🌗 Why do solar and lunar eclipses come in pairs?

Why don't eclipses occur, somewhere on Earth, at every full or new moon? And why, when they do occur, is a solar eclipse usually followed or preceded by a lunar eclipse?

The Moon does not follow exactly the same path as the Sun in the sky. Its orbit is inclined by about five degrees relative to the plane of the ecliptic, the apparent path of our star. Thus, during the new moon, our satellite most often slips too high or too low to cast its shadow on Earth. Similarly, at the full moon, our satellite usually passes outside the shadow of our planet. This slight inclination therefore prevents the perfect alignments necessary for eclipses from occurring at every cycle.


However, favorable periods, called eclipse seasons, allow these phenomena to occur. Lasting about a month, they occur approximately twice a year. During these windows, the Sun positions itself near the points where the lunar orbit crosses the plane of the ecliptic, called lunar nodes. This geometric configuration then allows an eclipse, provided that a new moon or a full moon coincides with this opportunity.

Solar and lunar eclipses frequently occur in pairs, a few weeks apart. When a new moon occurs near a lunar node, it can generate a solar eclipse. About fifteen days later, the full moon located near the opposite node can in turn cross Earth's shadow, giving rise to a lunar eclipse.

The year 2026 will perfectly illustrate this mechanism. The first season began on February 17 with an annular solar eclipse, mainly visible from Antarctica in the form of a thin bright ring. It will be followed, on March 3, by a total lunar eclipse, observable from East Asia, Australia, and western North America, where the Moon will take on a coppery hue.

The second eclipse season of 2026, in August, promises equally remarkable spectacles. On August 12, a total solar eclipse will plunge into darkness a narrow band crossing Greenland, Iceland, and northern Spain. A partial lunar eclipse will close this window on August 28, visible from the Americas, Europe, and Africa. These events confirm that the sky follows predictable cycles, regularly offering unique observations.

Lunar nodes and their movement

Lunar nodes are two invisible points in space where the Moon's orbit crosses the plane of the ecliptic. They play a fundamental role in triggering eclipses. Without their existence, perfect alignments between Earth, the Moon, and the Sun would be even rarer.

These nodes are not fixed. They move slowly along the lunar orbit in a retrograde motion, that is, towards the west, completing a full cycle in about 18.6 years. This slow migration gradually changes the times when the Sun approaches them, thus shifting the eclipse seasons from year to year.

This movement explains why eclipses do not repeat exactly at the same location or on the same date. It contributes to the diversity of shadow paths on Earth. Understanding this cycle allows astronomers to predict eclipses with great accuracy over centuries.

The study of lunar nodes dates back to antiquity, where early astronomers observed their patterns. Today, orbital calculations, such as those conducted by NASA, use this information to establish detailed calendars of future eclipses, helping observers plan their sessions.