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Sept-Oct 2009

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Twice in a Blue Moon

Even in 2009, the International Year of Astronomy, it is only “once in a blue moon” that you will read an article about blue moons. On rare occasions, the moon does appear blue, and this color lies at the origin of the expression “once in a blue moon.” Curiously, the astronomer’s blue moon simply denotes the third full moon in a season that has four full moons and has nothing to do with color. But the popular meaning of the expression refers both to color and to something unusual. In 1528, William Barlow, the Bishop of Chichester, wrote: “If they say the moon is blue / We must believe that it is true.”

Blue moons are so infrequent that at the time that Barlow wrote his verse, many people did not even know they existed. As interest in science and astronomy increased after the sixteenth century, blue moons were recognized as real but uncommon phenomena. Folklorist Philip Hiscock noted that the expression, “once in a blue moon,” assumed its current meaning as denoting something infrequent in the midnineteenth century.

Earth as seen from the moon by the Apollo 11 astronauts. Notice that the clouds on Earth are much brighter than the moon. (source: NASA)

There are two distinct and infrequent situations that turn the moon blue, and both of these occur when light is filtered as it passes through Earth’s atmosphere. In one situation, moonlight turns blue as it passes through the atmosphere on the way to our eyes. In the other, sunlight that illuminates the moon turns blue as it passes through Earth’s atmosphere on its way to the moon.

In fact, any time the moon appears to be a color other than white or gray, the atmosphere is responsible. So, while blue moons may be of great interest to astronomers, they must be explained using atmospheric optics, which treats the transmission of light and radiation through the atmosphere. It is a venerable branch of meteorology, concerned not only with phenomena such as rainbows, halos, and sky colors, but also with global warming via the atmospheric greenhouse effect and how the ozone layer protects us from harmful ultraviolet radiation.

Color of scattered light and light scattering efficiency as a function of particle size.

Atmospheric Optics by Day and Night

Like Earth, the moon shines by reflecting sunlight. Because sunlight covers the spectrum of electromagnetic radiation from ultraviolet through visible to infrared, the moon usually appears white—or, more accurately, gray—because it is largely covered by dark rocks, such as the basalt lavas that flooded the moon’s Maria or “Seas” about 3.9 billion years ago. The dark rocks make the moon a very poor reflector. It absorbs 93 percent of the sunlight that strikes it and has an albedo or reflection efficiency of only 7 percent. Earth, enveloped in an atmosphere with abundant aerosols and clouds, and with large areas of its surface covered by sand, ice, and snow, reflects sunlight much more efficiently and has an albedo of 30 percent. That is why the moon appears much darker than the Earth when both are photographed together from space. If the moon’s albedo were much higher, we could easily read by moonlight.

Atmospheric optics consider the interactions of light with the atmosphere. When light strikes any medium, such as the atmosphere, it can be transmitted directly, reflected, and scattered in other directions (deflected), or it can be absorbed (extinguished). Moonlight reaching the ground is light that has been transmitted directly through the atmosphere. Whenever the moon does not appear white or gray, it is because the atmosphere has filtered the light that reaches our eyes by scattering or absorbing some colors more than others. Skylight is scattered sunlight. In the atmosphere, light is scattered or absorbed by air molecules, aerosol particles (which include pollen, soot, dust, smoke, volcanic ash, and a wide range of natural and manmade chemicals), cloud droplets, ice crystals, and hydrometeors (raindrops, snowflakes, hail, etc.). On the moon, the sky appears black because there are no clouds, air, or aerosol particles to scatter light and spread it around the sky.


A jet streaks across the crimson red predawn sky over New York City on October 18, 1991, after the eruption of Mount Pinatubo. (source: Stanley David Gedzelman)

The moon appears colored when some colors and wavelengths of light are transmitted more efficiently and are therefore scattered or absorbed less than others. The fraction of scattered or absorbed light depends on the number, composition, and size (relative to the wavelength) of the particles in the atmosphere. The more particles along the path of light, the more light will be blocked, and the less will be transmitted. Particle chemistry determines how efficiently light is absorbed. For example, soot particles absorb light very efficiently but water droplets do not. Particle size is extremely important, not only because large particles block more light than small particles, but also because the efficiency of scattering depends on particle size.

Particles scatter light according to three general size classes—small, medium, and large. Particles that are much smaller than the wavelength of light (from 0.4 to 0.7 µm), such as air molecules (0.0001 micrometers, or µm), scatter the shortest waves of visible light (violet and blue) much more efficiently than the longest waves (orange and red). Scattering by air molecules makes the sky blue.

