Eclipses Deep Dive | Lunacliptica

Shadows

Earth and the Moon are always casting shadows into space. Most of the time, those shadows meet nothing. Here, the Moon’s shadow misses Earth.

In a solar eclipse, the Moon passes between the Sun and Earth, and the Moon’s shadow falls across Earth. The Lunation and Year Charts mark its global maximum with a Solar Eclipse Rare Event.

In a lunar eclipse, Earth passes between the Sun and Moon, and the Moon moves through Earth’s shadow. The charts mark its maximum with a Lunar Eclipse Rare Event.

East-west alignment

For either shadow to reach the other body, the Sun, Earth, and Moon must align in two dimensions. The first alignment is east to west, along the ecliptic. The Moon reaches this phase alignment at New Moon, when it lies in the Sun’s direction, or at Full Moon, when it lies in the opposite direction. This determines whether the Moon’s shadow points toward Earth or Earth’s shadow points toward the Moon.

Before alignment. The Moon approaches the Sun’s direction along the ecliptic, but the bodies are still separated east to west.

At alignment. At New Moon, the Moon lies between Earth and the Sun. At Full Moon, Earth lies between the Moon and Sun.

After alignment. The Moon continues past the Sun’s direction and the east-west separation opens again.

The Lunation Chart’s Moon Phases Band places each New and Full Moon at the time of this east-west alignment and marks the phase with a filled diamond.

North-south alignment

The second alignment is north to south. The Moon’s tilted orbit crosses the ecliptic at the ascending node ☊, where the Moon passes from south to north, and the descending node ☋, where it passes from north to south. Most New and Full Moons occur while the Moon is north or south of the ecliptic, so the relevant shadow misses its target. This is why not every New or Full Moon produces an eclipse. The Lunation Chart’s central Orbit Diagram shows the tilted orbit, ecliptic plane, and nodes directly.

South of the ecliptic. Even at New Moon, the Moon’s shadow misses Earth when the Moon is too far below the ecliptic.

At the node. The Moon reaches the ecliptic, removing the north-south separation.

North of the ecliptic. Past the crossing, the Moon’s shadow begins to miss Earth on the other side.

Two alignments

An eclipse requires both alignments to be close enough, but neither has to be exact. The Sun, Moon, Earth, and their shadows have apparent width, so a New or Full Moon can occur a little before or after a node and still produce an eclipse. A closer alignment generally produces a deeper eclipse; beyond the allowable range, the shadow misses. Here, the figure shows the clearest case: the Moon is at New Moon and at a node, so the two alignments coincide and the Moon’s shadow reaches Earth. In the Lunation Chart, the Moon Phases Band shows the phase alignment, the central Orbit Diagram shows the node alignment, and the eclipse Rare Event records the result in time.

Two rhythms

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. As the Moon moves, the two alignments slowly slip past one another.

The Year Chart’s Eclipse Band shows the same rhythms across a year. Its waveform traces the draconic cycle as the Moon moves north and south of the ecliptic. Dots and radial lines mark the New and Full Moons of the synodic cycle. A dot is circled when the two alignments fall close enough together to produce an eclipse.

Follow the two rhythms around the year. Most New and Full Moons pass well north or south of a node. When a phase alignment falls close enough to a crossing, its eclipse marker remains illuminated. The corresponding Solar Eclipse or Lunar Eclipse Rare Event appears with it, locating the same geometry in time.

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. Each highlighted span runs from the first eclipse in a season to the last, gathering the year’s five eclipses into two groups.

The Moon’s hourglass shadow

The Moon’s shadow has three regions. The dark umbra narrows behind the Moon toward a tip. Beyond that tip, the antumbra widens again. Together, they form an hourglass. The faint outer penumbra surrounds both central regions.

The hourglass does not meet Earth’s surface in the same way at every solar eclipse. Its shape and position vary, and Earth presents a curved surface for it to cross. These differences determine which region of the Moon’s shadow reaches the ground, and therefore what kind of eclipse occurs moment to moment over the duration of the event.

The umbra reaches Earth. When Earth’s surface cuts through the narrowing umbra, the result is a total solar eclipse. The Lunation and Year Charts represent it with a Solar Eclipse Rare Event marked TOT.

The antumbra reaches Earth. When Earth lies beyond the umbral tip and its surface cuts through the widening antumbra, the result is an annular solar eclipse. The charts represent it with a Solar Eclipse Rare Event marked ANN.

Only the penumbra reaches Earth. When the central hourglass misses Earth but the surrounding penumbra reaches some part of its surface, the result is a partial solar eclipse. Its Solar Eclipse Rare Event gives the maximum obscuration percentage. That percentage reports the greatest obscuration somewhere on Earth, not the maximum seen from the chart location.

What controls the meeting between Earth and the hourglass? The strongest influence is the distance between the Moon and Earth. As the Moon moves along its elliptical orbit, that changing distance determines where Earth’s surface cuts across the narrow waist of the hourglass.

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.

The distance between the Sun and the Moon also changes, slightly altering the angle at which the umbra narrows. The closely related change in the distance between Earth and the Sun appears as the Earth Perihelion or Aphelion Rare Event and has a smaller effect on the shape of the shadow. The exact place where the shadow crosses Earth’s curved surface can also affect which shadow region reaches the ground.

View from the ground

From the ground, the Sun and Moon appear as disks in the sky. How those disks overlap depends on which portion of the Moon’s shadow the viewer is in. The main figure locates the viewer within the shadow, while the inset magnifies the corresponding view of the Sun and Moon.

Inside the umbra. The Moon’s disk completely covers the Sun’s disk, and a total solar eclipse is visible.

Inside the antumbra. The Moon’s disk is centered on the Sun’s but does not cover it completely, leaving a bright ring. An annular solar eclipse is visible.

Inside the penumbra. The two disks are offset and overlap only partly, so a partial solar eclipse is visible.

Outside the penumbra. The disks do not overlap, so no eclipse is visible from this location even though one is occurring elsewhere on Earth.

If the eclipse can not be seen from the chart location, the Rare Event is marked ⊘. If it is visible, the Rare Event is marked ⏿, the duration of visibility is marked on the Gregorian Date Band, and a small diagram of the eclipse is included in the Lunation Chart’s Moon Faces Band. The small diagram for a solar eclipse is enclosed in a gold circle and shows the sun and moon at their relative sizes and positions. For a lunar eclipse it is enclosed in a blue circle and shows the moon tinted with Earth’s shadow.

Through Earth’s shadow

Around Full Moon, the alignment reverses: Earth lies between the Sun and Moon, casting its shadow toward the Moon. Earth’s shadow has two relevant regions, a faint outer penumbra and a dark inner umbra. A lunar eclipse unfolds as the Moon travels through these regions. The figure tightly frames the shadow near the Moon’s orbit; the inset magnifies the corresponding appearance of the lunar disk.

Inside the penumbra. Some sunlight still reaches every part of the Moon, so the dimming is subtle. If the Moon goes no deeper, the event is a penumbral lunar eclipse.

Entering the umbra. Earth’s dark shadow covers part of the lunar disk, producing the partial phase. If this is the deepest stage, the event is a partial lunar eclipse.

Inside the umbra. When the entire lunar disk is immersed in the umbra, the eclipse reaches totality and the event is a total lunar eclipse.

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.