Lunar Eclipse

Earth's Shadow: Umbra, Penumbra, and Geometry

The Sun is not a point source of light but a disk about half a degree across as seen from Earth. This finite angular size means Earth's shadow is not a simple cylinder but a system of nested cones. Understanding those cones is the key to understanding every type of lunar eclipse.

The umbra is the inner, darkest cone. Rays traced from opposite edges of the Sun to opposite edges of Earth converge to a point behind Earth — the umbral tip — defining a region where Earth completely blocks the Sun's disk. Any part of the Moon within the umbra receives no direct sunlight at all. The umbra is a converging cone: it narrows with distance from Earth.

The penumbra is the outer, lighter region that surrounds the umbra. Rays from opposite edges of the Sun to the same edge of Earth define a diverging cone where the Sun is only partially obscured by Earth. A point on the Moon inside the penumbra still receives some direct sunlight, so the Moon dims there but does not go dark.

At the Moon's average distance from Earth, the umbra's diameter is a little less than three times the Moon's diameter, while the penumbral annulus (the ring of penumbra around the umbra) is roughly one lunar diameter wide. This geometry is why the Moon can fit entirely within the umbra during a total eclipse and still be surrounded by penumbra at the same time.

A lunar eclipse can occur only at full Moon, when Earth lies roughly between the Sun and Moon. However, the Moon's orbit is tilted by about 5° to the ecliptic — Earth's orbital plane — so most full Moons pass above or below Earth's shadow entirely. An eclipse happens only when the full Moon is close to one of the two lunar nodes, the points where the Moon's orbit crosses the ecliptic. These favorable alignments recur in eclipse seasons spaced roughly 173 days apart.

Types of Lunar Eclipse

Three main types of lunar eclipse are defined by how far the Moon penetrates into Earth's shadow system, with a notable subtype within the total category.

A penumbral lunar eclipse occurs when the Moon passes only through Earth's penumbra, never entering the umbra. From any point on the Moon's visible face, Earth only partially blocks the Sun, so the Moon receives reduced but not zero sunlight. The visible effect on Earth is a subtle overall dimming of the lunar disk, often so faint that casual observers do not notice it. If the Moon lies entirely inside the penumbra at maximum, the event is called a total penumbral eclipse — a rare subtype.

A partial lunar eclipse occurs when part of the Moon dips into the umbra while the rest remains in the penumbra or in full sunlight. The umbra-covered portion appears distinctly and strikingly darker than the rest of the disk, as though a dark curved bite has been taken out of the Moon. The sharpness of this boundary — the clear contrast between umbral dark and penumbral dim — is one of the most visually striking aspects of a partial eclipse.

A total lunar eclipse occurs when the entire Moon passes within Earth's umbra at mid-eclipse. The sequence has five distinct stages: penumbral ingress, when the Moon first touches the penumbra; umbral ingress (partial eclipse begins), when the Moon's limb first enters the umbra; totality, when the Moon is completely immersed in the umbra; umbral egress (partial eclipse ends), as the Moon exits the umbra; and penumbral egress, as it clears the penumbra entirely.

Within the total category, a central lunar eclipse is one where the Moon passes close to the geometric center of Earth's umbra — near the antisolar point, the point directly opposite the Sun. Central eclipses tend to have the longest durations of totality and the deepest, darkest coloration. They occur in approximately 59.6% of all total lunar eclipses.

Why the Moon Turns Red: The Blood Moon Explained

During a total lunar eclipse, the Moon does not go black. Instead it glows in shades of copper, orange, or deep red — a phenomenon widely called a "blood moon." The explanation lies in Earth's atmosphere acting as a giant lens and color filter simultaneously.

If Earth had no atmosphere, the umbra would be a region of near-perfect darkness and the eclipsed Moon would be almost invisible to the naked eye. In reality, Earth's atmosphere refracts (bends) sunlight around the curve of Earth and redirects it into the umbral cone. As this light travels through a long, dense path of air at Earth's limb, shorter wavelengths — blue and violet — are strongly scattered away by Rayleigh scattering, the same process that makes the daytime sky blue. The longer wavelengths — red and orange — are scattered far less and thread through the atmosphere to illuminate the Moon from within the umbra.

The result is that an observer on the Moon during totality would see a thin ring of vivid red and orange light surrounding a dark Earth — all of Earth's sunrises and sunsets, projected simultaneously onto the Moon's surface. The same light, viewed from Earth, gives the Moon its characteristic blood-red hue.

The exact color and brightness vary considerably from one eclipse to the next, controlled by the state of Earth's atmosphere at the time. A clear, aerosol-free atmosphere produces a brighter, more orange eclipsed Moon. After major volcanic eruptions, when fine ash and sulfur aerosols load the stratosphere, the Moon can turn a dark brownish-gray and nearly disappear at totality. Because atmospheric conditions are never identical, no two total lunar eclipses look exactly alike in color or brightness.

