them.
Biodynamic gardeners add a fifth rhythm, the Moon's ascending and descending course, which follows the swing of her declination north and south over about 27⅓ days — see the biodynamic calendar.
In the tradition
Ptolemy already distinguished the Moon's phases from her course through the zodiac, and the old astrologers read both: her phase for growth and decline, her sign for the matter and the part of the body. Medieval tables of eclipses rested on the interplay of these months long before anyone could explain it by orbits.
In the almanac
The Almanac's Moon panel shows two of these rhythms side by side: the phase, from the synodic month, and the sign, from the Moon's passage through the zodiac — with the next change of sign, and her distance from the Earth, which follows the anomalistic month.