Particles that are much larger than the wavelength of light—such as cloud droplets (diameter, d >≈10 µm), ice crystals (d >≈50 µm), hydrometeors (d >≈ 100 µm), and large aerosols (d >≈ 2 µm)—scatter light with almost no color bias but deflect most of the light by only small angles. On humid days, salt-loving (hygroscopic) aerosols get wet and swell. These engorged aerosols make the sky hazy and much whiter and brighter than usual, especially near the sun. Cloud droplets or ice crystals make thin clouds near the sun very bright. They also make the sunlit sides of thick clouds white and the shaded parts gray when the sun is high in the sky. When the sun nears the horizon and its light turns from yellow to orange and then red, thin, sunlit clouds appear golden yellow or rosy because they scatter light with no color bias.

Particles about the same size as the waves produce the most complex scattering behavior. Particles either about 1.5 µm or about 3.0 µm in diameter are most unusual in that they scatter long waves more efficiently than short waves. Thus they tend to scatter orange and red light and transmit violet and blue light. Particles of these sizes can produce crimson twilights and blue moons.


A blue-green sun appears through thin altocumulus clouds on November 11, 1986, over Boulder, Colorado. The authors cannot explain this case.

Red and Blue Moons

Most aerosol particles are smaller than 1.0 µm in diameter. But cataclysmic volcanic eruptions eject larger-than-normal aerosol particles into the stratosphere. The largest of these fall out quickly as ash, but the particles around 1.5 µm in diameter, which tend to be droplets of sulfuric acid, settle so slowly that they persist in the stratosphere for months to a few years. It is these stratospheric sulfuric acid droplets that turn the sun and moon blue in the months after huge volcanic eruptions. For nearly 2 years after the enormous eruption of the Indonesian volcano Krakatau in 1883, the moon appeared blue. Blue moons were also seen after the eruption of El Chichón in Mexico in 1982. And whenever volcanic blue moons are seen, twilight skies are a brilliant crimson red because mostly red light is scattered in the thin air of the stratosphere. In the months following the eruption of Mount Pinatubo in June 1991, crimson twilights were seen around the world.

Large forest fires produce smoke particles that can also produce blue moons. Fires in western Canada and Sweden produced blue moons in September 1950 as far away as the eastern United States and Western Europe, and blue moons are seen over China when farmers plow the fine loess particles from the Gobi Desert into the atmosphere.

Curiously, the only time either of the authors recalls seeing a blue-green sun (neither of us has seen a blue moon) was through a thin altocumulus cloud on November 11, 1986, over Boulder, Colorado, and we cannot identify any unusual event that might have injected particles in the atmosphere. Even in thin clouds, most cloud droplets are much larger than 3 µm in diameter, so the cause of the blue color remains a mystery to us.

In order to produce colors in the atmosphere, a significant percentage of the light must be scattered or absorbed. The more particles in the path of the light, the more light is scattered or absorbed, and the less is transmitted. The standard measure of how much light is blocked along a path is the optical thickness, which is simply a number. When the optical thickness is 1, particles allow a fraction equal to 1/e1 (e ≈ 2.718), or about 36.8 percent of a light beam to pass and block the remaining 63.2 percent. When the optical thickness is 2, particles allow only a fraction of 1/e2, or 13.5 percent, of the light to pass, and so on. By the time optical thickness is greater than about 5, very little light is transmitted directly. Thick clouds such as nimbostratus and cumulonimbus have very large optical thickness (>> 10 100) and are completely opaque. The clean, molecular atmosphere has small optical thickness and is therefore relatively transparent. When the sun is overhead, about 30 percent of its violet light and only about 3 percent of its red light is scattered, while the rest reaches sea level. Thus, the optical thickness of the dry atmosphere is about 0.3 for violet light and 0.03 for red light.

When optical thickness is small, little light is scattered and most is transmitted, so the moon appears white. As optical thickness increases, more of the light is scattered and less is transmitted. If one color is transmitted much more than another, the moon and sun will assume that color.

The atmosphere is a wafer-thin veneer covering the Earth. As the moon approaches the horizon, moonbeams pass through the thin atmosphere more obliquely. This increases the length of a moonbeam within the atmosphere so much that when the moon rests on the horizon, a moonbeam must pass through 38 times as much atmosphere as when the moon is directly overhead. The result is that even in clean air, about 1/3 of the moon’s red light but only about 1/100,000 of its violet light reach the ground at moonset or moonrise. Deprived of violet and blue light, the sun, the moon and even the twilight sky near the horizon turn orange or red.