"Blood moon" is a popular term, not a formal astronomical designation. Astronomers describe such events simply as total lunar eclipses and characterize their appearance qualitatively or using the Danjon scale of lunar eclipse brightness. Some writers also use "blood moon" in reference to tetrads — sequences of four consecutive total lunar eclipses — sometimes with biblical or prophetic associations, but this usage is not standard in astronomy.

Duration, Frequency, and the Saros Cycle

Lunar eclipses of all types occur on average about three times per year worldwide, constrained by orbital geometry to intervals of 1, 5, or 6 synodic months between successive events. Approximately 29% of all lunar eclipses are total. In the 21st century (2001–2100), there are 228 lunar eclipses: 86 penumbral, 57 partial, and 85 total — about 0.85 total lunar eclipses per year. Any given location on Earth, assuming clear skies, can expect to see a total lunar eclipse on average roughly once every 2.5 years.

The duration of totality — the interval during which the Moon lies entirely within the umbra — depends on how centrally the Moon crosses the shadow and how fast it is moving. A Moon near apogee (farthest from Earth) moves more slowly and subtends a smaller angle, both of which favor longer totalities. A Moon passing through the center of the umbra travels the maximum chord length. The theoretical maximum totality is close to 107 minutes. The longest measured totality of the 20th century occurred on 16 July 2000 at approximately 1 hour 46.4 minutes; the longest of the 21st century so far was 27–28 July 2018, at 1 hour 42 minutes 57 seconds. At the opposite extreme, the shortest totality of the 21st century was 4 April 2015, at about 4 minutes 48 seconds — the Moon barely grazed the edge of the umbral cone. The historical record appears to include extremes as short as roughly 1 minute 42 seconds, computed for 17 October 1529.

Eclipse recurrence is governed by the Saros cycle, a period of approximately 18 years 11⅓ days (about 6,585.32 days). After one Saros, the geometry of the Sun–Earth–Moon system at full Moon nearly repeats because three lunar periods come back into close alignment: 223 synodic months, 242 draconic months, and 239 anomalistic months all equal roughly 6,585.32 days. The result is a nearly identical eclipse — same type, similar magnitude, similar position of the Moon within Earth's shadow.

Every lunar eclipse belongs to a numbered Saros series. Each series contains roughly 70–80 eclipses and spans about 12–13 centuries from its first member to its last. Modern catalogs such as NASA's Five Millennium Catalog of Lunar Eclipses list the Saros number for each eclipse, linking every event to its ancient and future cousins.

History of Observation and Prediction

Systematic observation of lunar eclipses began in Mesopotamia no later than the 8th century BCE. One of the earliest securely dated records is a partial lunar eclipse on 6 February 747 BCE, in the first year of the reign of Babylonian king Nabonassar. This epoch was later chosen by Ptolemy as the starting point for his astronomical calculations, a sign of how central these records were to ancient science. Babylonian astronomical diaries from the 8th through 1st centuries BCE contain systematic eclipse records giving dates, times, and qualitative descriptions such as "total" or "north part dark."

Chinese chronicles are also among the oldest sources. The Zhou-Shu (Book of Zhou), reportedly recovered from a tomb in 280 CE, contains a lunar eclipse reference associated with 29 January 1137 BCE, making it one of the oldest known eclipse mentions in any written record. Subsequent Chinese dynasties maintained eclipse records that were both astronomical observations and political omens, since an eclipse was often read as a heavenly warning about imperial conduct.

In the Greek world, lunar eclipses appear as both historical anchors and literary references. A total lunar eclipse on 28 August 413 BCE occurred during the Athenian siege of Syracuse; the Athenian commander Nicias, unnerved by what he took as a bad omen, delayed the fleet's departure — a delay that led to the destruction of the expedition. This eclipse is now used by historians to anchor the chronology of the Sicilian Expedition. A total lunar eclipse before the Battle of Gaugamela on 20 September 331 BCE was recorded as the Moon being "suffused with the hue of blood," and it helps confirm the dating of Alexander the Great's campaign. The playwright Aristophanes alluded to a lunar eclipse in his comedy The Clouds, while the Fall of Constantinople in 1453 was preceded by a partial lunar eclipse interpreted by Byzantine chroniclers as the fulfillment of prophecy.

By the time of Aristotle, Hipparchus, and Ptolemy, Greek astronomers had arrived at a correct geometric model: a lunar eclipse occurs when Earth passes between the Sun and Moon at full Moon and the Moon lies near one of its orbital nodes, explaining why eclipses do not happen every month. Combined with Babylonian numerical records, this understanding enabled increasingly accurate eclipse prediction. The Babylonians themselves had already empirically discovered the 18-year pattern — what we now call the Saros — by the 7th–5th centuries BCE, as evidenced by cuneiform texts listing eclipse intervals of 223 months. The word "Saros" in its modern sense was popularized by the astronomer Edmund Halley, but the underlying cycle was an ancient Babylonian discovery.