The eighteen-year wobble, the gist

The eighteen-year wobble

Eclipses come in pairs a fortnight apart, in short seasons about 173 days apart, because two points where the moon's orbit crosses the earth's slide slowly backwards around the sky, and you can work out how fast from two numbers printed in any astronomy reference.

This is the short way in. The essay has the working, the sources and the sky.

The moon passes through the exact plane of the earth's orbit about 27 times a year. Almost every one of those crossings happens in an empty sky with nothing to see. Only four to seven of them a year turn into an eclipse.

The earth goes round the sun. Draw that path flat on a table. The moon goes round the earth on a path tilted about 5 degrees from the table, so half of each month it is above the table and half below. It cuts through the table twice a month, at two opposite spots. Those spots are the nodes.

An eclipse needs the sun, the earth and the moon in a straight line. Lining them up sideways is easy: that is what a new moon and a full moon are, twice a month. The hard part is height. Unless the moon is near a node at that moment, it sits above or below the line and slides past.

So eclipses come only when the sun is pointing along the line the nodes sit on. That happens twice a year and opens a window about a month long. Inside it there is always a new moon and usually a full moon, so you get two eclipses about a fortnight apart: a solar eclipse on 12 August 2026 and a lunar one on the 28th.

Now the interesting part. The nodes do not stay still. The essay compares two measured lengths of a month: the moon's return to the same star, 27.321662 days, and its return to the same node, 27.212221 days. The second is 2 hours and 38 minutes shorter. The only explanation is that the node moved backwards to meet the moon.

Turn those two months into speeds, subtract, and the nodes retreat 0.052992 degrees a day: 19.355 degrees a year, one full circuit in 18.6 years. Feed that into the sun's motion and the sun gets back to the same node every 346.62 days, so eclipse seasons come every 173.31 days.

The essay checks that against four real seasons from 2025 to 2027, where the gaps run 162.7, 177.0 and 177.4 days, and explains the difference honestly. The arithmetic says when the sun reaches the node. An eclipse also has to wait for the nearest new or full moon, and those come only every 14.77 days.

Words decoded

ecliptic: the flat plane of the earth's orbit round the sun, drawn as a line across the sky
node: one of the two points where the moon's tilted orbit crosses that plane
lunation, or synodic month: one new moon to the next, about 29.53 days
sidereal month: the moon's return to the same star, 27.321662 days
draconic month: the moon's return to the same node, 27.212221 days
syzygy: the sun, earth and moon lined up, which is what a new moon or a full moon is
eclipse season: the roughly month-long window, twice a year, when the sun is lined up with the nodes and eclipses become possible
eclipse year: the sun's return to the same node, 346.62 days, shorter than an ordinary year by 18.62 days
arcsecond: one 3600th of a degree, a very small angle
greatest eclipse: the instant an eclipse is at its deepest, which is how catalogues date it
Rahu (राहु) and Ketu (केतु): the traditional Indian names for the ascending and descending nodes; the essay names them and leaves what they are held to mean to other writers
mean node: the smoothed average position of a node, as against its slightly wobbling true position

Why anyone should careEclipses have carried more fear per event than almost anything else in the sky, and the fear survives partly because the timing looks arbitrary. It is not. Two numbers you can look up give you the whole schedule, including why the seasons drift about 18 days earlier each year and why the pairs sit a fortnight apart. Knowing where the pattern comes from is not the same as knowing what it means, and the essay is careful to say so, but it does remove the one thing that made eclipses look like an interruption rather than a clock.

Read it properly, 7 min The eighteen-year wobble