The Lunar Nodes

What the nodes are

Picture two great rings sharing the same center. One is the flat plane of Earth's orbit around the Sun, which we call the ecliptic (it is also the Sun's apparent path through the year). The other is the Moon's orbit around Earth, tilted about 5 degrees to the first. Two rings tilted against each other can only meet at two points, on opposite sides. Those two meeting points are the lunar nodes. At the ascending node (☊) the Moon is climbing from south of the ecliptic to north of it; half an orbit later, at the descending node (☋), it is sinking back from north to south. Halfway between the nodes the Moon rides at its full 5 degrees above or below the Sun's path. The nodes matter for one big reason: they are the only points where the Moon can sit exactly level with the line from Earth to the Sun, so when a new or full moon happens to fall on a node, the three bodies line up and an eclipse can occur.

The line of nodes turns: 18.6 years

The nodes do not stay fixed among the stars. The whole line of nodes slowly rotates westward, opposite to the Moon's monthly motion, completing one full circle around the ecliptic every 18.6 years. Astronomers call this the regression of the nodes. It happens because the Sun's gravity is constantly tugging on the Moon's tilted orbit, swinging it around in much the same way gravity makes a leaning, spinning top wheel its axis in a slow circle. When you scrub the years in the view above, that 18.6-year drift is what you are watching, sped up enormously.

Why it matters

• Eclipses. An eclipse needs a new or full moon to happen near a node. As the nodes turn, the eclipse seasons creep earlier each year, by about 19 days a year. See Eclipses for the full picture.
• The draconic month. The Moon returns to the same node every 27.21 days, a touch shorter than the 27.32-day sidereal month, because the node has shifted to meet it.
• Lunar standstills. The Moon's 5-degree tilt adds to or subtracts from Earth's 23.4-degree axial tilt as the nodes turn. So the Moon's monthly swing in the sky grows and shrinks: a major standstill of about ±28.6 degrees and, 9.3 years later, a minor standstill of about ±18.3 degrees. Ancient sites such as Callanish and the standing stones at Stonehenge appear to track this rhythm.
• The 18.6-year nodal tide. The same cycle gently modulates the tides and even long-term sea level.

You will find the nodal cycle, the draconic month, and the eclipse year on the cycles by length page.