The Moon's orbit: phases, nodes and eclipses

Derive the lunar phases as an aspect, explain why the nodes move backwards, and say exactly what an eclipse needs that an ordinary New Moon does not.

Lesson 6 of 10 in Astronomy for Astrologers · free to read · no account, no email

A tilted orbit, and two crossing points

The Moon does not travel along the ecliptic. Its orbit is tilted about 5.1° to it, so for half of each month the Moon is north of the Sun's path and for the other half it is south. Twice a month it crosses.

Those two crossings are the lunar nodes. The one where the Moon is heading north is the North Node; the opposite crossing is the South Node. They are not objects. There is nothing there. They are the two places where two circles intersect, which is why a chart marks them with a point rather than a body, and why no telescope will ever photograph one.

The nodal axis does not hold still. The Sun's pull drags it backwards around the ecliptic, one full circuit in 18.6 years — a little over 19° a year, about three arcminutes a day. Backwards is the normal direction for the nodes, which is why they are nearly always shown retrograde, and why “a retrograde node” is not the remarkable event a retrograde planet is.

That 18.6 years is a real rhythm in a life. The nodes return to their birth positions at about 18½, 37 and 55½, and reverse — North Node where South was — at roughly 9¼, 27¾ and 46.

A phase is an aspect

The Moon returns to the same star in 27.32 days — the sidereal month, one real orbit. But it returns to the same phase in 29.53 days, the synodic month, because while the Moon went round, the Sun moved on about 27° and the Moon has to catch it up.

And that gap is the whole of phase. A lunar phase is nothing but the angular distance between the Moon and the Sun. New Moon is 0°, which is to say a conjunction. First quarter is 90° — a square. Full Moon is 180°, an opposition. Last quarter is 270°, the other square. The eight named phases are simply 45° steps around that one angle.

So the phase you were born under is not an extra system bolted onto your chart. It is a reading of a single aspect you already have: your Sun–Moon angle. Subtract your Sun's longitude from your Moon's, take the result modulo 360, and you have the phase, exactly. Someone born at 45° is a crescent; at 135°, a gibbous Moon building to full.

One practical consequence of the Moon's speed: it is not constant. The orbit is an ellipse, so the Moon covers between about 11.8° and 15.4° a day. The same orb of aspect can therefore last a third longer or shorter depending where in the month it falls — which matters for the void-of-course window and for anyone timing by the Moon.

What an eclipse needs that a New Moon does not

Every month there is a New Moon: the Moon at 0° from the Sun. Every month there is a Full Moon at 180°. Eclipses are far rarer than that, and the tilt is the reason.

An eclipse needs two conditions at once. The Moon must be new or full, and it must be close to a node, so that it is actually on the ecliptic rather than 5° above or below it. A New Moon away from the nodes passes above or below the Sun and nothing happens. A New Moon at a node passes in front: a solar eclipse. A Full Moon at a node passes into the Earth's shadow: a lunar eclipse.

Because the nodal axis slides backwards, the two stretches of the year when eclipses are possible — the eclipse seasons — arrive about nineteen days earlier each year, and they come around every 173 days or so rather than every six months. Eclipses on the same axis repeat with a period of 18 years and 11 days, the saros, and that is old enough knowledge that Babylonian astronomers were predicting eclipses with it.

Two more things live on this orbit, and you should know where they come from. Because the orbit is an ellipse with the Earth at one focus, there is a near point and a far point: perigee and apogee. The direction of apogee is what Black Moon Lilith marks — an empty place rather than a body, which is why there are two defensible versions of it, a smoothed mean one and a true one that wobbles, and why two pieces of software can put Lilith 30° apart and both be right about the thing they chose to compute.

What that looks like

Take a month with an eclipse in it. The New Moon falls within a few degrees of the nodal axis, so the three bodies line up in three dimensions rather than only in longitude, and the Moon's shadow lands somewhere on the Earth. Two weeks later the Full Moon falls near the opposite node and there is a lunar eclipse to match — which is why eclipses come in pairs a fortnight apart, and why the pair sits on one axis of your chart rather than scattered.

What to take away

• The Moon's orbit is tilted 5.1°, so it crosses the ecliptic twice a month; those two crossings are the nodes, and they are intersections rather than objects.
• The nodal axis slides backwards once every 18.6 years, which is why the nodes are normally retrograde and why nodal returns land near 18½, 37 and 55½.
• A lunar phase IS the Sun–Moon angle: New is a conjunction, quarters are squares, Full is an opposition. Your birth phase is an aspect you already have.
• An eclipse needs a New or Full Moon AND proximity to a node — which is why they are rare, why they come in pairs, and why they sit on one axis.

The common mistake

Expecting an eclipse whenever there is a New Moon, or treating the nodes as bodies with a position of their own. Both come from forgetting the 5° tilt — the one fact that makes eclipses special and makes the nodes exist at all.

Try it on your own chart

Work out your own Sun–Moon angle: Moon's longitude minus Sun's longitude, and add 360 if it comes out negative. Under 45° and you were born at a New Moon; near 90° a first quarter; near 180° a Full Moon. Then check it against the Moon phase your chart already shows. You have just derived one of the most-quoted things about a birth chart from two numbers and a subtraction — and you now know it is an aspect wearing a different name.

Check yourself

The same questions the lesson asks in the app. Open one to see the answer.

Keep going

← Umbriel University

“University” is Umbriel’s name for a real, sequenced course, not an accredited academic institution. Anything you earn here is Umbriel’s own record of course completion, recognised by us — not a licence, a diploma or a government-accredited qualification. Astrology is offered for reflection and enjoyment, not as a regulated profession.