On the evening of January 20 to 21, the entire Western Hemisphere will be treated to a more-than-hour-long sky show: a total eclipse of the moon. And the eruption of a volcano half a world away could make this particular event especially colorful — as in deep red.
Total lunar eclipses aren’t rare. They happen somewhere across the globe once a year or so. But not everybody can see them. People have to be on the night side of Earth to catch the dark shadow fall over the moon’s face.
A lunar eclipse occurs when Earth moves between the sun and a full moon. At first, the moon appears to darken a bit as the shadow cast by Earth sweeps across the lunar surface. At what’s known as totality, Earth blocks out most moon-bound sunlight. This casts the darkest shadow. With only a fraction of the light passing through Earth’s razor-thin atmosphere, the moon plunges into sudden darkness.
But it doesn’t vanish entirely. That blanket of shadow, known as an umbra, lights up the moon enough that it remains slightly visible. It also turns the moon red. The reason has to do with optical physics. Most of the sunlight passing through our atmosphere will be scattered. Only the long-wavelength radiation — red light — penetrates. (It’s the same reason why sunsets appear red, orange and maroon.)
The moon rises Sunday (around 5:30 p.m. along the Atlantic coast, reaching its maximum eclipse near midnight for the Eastern Seaboard). Times will vary elsewhere, so be sure to check local information. Throughout North America, the moon will be high in the sky during the eclipse. That, plus a generally dry and clear night sky in winter, could make this the best viewing opportunity in years.
You might call this eclipse Big Red
This eclipse also will be supersized. That’s due to what some are calling a supermoon. That, too, is due to physics. The moon appears larger because it has been getting closer to Earth. At 357,342 kilometers (222,042 miles) away, it’s approaching its perigee — that point in its orbit when the moon will be nearest our planet. (If you’re looking for a super supermoon, you’ll have to wait until November 25, 2034. That’s when the moon will reach its closest in more than 85 years, according to TimeandDate.com.)
The January total lunar eclipse will be special for another reason: its color. One can never predict the specific red hue that will develop. But there is reason to suspect that this year’s will be deep. And that has to do with Anak Krakatau (AN-ak Kraa-kah-TAU). This volcano in Indonesia erupted with explosive force on December 22, 2018. The shock was so powerful that it sent a tsunami — a wall of water — racing ashore the beaches of the Sunda Strait. (This sudden, forceful flooding killed more than 400 people, according to the Red Cross.)
The volcano also lost 110,000,000 cubic meters in volume, dropping 229 meters (750 feet) in height. That’s according to Indonesia’s Centre for Volcanology and Geological Hazard Mitigation. All that ash and other debris had to go somewhere. Much of it shot high into the atmosphere. Along with it came copious smoke and airborne particles spewed by erupting lava itself.
This plume of ash and more towered to 16.1 kilometers (10 miles), according to an analysis of data collected by the Japanese weather satellite Himawari-8. Much of the aerosol and particulate matter was swept up by the jet stream. That’s a current of air that snakes around Earth some 10 kilometers (6 miles) above the surface. From Indonesia, it takes about two weeks for this high-altitude air pollution to make one round-the-world trek. So by mid-January, all the aerosols were fully mixed within the Southern Hemisphere’s upper atmosphere. (Mixing between the two hemispheres takes much longer.)
Since it’s summer there, the Southern Hemisphere is pointed toward the sun. That means sunlight passing through Earth’s atmosphere during totality will stream through more of the Southern Hemisphere’s, where all of that volcanic pollution is still hanging out. That should filter out even more sunlight than during a typical eclipse. If Sunday night’s moon looks darker, browner or more obscured than usual for an eclipse, you’ll know why.
Turning to history
This isn’t the first time a volcano has impacted eclipse viewing.
The late Richard B. Stothers worked at NASA’s Institute for Space Studies at the Goddard Space Flight Center in New York. He reviewed data on volcanic eruptions that had altered the color of eclipses. In 2005, he reported finding that recent volcanic activity seemed to darken a lunar eclipse. Indeed, he noted, “A dark eclipse on a clear night usually implies the presence of significant turbidity in the Earth's stratosphere arising from a recent volcanic eruption.” By turbidity, he was referring to the ability of particles in the air to make it hard to see through to the sky above.
