Lunation Calendar — Deep Dives — Eclipses
Eclipses occur when two rhythms overlap.
One rhythm brings the Moon to New or Full phase, lining up the Sun, Moon, and Earth. Another brings it to a node, where its tilted orbit crosses the ecliptic. When the rhythms coincide, one body’s shadow can fall across another. The charts show how this alignment recurs in eclipse seasons—and how apparent size and point of view shape what follows.
Shadows and alignment
Earth and the Moon are always casting shadows into space. Most of the time, those shadows meet nothing. But when the alignment is just right, the Moon’s shadow falls across Earth and produces a solar eclipse, or the Moon passes into Earth’s shadow and produces a lunar eclipse.
The alignment must be right in two dimensions. The east-west alignment occurs at New Moon, when the Moon is between Earth and the Sun, and at Full Moon, when Earth is between the Moon and Sun.
The north-south alignment occurs where the Moon’s tilted orbit crosses the ecliptic. These crossings are the ascending node ☊, where the Moon passes from south to north, and the descending node ☋, where it passes from north to south. The Lunation Chart’s central Orbit Diagram shows the tilted orbit, ecliptic plane, and nodes directly.
An eclipse requires both alignments at once: a New or Full Moon must occur close enough to a node. The Year Chart’s Eclipse Band makes this overlap visible. Its waveform traces the Moon north and south of the ecliptic, while dots and radial lines mark each New and Full Moon. When one falls close enough to the ecliptic to produce an eclipse, the dot is circled. The Lunar Node Crossing, Solar Eclipse, and Lunar Eclipse Rare Events mark the same geometry in time. The Orbit Diagram reveals the alignment when a node is near the New Moon or Full Moon.
The two dimensions run on different clocks. The north-south cycle, called the draconic month, returns the Moon to the same node in about 27.2 days. The east-west cycle, called the synodic month, returns the Moon from one New Moon to the next in about 29.5 days. The Year Chart’s Eclipse Band reveals both clocks at once: its repeating waveform traces the draconic cycle, while the rhythm of its New and Full Moon markers traces the synodic cycle.
Because these cycles have different lengths, they drift into and out of alignment. Roughly every 173 days—a little less than six months—successive New and Full Moons can again fall close enough to a node for eclipses to occur. This several-week interval is an eclipse season.
(Apparent) size matters
The Sun is roughly 400 times wider than the Moon, but it is also roughly 400 times farther from Earth. From here, their disks therefore appear nearly the same size. That resemblance is not exact or constant: the Moon’s apparent size changes as its distance from Earth changes, and the Sun’s apparent size varies to a lesser degree.
The Lunation Chart’s central Orbit Diagram depicts the Moon’s changing distance from Earth, marking its nearest point, perigee ⯝, and its farthest, apogee ⚸. The same extrema appear as Lunar Perigee or Apogee Rare Events. Earth’s changing distance from the Sun appears as the Earth Perihelion or Aphelion Rare Event and contributes to the Sun’s smaller variation in apparent size.
When the Sun and Moon align center-to-center somewhere on Earth, their relative apparent sizes determine the eclipse type. A Moon that appears large enough to cover the Sun produces a total eclipse. If the Moon appears smaller, a bright rim of the Sun remains, producing an annular eclipse. If the disks overlap but never align center-to-center from anywhere on Earth, the event is a partial eclipse. Away from the central path, even a globally total or annular eclipse appears partial. The Solar Eclipse Rare Event summarizes the global eclipse as total TOT, annular ANN, or partial with the maximum obscuration percentage. For a partial eclipse, that percentage reports the greatest obscuration somewhere on Earth, not the maximum seen from the chart location.
Through the shadow
An eclipse unfolds as the bodies move into and out of alignment. Its contacts mark the moments when a body or shadow reaches a new boundary; its maximum is the moment of deepest alignment. The eclipse type describes which shadow regions are crossed and how centrally the bodies align.
Earth’s shadow has two regions: the faint outer penumbra and the dark inner umbra. If the Moon passes only through the penumbra, it is a penumbral lunar eclipse. If part of the Moon enters the umbra, it is a partial lunar eclipse; if the entire Moon enters the umbra, it is a total lunar eclipse.
The Solar Eclipse and Lunar Eclipse Rare Event labels identify the eclipse type and maximum. A solar-eclipse label begins with the Sun ☉; a lunar-eclipse label begins with the Moon ☽. Solar-eclipse labels appear in gold and lunar-eclipse labels in blue, keeping each event visually connected to the Sun or Moon elsewhere in the charts.
A Lunar Eclipse label uses PEN for a penumbral eclipse and TOT for a total eclipse. For a partial eclipse, its percentage reports how much of the Moon’s diameter is immersed in the umbra. It describes the eclipse geometry, not how much the Moon’s brightness will dim.
A global event, a local view
An eclipse is one global alignment, but its appearance depends on where it is observed. The Rare Event label is placed at the globally timed eclipse maximum. Its visibility symbol is ⏿ when the eclipse can be seen from the chart location and ⊘ when it cannot. When an eclipse is visible, an interval marker in the Lunation Chart’s Gregorian Date Band shows the locally visible partial phase of a solar eclipse or umbral phase of a lunar eclipse.
When an eclipse is visible from the chart location, the Moon Faces Band includes an eclipse graphic. For a solar eclipse, it depicts the Sun and Moon at their relative apparent sizes and offset as seen from that place. For a lunar eclipse, it shows the Moon at the most advanced stage visible from the chart location.