The sky — astronomy

Lunar Eclipse and the “Blood Moon”

A lunar eclipse happens when the full Moon passes into Earth’s shadow. The red colour nicknamed the “Blood Moon” comes from Earth’s atmosphere, which scatters away blue light and bends red light onto the Moon. It is completely safe to watch with the naked eye.

Earth’s shadow: penumbra and umbra

A lunar eclipse is a line-up of three bodies: the Sun, Earth and the Moon, arranged almost in a straight line. Earth is lit by the Sun and casts a long shadow into space. When the Moon travels through that shadow, it loses some or all of its direct sunlight. That is why lunar eclipses happen only at full Moon (see the lunar phases).

Earth’s shadow has two parts:

• Penumbra (the outer, lighter region): Earth blocks only part of the Sun’s disk. The Moon dims slightly, often too subtly to notice.
• Umbra (the inner, dark core): Earth blocks the whole Sun. When the Moon enters it, the eclipse becomes obvious.

Why isn’t there an eclipse every full Moon? The Moon’s orbit is tilted by about 5° relative to the plane of Earth’s orbit around the Sun (the ecliptic). Most months the full Moon passes above or below Earth’s shadow. An eclipse needs the full Moon to coincide with the crossing of the two planes.

A lunar eclipse can be seen from the entire night side of Earth. That makes it far more widely accessible than a solar eclipse, which is visible only along a narrow path.

Why the Moon turns red

Many people expect the Moon to vanish in the shadow. Usually it stays visible, glowing anywhere from orange to deep red. The reason is Earth’s atmosphere.

Sunlight grazing the edge of our planet passes through the air and is refracted, bent slightly inward toward the shadow. At the same time, air scatters short-wavelength blue light far more strongly than long-wavelength red light. This is called Rayleigh scattering, the same effect that makes the daytime sky blue and sunsets red. Blue light is filtered out along the way, while red and orange light reaches the Moon.

Imagine standing on the Moon during the eclipse: you would see a dark Earth surrounded by a glowing red ring. That ring is all of Earth’s sunrises and sunsets at once, and it is this light that illuminates the lunar surface.

The exact shade differs from eclipse to eclipse. It depends on the state of the atmosphere, including clouds and the amount of dust or volcanic ash in the upper layers. After major volcanic eruptions, the eclipsed Moon can look much darker. Astronomers use the Danjon scale, with levels from 0 to 4, to describe how dark or bright an eclipse appears, from very dark to bright copper-red.

The phrase “Blood Moon” is not a scientific term. It is a popular, poetic nickname for the red Moon of a total lunar eclipse. It is unrelated to a supermoon or a blue moon, unless they happen to coincide in time. These are different phenomena.

Types of lunar eclipses

Depending on how the Moon crosses the shadow, there are three types:

1. Penumbral eclipse. The Moon passes only through the penumbra. The dimming is faint and easy to miss.
2. Partial eclipse. Part of the Moon enters the umbra. You see a dark “bite” on the disk while the rest stays bright. The curved edge of that shadow is a classic demonstration that Earth is a sphere.
3. Total eclipse. The whole Moon enters the umbra. This stage is called totality and can last up to a little under two hours. The whole event, from the first touch of the penumbra to its end, takes several hours.

During totality the Moon looks its most striking: red, often with a brighter rim on one side, where it lies nearer the edge of the shadow.

How to watch a lunar eclipse

A lunar eclipse is among the easiest astronomical events to enjoy:

• No special glasses needed. The Moon reflects only weak light, so it is safe to view with the naked eye, binoculars or a telescope. This is a crucial difference from a solar eclipse, which requires proper eye protection (except during the brief totality of a total solar eclipse).
• Find a dark spot with a clear view toward where the Moon will be. City lights matter less than for faint objects, but a darker sky reveals more stars around the red Moon.
• Use binoculars. Even a simple pair shows more subtle colour shades across the disk. For help choosing, see binoculars or telescope.
• Check timing and visibility for your location. Not every eclipse is visible everywhere, because the Moon must be above your horizon. NASA and other scientific institutions publish eclipse calendars.
• Be patient. The eclipse unfolds slowly. Bring warm clothes and look up at intervals. The colour is usually richest in the middle of totality.

General tips for night observing are in how to watch the sky, and the Moon has its own article. To see how science differs from cultural interpretations of sky events, read about the difference between astronomy and astrology.

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