Lunar Eclipse

A lunar eclipse is a celestial syzygy event that occurs when Earth passes directly between the Sun and the Moon during the full moon phase while the Moon is near one of its two orbital nodes (the intersections of the lunar orbital plane with Earth's ecliptic plane), causing the Moon to traverse Earth's cast shadow in space. Unlike a solar eclipse, which can only be viewed along a narrow ~100–250 km ground track swept by the Moon's small umbral shadow, a lunar eclipse is a physical darkening of the lunar surface itself and is simultaneously visible from the entire nightside hemisphere of Earth—anywhere the Moon is above the local horizon.

Because Earth's equatorial diameter (D_⊕ = 12,756 km) is 3.67 times larger than the Moon's diameter (D_☾ = 3,474.8 km), Earth casts a wide inner shadow cone (umbra) that remains roughly 2.6 times wider than the Moon (~9,200 km across) at the Moon's mean orbital distance of 384,400 km. Consequently, a total lunar eclipse can sustain totality for up to 107 minutes (1 hour 47 minutes), during which the Moon does not vanish into blackness but instead glows a deep coppery crimson—colloquially termed a "Blood Moon"—illuminated exclusively by long-wavelength red sunlight refracted through Earth's terminator atmosphere after short-wavelength blue light has been removed by Rayleigh scattering.

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Why Doesn't a Lunar Eclipse Occur at Every Full Moon?

The Moon completes one synodic phase cycle (new moon to full moon to new moon) every 29.53059 days (the synodic month). However, the Moon's orbital plane is inclined at a mean angle of i ≈ 5.145° relative to Earth's orbital plane around the Sun (the ecliptic). At the Moon's mean distance (r = 384,400 km), a 5.145° tilt carries the full Moon up to r sin(5.145°) ≈ 34,470 km north or south of the ecliptic axis—passing well above or below Earth's ~9,200 km umbral shadow during most months.

A lunar eclipse can occur only during an eclipse season (recurring every 173.31 days, half of an eclipse year of 346.62 days), when the line of ascending and descending orbital nodes aligns sufficiently close to the Sun–Earth axis. Specifically, at full moon the Moon's ecliptic latitude β must be less than ~1.5°, corresponding to an angular distance from the nearest node (the lunar ecliptic limit) of ≤ 12.2° for a penumbral eclipse, ≤ 9.5° to 11.0° for a partial umbral eclipse, and ≤ 4.6° to 6.0° for a total lunar eclipse.

Umbra, Penumbra, and the Three Types of Lunar Eclipses

Because the Sun is an extended disk of angular diameter θ_☉ ≈ 0.533° (32 arcminutes) rather than a point source, Earth's shadow divides into two concentric geometric cones:

1. The Penumbra (Outer Diverging Cone): The region of space where Earth blocks only a fraction of the solar disk. At the Moon's distance, the penumbra has a diameter of ~16,000 km (~2.35° angular radius).
2. The Umbra (Inner Converging Cone): The dark central cone where Earth completely ocults the photosphere of the Sun. Extending ~1.38 million km antisunward of Earth, the umbra has a diameter of ~9,000–9,400 km (~0.68°–0.76° angular radius) at the lunar orbit.

*(Here gamma (γ) is the minimum perpendicular distance from the center of Earth's shadow axis to the center of the Moon at greatest eclipse, expressed in units of Earth's equatorial radius R_⊕).*

Physics of the "Blood Moon" Spectrum

If Earth lacked an atmosphere, the Moon would turn completely black during totality (U₂ to U₃), illuminated only by the faint solar corona. Instead, sunlight grazing Earth's limb around the entire 40,000-kilometer ring of simultaneous sunrises and sunsets (the terminator) passes through the troposphere and stratosphere along a long tangential optical path length (airmass ~35–40 at sea level).

Two optical processes act on this grazing sunlight:

1. Differential Atmospheric Refraction: Density gradients in Earth's lower atmosphere act as a giant spherical prism, bending grazing solar rays inward toward the shadow axis by a deflection angle of 2*ω* ≈ 1.1° to 1.4°, directing residual light directly into the geometrical umbra at the Moon's distance.
2. Rayleigh Scattering (σ_R ∝ λ⁻⁴): Atmospheric N₂ and O₂ molecules scatter light with a cross-section inversely proportional to the fourth power of wavelength:

σ_R(λ) = [ 8*π*³ (n² − 1)² ] / [ 3 N² λ⁴ ]

Because violet and blue photons (λ ≈ 400–480 nm) are scattered out of the direct beam 5 to 9 times more strongly than deep red photons (λ ≈ 650–700 nm), the transmitted sunset-ring light reaching the lunar regolith is strongly depleted below 550 nm and peaks in the red/near-infrared, casting a reddish-copper glow across the eclipsed Moon (while a narrow turquoise-blue rim is sometimes visible along the outer edge of the umbra due to Chappuis-band ozone absorption of 500–700 nm light in the high stratosphere at 20–40 km altitude).

