Lunar Eclipse
A lunar eclipse occurs when the Earth passes directly between the Sun and the Moon, casting Earth's shadow on the Moon's surface. Unlike a solar eclipse, a lunar eclipse is visible from the entire night side of Earth simultaneously. The Moon can turn a dramatic blood-red colour during totality — an effect produced by Earth's atmosphere refracting red sunlight around the planet onto the Moon's surface.
How a Lunar Eclipse Occurs
The Moon orbits Earth every 27.3 days (sidereal month) and reaches Full Moon phase every 29.5 days (synodic month). A lunar eclipse can only happen at Full Moon — when the Moon is on the opposite side of Earth from the Sun. However, because the Moon's orbital plane is tilted 5.1° relative to the ecliptic, most Full Moons pass above or below Earth's shadow. An eclipse only occurs when the Full Moon is close to an orbital node — where the Moon's orbit crosses the ecliptic plane.
Earth's Shadow: Umbra and Penumbra
| Shadow Zone | Description | Effect on Moon | Radius at Moon's Distance |
|---|---|---|---|
| Umbra | Full shadow — no direct sunlight | Moon darkens dramatically, turns red/orange | ≈ 4,600 km |
| Penumbra | Partial shadow — some sunlight blocked | Subtle dimming, barely visible | ≈ 13,400 km |
Earth's umbra is about 2.65× the Moon's diameter (Moon radius = 1,737 km), so the Moon can fit completely inside the umbra — producing a total lunar eclipse. This is very different from solar eclipses, where the Moon's shadow is only ~100–270 km wide on Earth.
Types of Lunar Eclipse
Total Lunar Eclipse
The entire Moon passes through Earth's umbra. The Moon turns blood red or orange (Blood Moon) due to atmospheric refraction of red wavelengths. Totality can last up to 1 hour 47 minutes.
Partial Lunar Eclipse
Only part of the Moon enters Earth's umbra. One section remains brightly lit while another turns dark and reddish. The exact appearance depends on how deeply the Moon enters the shadow.
Penumbral Lunar Eclipse
The Moon passes only through Earth's outer penumbra. The dimming is so subtle (< 10% brightness reduction) it is often undetectable to the naked eye. No red colour appears.
Why Does the Moon Turn Red? (Blood Moon)
During totality, the Moon is not completely dark. Earth's atmosphere acts like a lens, refracting sunlight around the planet. Blue and violet wavelengths are scattered away by atmospheric particles (Rayleigh scattering), while longer red and orange wavelengths bend through the atmosphere and reach the Moon. The Moon therefore glows with the combined light of all Earth's sunrises and sunsets projected simultaneously onto its surface.
The Danjon Scale
| Scale (L) | Appearance | Atmospheric Conditions |
|---|---|---|
| L = 0 | Very dark eclipse — Moon almost invisible | Heavy volcanic dust / thick clouds |
| L = 1 | Dark grey or brownish | Moderately turbid atmosphere |
| L = 2 | Deep red or rust coloured, dark centre | Average conditions |
| L = 3 | Brick red, bright yellow rim | Clear atmosphere |
| L = 4 | Bright orange or copper, bluish outer rim | Very clean, transparent atmosphere |
Key Formulas
Solved Examples
Example 1 — Calculate Umbra Radius at Moon's Distance
Given: R⊕ = 6,371 km, R☀ = 696,000 km, dEM = 384,400 km, dES = 149,600,000 km
Step 1: rU = 6,371 − (6,371 − 696,000) × (384,400 / 149,600,000)
Step 2: rU = 6,371 + 689,629 × 0.002569 = 6,371 + 1,771 = 4,600 km
Result: Earth's umbra at Moon's distance has radius ≈ 4,600 km — about 2.65× the Moon's radius (1,737 km), confirming a total eclipse is possible.
Example 2 — Maximum Duration of Totality
Given: Moon's orbital speed ≈ 1.023 km/s, umbra diameter ≈ 9,200 km
Step 1: Time = diameter / speed = 9,200 / 1.023 ≈ 8,993 s
Result: Maximum possible totality ≈ 107 minutes (actual record: 106 min 23 s on 16 July 2000, when Moon near apogee — slower orbital speed)
Practice Questions
- Why does a lunar eclipse last much longer than a total solar eclipse?
- Calculate the penumbra radius at the Moon's distance using the formula above.
- Why doesn't a lunar eclipse happen every Full Moon?
- A lunar eclipse has umbral magnitude U = 1.45. What type of eclipse is it, and what percentage of the Moon is inside the umbra?
- During a total lunar eclipse, an astronaut on the Moon would see: (a) a solar eclipse, (b) a lunar eclipse, (c) the Sun behind Earth. Explain.
- Why does the Moon appear brighter orange at L = 4 than at L = 0 on the Danjon scale?
Frequently Asked Questions
Is a lunar eclipse dangerous to look at?
No — unlike a solar eclipse, a lunar eclipse is completely safe to observe with the naked eye, binoculars, or a telescope. The Moon only reflects sunlight that has passed through Earth's atmosphere; it is not bright enough to damage your eyes.
How often do lunar eclipses occur?
About 1.5 lunar eclipses per year on average (total + partial + penumbral). Total lunar eclipses occur roughly once every 1.5 years. Because they are visible from the entire night side of Earth, any given location sees them far more frequently than solar eclipses.
Why is it called a "Blood Moon"?
The popular name refers to the deep red or copper colour the Moon takes during totality. Earth's atmosphere scatters blue light and refracts (bends) red/orange wavelengths around the planet. These red rays illuminate the Moon — effectively projecting all of Earth's sunrises and sunsets onto the lunar surface simultaneously.
Can a lunar eclipse occur without a full moon?
No. A lunar eclipse requires the Moon to be on the opposite side of Earth from the Sun, which is by definition Full Moon phase. However, not every Full Moon produces an eclipse because the Moon's orbit is tilted 5.1° relative to the ecliptic.
What would a lunar eclipse look like from the Moon?
An astronaut on the Moon would see a total solar eclipse — Earth blocking the Sun. They would see Earth's atmosphere glowing with a brilliant red-orange ring (all the sunrises and sunsets simultaneously), which is precisely the light that gives the Moon its blood-red colour during totality.