Lunar Eclipse and the Blood Moon

What Is a Lunar Eclipse?

A blood moon is one name for the reddish moon viewed during a total lunar eclipse.
A blood moon is one name for the reddish moon viewed during a total lunar eclipse. a. v. ley / Getty Images

A lunar eclipse is the eclipse of the Moon, which occurs when the Moon is directly between the Earth and its shadow or umbra. Because the Sun, Earth, and Moon have to be aligned (in syzgy) with the Earth between the Sun and Moon, a lunar eclipse only occurs during a full moon. How long an eclipse lasts and the type of eclipse (how full it is) depends on where the Moon is in relation to its orbital nodes (points where the Moon crosses the ecliptic). The Moon has to be near a node for any visible eclipse to occur. Although the Sun can appear completely blotted out during a total solar eclipse, the Moon remains visible throughout a lunar eclipse because sunlight is refracted by the Earth's atmosphere to light the Moon. In other words, the Earth's shadow on the Moon is never completely dark.

How a Lunar Eclipse Works

A diagram illustrating how eclipses are created.
A diagram illustrating how eclipses are created. Ron Miller/Stocktrek Images / Getty Images

A lunar eclipse occurs when the Earth is directly between the Sun and the Moon. The shadow of the Earth falls across the face of the Moon. The type of lunar eclipse depends on how much of the Earth's shadow covers the Moon.

The Earth's shadow consists of two parts. The umbra is the portion of the shadow that has no solar radiation and is dark. The penumbra is dim, but not completely dark. The penumbra gets light because the Sun has such a large angular size the sunlight isn't totally blocked. Instead, light is refracted. In a lunar eclipse, the color of the Moon (refracted light) depends on the alignment between the Sun, Earth, and Moon.

Types of Lunar Eclipses

Penumbral Eclipse - A penumbral eclipse occurs when the Moon passes through the Earth's penumbral shadow. During this type of lunar eclipse, the portion of the Moon that is eclipsed appears darker than the rest of the Moon. In a total penumbral eclipse, the full moon is entirely shadowed by the Earth's penumbra. The Moon dims, but it's still visible. The Moon may appear gray or golden and may almost completely disappear at totality. In this type of eclipse, the dimming of the Moon is directly proportional to the area of sunlight blocked by the Earth. A total penumbral eclipse is rare. Partial penumbral eclipses occur more often, but they tend not to be very well publicized because they are difficult to see.

Partial Lunar Eclipse - When part of the moon enter the umbra, a partial lunar eclipse occurs. The part of the Moon falling within the umbral shadow dims, but the rest of the Moon remains bright.

Total Lunar Eclipse - Generally when people talk about a total lunar eclipse, they mean the type of eclipse where the Moon travels fully into the Earth's umbra. This type of lunar eclipse occurs about 35% of the time. How long the eclipse lasts depends on how close the Moon is to the Earth. The eclipse lasts longest when the Moon is at its furthest point or apogee. The color of the eclipse can vary. A total penumbral eclipse can precede or follow a total umbral eclipse.

Danjon Scale for Lunar Eclipses

All lunar eclipses don't look the same! Andre Danjon proposed the Danjon scale to describe the appearance of a lunar eclipse:

L = 0: Dark lunar eclipse where the Moon becomes nearly invisible at totality. When people imagine what a lunar eclipse looks like, this is probably what they envision.

L = 1: Dark eclipse in which the details of Moon are hard to distinguish and the Moon appears brown or gray at totality.

L = 2: Deep red or rusty eclipse at totality, with a dark central shadow but a bright outer edge. The Moon is relatively dark at totality, but easily visible.

L = 3: Brick red eclipse where the umbral shadow has a yellow or bright rim.

L = 4: Bright copper or orange lunar eclipse, with a blue umbral shadow and bright rim.

When a Lunar Eclipse Becomes a Blood Moon

The moon appears most red or "bloody" at and near totality of a lunar eclipse.
The moon appears most red or "bloody" at and near totality of a lunar eclipse. DR FRED ESPENAK/SCIENCE PHOTO LIBRARY / Getty Images

The phrase "blood moon" is not scientific terminology. The media started referring to total lunar eclipes as "blood moons" around the year 2010, to describe a rare lunar tetrad. A lunar tetrad is a series of four consecutive total lunar eclipses, six months apart. The moon appears reddish only at or near the total umbral eclipse. The red-orange color happens because sunlight passing through the Earth's atmosphere is refracted. Violet, blue, and green light is scattered more strongly than orange and red light, so the sunlight illuminating the full moon appears red. The red color is most noticeable during a total lunar eclipse of the Super Moon, which is the full moon when the Moon is closest to Earth or at perigee.

Dates of Blood Moons

Lunar typically occur 2-4 times each year, but total eclipses are relatively rare. In order to be a "blood moon" or red moon, the lunar eclipse needs to be total. Dates of total lunar eclipses are:

  • January 31, 2018
  • July 27, 2018
  • January 21, 2019

No lunar eclipse in 2017 is a blood moon, two eclipses in 2018 are, and only one of the eclipses in 2019 is. The other eclipses are either partial or penumbral.

While a solar eclipse can only be viewed from a small portion of the Earth, a lunar eclipse is visible anywhere on Earth where it's night. Lunar eclipses may last for a few hours and are safe to view directly (unlike solar eclipses) at any point in time.

Bonus Fact: The other colored moon name is the blue moon. However, this only means two full moons occur within a single month, not that the Moon is actually blue or that any astronomical event occurs.

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Helmenstine, Anne Marie, Ph.D. "Lunar Eclipse and the Blood Moon." ThoughtCo, Oct. 3, 2017, thoughtco.com/lunar-eclipse-and-the-blood-moon-4135955. Helmenstine, Anne Marie, Ph.D. (2017, October 3). Lunar Eclipse and the Blood Moon. Retrieved from https://www.thoughtco.com/lunar-eclipse-and-the-blood-moon-4135955 Helmenstine, Anne Marie, Ph.D. "Lunar Eclipse and the Blood Moon." ThoughtCo. https://www.thoughtco.com/lunar-eclipse-and-the-blood-moon-4135955 (accessed October 24, 2017).