And activity at Krakatau (also known as Krakatoa) has dimmed lunar eclipses before. A great eruption there in 1883 pumped a huge amount of dust and gas into the upper atmosphere. Noted Stothers, this led “to a series of very dark lunar eclipses in the following 2 years.”
One of the darkest eclipses on record occurred on June 10, 1816. This was shortly after the eruption of Mount Tambora, another Indonesian volcano. Henrik B. Claussen was an astronomer at the University of Copenhagen in Denmark. His team studied Greenland ice cores using electrical conductivity. This let them gauge the amount of acid in different air pockets within the ice. They found an “acid signal” that came from sulfate pollution. Volcanoes often release lots of sulfates. And this sulfate was abundant in the ice that formed in 1816. That’s a year after Tambora’s 1815 eruption. And that makes sense: It would have taken volcanic pollutants that long to make it to the Northern Hemisphere and settle out in Greenland.
Sulfates are known to seed clouds. They can serve as a core around which supercooled water droplets collect. This can boost the number of small water droplets in the atmosphere. And those tiny droplets effectively scatter light, which can make the moon’s apparent change in color even more dramatic. It’s possible Anak Krakatau’s emissions could have that effect.
So when the moon goes red or brown on Sunday night, keep a watchful eye. It may be as informative about our planet as it is beautiful.
aerosol A group of tiny particles suspended in air or gas. Aerosols can be natural, such as fog or gas from volcanic eruptions, or artificial, such as smoke from burning fossil fuels.
Anak Krakatau A volcano in Indonesia that in 1929 rose above the sea to form an island at the site of Krakatau, or Krakatoa (a volcano that erupted violently in 1883, sending shock waves and ash around the world). Anak Krakatau emerged above sea level in 1929. It had been growing ever since. In December 2018 it erupted, losing much of its cone and rocky mass. This event also triggered an underwater landslide, which caused a tsunami that killed more than 400 people.
ash (in geology) Small, lightweight fragments of rock and glass spewed by volcanic eruptions.
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
Atlantic One of the world’s five oceans, it is second in size only to the Pacific. It separates Europe and Africa to the east from North and South America to the west.
atmosphere The envelope of gases surrounding Earth or another planet.
core Something — usually round-shaped — in the center of an object.
debris Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.
eclipse This occurs when two celestial bodies line up in space so that one totally or partially obscures the other. In a solar eclipse, the sun, moon and Earth line up in that order. The moon casts its shadow on the Earth. From Earth, it looks like the moon is blocking out the sun. In a lunar eclipse, the three bodies line up in a different order — sun, Earth, moon — and the Earth casts its shadow on the moon, turning the moon a deep red.
electrical conductivity The ability of some substance (such as water or metals) to transport an electrical charge or current.
equator An imaginary line around Earth that divides Earth into the Northern and Southern Hemispheres.
eruption (in geoscience) The sudden bursting or spraying of hot material from deep inside a planet or moon and out through its surface. Volcanic eruptions on Earth usually send hot lava, hot gases or ash into the air and across surrounding land. In colder parts of the solar system, eruptions often involve liquid water spraying out through cracks in an icy crust. This happens on Enceladus, a moon of Saturn that is covered in ice.
filter (in physics) A screen, plate or layer of a substance that absorbs light or other radiation or selectively prevents the transmission of some of its components.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
gauge A device to measure the size or volume of something. (v. to gauge) The act of measuring or estimating the size of something.
Greenland The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Ice covers roughly 80 percent of Greenland. Indeed, the Greenland ice sheet is the world’s largest. If its frozen water were to melt, it could raise sea levels around the world by 6 meters (about 20 feet). Although this is the 12th biggest nation (based on surface area), Greenland averages the fewest people per square kilometer of its surface area.
hue A color or shade of some color.
jet stream A fast-flowing, high-altitude air current. On Earth, the major jet streams flow from west to east in the mid-latitude regions of the Northern and Southern Hemispheres.
lava Molten rock that comes up from the mantle, through Earth’s crust, and out of a volcano.
lunar Of or relating to Earth’s moon.
orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.
particle A minute amount of something.
