What is a Lunar Eclipse?
On January 31, 2018, millions of people stayed up late or set early alarms for something that hadn't been seen in over 150 years: a Super Blue Blood Moon. The name is wonderfully dramatic, combining three separate lunar phenomena into one rare event. But to understand what actually happened that night — and why it was genuinely remarkable — we need to understand the geometry of the Earth-Moon-Sun system and the physics of light bending through our atmosphere.
Why Don't Eclipses Happen Every Month?
The Moon orbits Earth roughly every 27.3 days. So why isn't there a lunar eclipse every month? The answer is geometry.
Earth orbits the Sun in a plane called the ecliptic. If the Moon orbited in the exact same plane, we'd get a lunar eclipse every full moon and a solar eclipse every new moon. But the Moon's orbital plane is tilted by about 5.14° relative to the ecliptic. Most of the time, the Moon passes above or below Earth's shadow at full moon.
Eclipses only occur when the Moon crosses the nodes — the two points where its orbit intersects the ecliptic — near the time of a full moon. These alignments happen roughly twice a year, but not every full moon at a node produces a total eclipse.
Earth's Shadow: Umbra and Penumbra
When the Sun shines on Earth, it creates two distinct shadow regions:
The umbra is the region of total shadow, where Earth completely blocks all direct sunlight. It's shaped like a cone, narrowing away from Earth — at the Moon's average distance (384,400 km), the umbra is about 9,000 km in diameter.
The penumbra is the region of partial shadow surrounding the umbra. In the penumbra, Earth blocks only some of the Sun's disk, so reduced sunlight still reaches this zone.
Three Types of Lunar Eclipses
Penumbral Eclipse
The Moon passes through Earth's penumbra but misses the umbra entirely. The dimming is subtle — usually less than 10% — and often invisible to the naked eye. These are the most common type.
Partial Eclipse
Part of the Moon enters the umbra while the rest remains in the penumbra or direct sunlight. The boundary between the lit and shadowed portions is curved — and the curvature was recognized by ancient Greek philosophers as evidence that Earth is spherical. These occur once or twice per year on average.
Total Eclipse
The entire Moon passes through the umbra. This is when things get spectacular.
The Blood Moon: Rayleigh Scattering at Work
During a total eclipse, the Moon doesn't disappear. It turns deep red — sometimes bright orange, sometimes so dark it nearly vanishes. This is the Blood Moon, and its cause is the same physics that makes sunsets red.
Earth's atmosphere acts as a lens, bending light around the planet. Not all wavelengths bend equally: blue light has a shorter wavelength and is scattered far more strongly than red light. This is Rayleigh scattering, and the scattering intensity follows:
Blue light ( nm) scatters roughly 5.5 times more strongly than red light ( nm). All that scattered blue light goes sideways — contributing to the blue sky during the day. The red light passes through, bends around Earth's edge, and continues into the umbra — illuminating the Moon with a deep red glow.
Put differently: every lunar eclipse is a simultaneous sunset and sunrise happening all around Earth, and the Moon is lit by the collective red glow of all of those sunsets and sunrises at once.
The exact color depends on how much dust, cloud, and aerosol is currently in Earth's atmosphere. After major volcanic eruptions (which inject enormous amounts of aerosols into the stratosphere), total lunar eclipses can be so dark the Moon nearly vanishes.
What is a Supermoon?
The Moon's orbit around Earth is not a perfect circle — it's an ellipse. This means the Moon's distance varies between extremes:
| Point | Name | Distance |
|---|---|---|
| Closest | Perigee | ~356,700 km |
| Farthest | Apogee | ~406,300 km |
A supermoon is a full moon occurring near perigee. At its closest, the Moon appears about 14% larger and 30% brighter than at apogee. The difference is subtle but noticeable to the careful observer. The term "supermoon" has no strict astronomical definition — it's a popular term for a full moon that appears unusually large.
What is a Blue Moon?
A blue moon has nothing to do with its color. It's the second full moon in a single calendar month — a consequence of the slight mismatch between the Moon's 29.5-day synodic period and calendar months of 28–31 days. This happens roughly once every 2.5 years, giving rise to the phrase "once in a blue moon" for something rare.
An older definition — the third full moon in a season that has four full moons — is still used by some.
The January 2018 Event
The January 31, 2018 event combined all three:
- Supermoon: The Moon was near perigee, appearing noticeably larger
- Blue Moon: It was the second full moon of January
- Blood Moon: A total lunar eclipse turned the Moon a deep coppery red
The last time all three coincided was March 31, 1866 — over 150 years earlier.
Future Eclipses
Key dates for notable lunar eclipses:
| Event | Date |
|---|---|
| Next Blue Blood Moon | December 31, 2028 |
| Next Super Blue Blood Moon | January 31, 2037 |
| Next total lunar eclipse visible from North America | March 3, 2026 |
The Saros Cycle
Lunar (and solar) eclipses repeat in an 18-year, 11-day, 8-hour cycle called the Saros cycle. This cycle was known to ancient Babylonian astronomers, who used it to predict eclipses centuries in advance without understanding the physics behind it. One Saros period is the interval after which the Earth-Moon-Sun geometry nearly exactly repeats, producing an eclipse of the same type and similar duration.
Ancient eclipses as a clock: The Saros cycle is so precise that historians and astronomers can use recorded ancient eclipse observations — from Babylonian clay tablets, Chinese chronicles, and Greek texts — to calculate the long-term deceleration of Earth's rotation. The Moon's tidal friction is slowing our planet by about 1.4 milliseconds per century, and old eclipse records let us measure this over millennia.