The Danjon Scale (L = 0 to 4) and Volcanic Aerosol Diagnostics

Introduced by French astronomer André Danjon (1921), the Danjon scale grades the visual brightness and color of the Moon at mid-totality from L = 0 to L = 4:

• L = 0 (Very Dark Eclipse): Moon is nearly invisible, charcoal-gray to brownish-black, especially at mid-totality. Occurs after major stratospheric volcanic eruptions (such as Mount Agung 1963, El Chichón 1982, and Mount Pinatubo 1991), when global stratospheric sulfate aerosol layers at 18–28 km altitude block the lower-stratospheric refraction window.
• L = 1 (Dark Eclipse): Dark gray or dull brownish coloration; lunar maria details are difficult to distinguish.
• L = 2 (Deep Red or Rust Eclipse): Dark red or rust-colored central umbra with a brighter yellowish-orange outer umbral rim.
• L = 3 (Brick-Red Eclipse): Bright crimson or brick-red umbra bordered by a yellow or bluish-white outer margin (typical of clean stratospheric conditions when the Moon passes moderately off-center through the umbra).
• L = 4 (Copper-Red or Bright Orange Eclipse): Brilliant copper-red or luminous orange eclipse with a bright turquoise-tinted outer zone (typical when the Moon grazes the northern or southern outer region of the umbra during low-aerosol periods).

Eclipses repeat with remarkable geometric fidelity according to the ancient Chaldean/Babylonian Saros cycle of 6,585.3211 days (18 years, 11 days, and ~8 hours, or 18 years 10 days if five leap years intervene). This interval arises from the near-exact harmonic commensurability of three lunar orbital periods:

• 223 synodic months (phase cycle: 223 × 29.53059 d = 6,585.321 d)
• 242 draconic months (node-crossing cycle: 242 × 27.21222 d = 6,585.357 d)
• 239 anomalistic months (perigee-to-perigee distance cycle: 239 × 27.55455 d = 6,585.537 d)

Because 223 synodic months, 242 draconic months, and 239 anomalistic months agree to within ~2 to 5 hours, two eclipses separated by one Saros share nearly identical Sun–Earth–Moon distances, shadow diameters, and node geometries. The residual 0.3211-day (~8-hour) fraction rotates Earth through ~120° of longitude between successive eclipses in a Saros series, shifting the visibility hemisphere westward by one-third of the globe at each 18-year return and returning to the same geographic longitudes every three Saros cycles (1 Exeligmos = 54 years, 33 days).

Why is it completely safe to view a lunar eclipse with the naked eye or binoculars, unlike a solar eclipse?

During a lunar eclipse, you are viewing reflected sunlight from the lunar regolith—which at full moon is already ~400,000 times fainter than direct sunlight (visual magnitude −12.7 vs. −26.74 for the Sun) and drops by another factor of 10,000 during totality. No optical filters are ever required to observe any phase of a lunar eclipse with the unaided eye, binoculars, or a telescope.

How did ancient Greek astronomers use lunar eclipses to prove Earth is a sphere and measure the Moon's distance?

In On the Heavens (De Caelo, ~350 BCE), Aristotle noted that during every partial lunar eclipse—regardless of the Moon's altitude above the horizon—the edge of Earth's umbral shadow cast across the Moon is always a circular arc, and a sphere is the only three-dimensional solid that casts a circular shadow from every orientation. Around 270 BCE, Aristarchus of Samos (and later Hipparchus in the 2nd century BCE) timed how long the Moon took to cross Earth's umbral shadow at orbital speed, using the shadow-to-Moon diameter ratio (~2.6:1) and solar/lunar angular diameters (~0.5°) to deduce that the Moon lies roughly 59 to 67 Earth radii away (remarkably close to the modern mean value of 60.27 R_⊕).

What would an astronaut standing on the Moon see during a total lunar eclipse?

An astronaut on the nearside of the Moon during a total lunar eclipse would witness a total solar eclipse by Earth: the dark nightside disk of Earth (appearing nearly 4 times wider in angular diameter than the Sun) would completely cover the Sun and its inner corona, encircled by a brilliant, thin 360-degree crimson ring of fire—the simultaneous glow of every sunrise and sunset occurring around Earth's atmospheric limb. Meanwhile, the lunar surface temperature around the astronaut would plummet by more than 150 to 200 K within two hours as the low-thermal-inertia regolith dust radiates heat into space.

1. Danjon, A. (1921). "Relation entre l'éclat de la Lune éclipsée et l'activité solaire." Comptes Rendus de l'Académie des Sciences, 171, 1127–1129. https://gallica.bnf.fr/ark:/12148/bpt6k3124c/f1127.item
2. Link, F. (1969). Eclipse Phenomena in Astronomy. Springer-Verlag. https://doi.org/10.1007/978-3-642-86475-9
3. Espenak, F., & Meeus, J. (2009). Five Millennium Canon of Lunar Eclipses: −1999 to +3000 (2000 BCE to 3000 CE). NASA Technical Publication TP-2009-214172. https://eclipse.gsfc.nasa.gov/SEpubs/5MCLE.html
4. García Muñoz, A., & Pallé, E. (2011). "Lunar Eclipse Theory Revisited: Refracted Sunlight, Atmospheric Transmission, and Ozone Absorption." Journal of Quantitative Spectroscopy and Radiative Transfer, 112(10), 1609–1621. https://doi.org/10.1016/j.jqsrt.2011.03.017