particulate A tiny bit of something. A term used by pollution scientists to refer to extremely tiny solid particles and liquid droplets in air that can be inhaled into the lungs. So-called coarse particulates are those with a diameter that is 10 micrometers or smaller. Fine particulates have a diameter no bigger than 2.5 micrometers (or 2,500 nanometers). Ultra-fine particulates tend to have a diameter of 0.1 micrometer (100 nanometers) or less. The smaller the particulate, the more easily it can be inhaled deeply into the lungs. Ultra-fine particulates may be small enough to pass through cell walls and into the blood, where they can then move throughout the body.
perigee That point when some orbiting satellite or celestial object (such as a moon, planet or comet) will be nearest the center of the object that it is orbiting.
physics The scientific study of the nature and properties of matter and energy. A scientist who works in such areas is known as a physicist.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
plume (in environmental sciences) The movement of some gas or liquid, under the direction of gravity, winds or currents. It may be in air, soil or water. It gets its name from the fact that it tends to be long and relatively thin, shaped like a large feather. (in geology) Fluids (air, water or magma typically) that move, largely intact, in a feather-like shape over long distances.
pollutant A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.
radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.
satellite A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.
smoke Plumes of microscopic particles that float in the air. They can be comprised of anything very small. But the best known types are pollutants created by the incomplete burning of oil, wood and other carbon-based materials.
stratosphere The second layer of the Earth’s atmosphere, just above the troposphere, or ground layer. The stratosphere stretches from 10 kilometers to 50 kilometers (about 6.2 to 31 miles) above sea level.
sulfate A family of chemical compounds that are related to sulfuric acid (H2SO4).
supercool (in chemistry) To cool liquid water below its freezing point without it turning into a solid or crystalizing.
supermoon A full moon that occurs at the time that the moon is at that point in its somewhat elliptical orbit that brings it closest to Earth.
totality (in astronomy) The brief period during an eclipse when one object totally obscures another. For a solar eclipse (when viewed from Earth), this would be when the moon appears to completely block out the sun’s light.
tsunami One or many long, high sea waves caused by an earthquake, submarine landslide or other disturbance.
umbra The darkest part of the moon's shadow during a solar eclipse. For people on Earth passing within the umbra, the moon will appear to totally cover the sun, briefly blacking out its light.
volcano A place on Earth’s crust that opens, allowing magma and gases to spew out from underground reservoirs of molten material. The magma rises through a system of pipes or channels, sometimes spending time in chambers where it bubbles with gas and undergoes chemical transformations. This plumbing system can become more complex over time. This can result in a change, over time, to the chemical composition of the lava as well. The surface around a volcano’s opening can grow into a mound or cone shape as successive eruptions send more lava onto the surface, where it cools into hard rock.
wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.
Journal: Henrik B. Claussen et al. A comparison of the volcanic records over the past 4000 years from the Greenland Ice Core Project and Dye 3 Greenland ice cores. Journal of Geophysical Research: Oceans. Vol. 102, November 30, 1997, p. 26707. doi: 10.1029/97JC00587.
News article: 22-23 Dec 2018 eruption & tsunami of Krakatoa - updates Volcanodiscovery.com Accessed January 4, 2019
News article: Drew Macfarlane. Indonesia volcano shrinks 750 feet in height after tsunami-triggering collapse Weather.com. Accessed January 4, 2019.
Journal: Richard B. Stothers. Stratospheric transparency derived from total lunar eclipse colors, 1801–1881. Publications of the Astronomical Society of the Pacific. Volume 117, December 2005, p. 1445.
Explainer: What Is a supermoon and when Is the next one? Timeanddate.com. Accessed January 4, 2019.
Article: Does air pollution alter lunar eclipses? SkyandTelescope.com Accessed January 4, 2019.
Data Source: Total lunar eclipse of 2019, Jan 21. Nasa.gov.
Animation: Eclipses visible in Boston, MA. Timeanddate.com. Accessed January 4, 2019.
Data source: Moon Distance Calculator – How Close is Moon to Earth? Timeanddate.com. Accessed January 4, 2019.
Article: Anak Krakatau volcano now a quarter of its pre-eruption size The Associated Press. December 29, 2018.