Anomalistic and draconic month: the hidden rhythms of the Moon
Perigee, apogee and nodes: what the anomalistic and draconic months measure, where their names come from and why they hold the key to eclipses.
Besides the month of the phases and the month of the stars, astronomers distinguish other lunar cycles, less well known but decisive. Two in particular explain why the Moon sometimes looks larger and why eclipses do not happen every month. They are called the anomalistic month and the draconic month, and their names preserve traces of Greek astronomy and of the medieval imagination.
Perigee and apogee
The Moon's orbit is not a circle but an ellipse. The point nearest the Earth is called perigee, the farthest apogee; the distance between the centres of the two bodies varies roughly between 356,000 and 406,000 kilometres. The anomalistic month is the time the Moon takes to pass through perigee twice in succession: on average 27.55 days, a little longer than the sidereal month.
The difference arises because the axis of the ellipse is not fixed but turns slowly in the same direction as the Moon's motion, completing a full revolution in about 8.85 years. The term «anomaly» comes from Greek astronomical vocabulary, where it denoted the irregularity of a body's motion compared with uniform circular motion. Hipparchus and later Ptolemy tried to explain it with the system of eccentrics and epicycles.
The way of the dragon
The plane of the lunar orbit is tilted by about five degrees with respect to the ecliptic, the plane along which the Sun appears to move. The two points where the two paths cross are called nodes. The draconic month, also called the nodal month, is the interval between two consecutive passages of the Moon through the same node and lasts on average 27.21 days.
In this case the month is shorter than the sidereal one, because the line of nodes regresses, that is, it moves in the direction opposite to the Moon's motion, completing a circuit in about 18.6 years. The name derives from an image, widespread in medieval astronomy and astrology, of a celestial dragon with its head and tail at the two nodes, which according to belief devoured the Sun or the Moon during eclipses.
Why they matter
The draconic month is the key to eclipses, which can occur only when a new moon or a full moon falls near a node. The anomalistic month instead determines the apparent size of the Moon and distinguishes, for example, a total solar eclipse from an annular one. The fact that these cycles fall almost exactly into step with the synodic month after a certain number of years makes possible the Saros cycle, used by Babylonian astronomers to predict eclipses.
Finally there is the tropical month, measured with respect to the point of the spring equinox, which is a few seconds shorter than the sidereal month because of the precession of the equinoxes. It is the reference month for Western astrology, whose zodiac is tied to the equinoxes rather than to the constellations, which over the centuries have shifted with respect to the signs.
A motion hard to tame
The existence of so many different months shows that the Moon's motion is the most complex of those visible to the naked eye. Already in antiquity astronomers realised that no simple model was enough to describe it, and lunar theory remained a testing ground for centuries. Isaac Newton, according to an account handed down by his contemporaries, is said to have called it the only problem that made his head ache.
Anomalistic and draconic months remain invisible to the casual observer, yet they govern phenomena everyone can see: the Moon that looks larger on certain nights, the shadow that sometimes darkens it, the Sun that goes out for a few minutes at midday. Behind the apparent simplicity of the night's lamp lies a web of rhythms of surprising precision.