The impact of particle size and optical thickness on the color of directly transmitted sunlight or moonlight reaching the ground. Red skies and blue moons can be produced by particles with radii around 0.75 and 1.5 µm, so long as the aerosol optical thickness is large enough.

Another Role for Ozone

Particle chemistry affects color because it determines how much light of each wavelength is absorbed. Soot particles, which are mainly carbon, absorb all colors of visible light and therefore appear black, while hematite particles (iron oxide) appear red because they absorb red least of all colors. Most other solid aerosol particles scarcely absorb any visible light. Many atmospheric gases absorb ultraviolet radiation, producing a shield that makes life on land possible, while all gases consisting of 3 or more atoms (for example, CO2, H2O, O3, and CH4) absorb infrared radiation and produce greenhouse warming.

The few gases that absorb visible light tend to be poisonous, such as chlorine gas and mustard gas, both used during World War I. Chlorine gas appears yellow-green because it absorbs violet, blue, and red light most efficiently and transmits green and yellow light. Fortunately, chlorine is not present in the atmosphere under normal conditions.



The length of a moonbeam through the thin atmosphere increases so much as the moon nears the horizon that scattering of short light waves by tiny particles along the long path turns the white moon red. (source: Stanley David Gedzelman)

Ozone (O3) is the only naturally occurring atmospheric gas that absorbs visible light appreciably. An acrid gas, ozone is blue because it absorbs orange light. It is extremely fortunate that most ozone is concentrated up in the stratosphere, where it protects us from lethal sunburn by absorbing ultraviolet radiation, but where we don’t have to breathe it. The amount of ozone in the atmosphere is tiny—about 1 part per 2 million—so if it were all brought to sea level, it would form a layer only about 3 millimeters deep, on average.

These 3 millimeters of ozone absorb about 1.5 percent of the orange light (and none of the blue or violet light) in a sunbeam or moonbeam that reaches ground level when the sun or moon is directly overhead, but more than 25 percent of the orange light when the moon or sun is at the horizon because of the long, oblique path. Ozone absorbs enough orange light to keep the zenith sky blue at sunset and sunrise. Without ozone, the zenith sky would be almost white at sunrise and sunset! And as we are about to see, absorption of orange light by ozone can also turn the moon blue.



Sunbeams change color as they pass through the atmosphere and refract into the umbra. The higher beam turns blue because orange light is absorbed by ozone while passing through the stratosphere. The lower beam, which refracts more, turns red because short waves are scattered more thoroughly as the beam passes through denser air near the ground in the troposphere. (source: Stanley David Gedzelman)

Lunar Eclipses

The second situation that can turn the moon blue is a total lunar eclipse. When the moon lies in the shadow or umbra of the Earth, it would seem that it should be pitch-black, but it is generally a dull red that varies greatly from one eclipse to another. This raises several questions. First, how does light get into Earth’s umbra? Second, why is it usually red? And third, under what conditions does it turn blue? The answers to all these questions involve the atmosphere. Sunlight passing through Earth’s atmosphere along the terminator or twilight zone (the narrow zone that separates day and night) is refracted or bent as much as 1.2° inward toward the umbra. The sunbeams that emerge from this grazing path must pass through exactly twice as much atmosphere as sunbeams reaching ground level at sunrise or sunset. With such a long path through the atmosphere, most of the transmitted sunlight that heads toward the moon is deep red, because only the longest waves can pass through the atmosphere near ground level without being scattered.

So what about the blue moonlight during a lunar eclipse? Anyone who looks at the eclipsed moon will notice the dull but dominant red light, but only an astute observer or a savvy amateur astronomer will notice that during many eclipses, the narrow region just inside the fringe of the umbra is turquoise blue. The blue fringe of the umbra was documented by many photographers during the total lunar eclipses of 2007 and 2008.

What causes the blue fringe? Because scattering almost invariably depletes the violet and blue light from sunbeams that pass through the atmosphere, only absorption of orange light by ozone in the stratosphere can explain the blue fringe. Now let us see how this works during a lunar eclipse.

The color and brightness at any point in the umbra depend on the angle by which the sunlight is refracted and how much light at each wavelength passes through the atmosphere at the terminator. Sunbeams that have passed through the atmosphere consist mostly of red light if scattering takes a greater toll than absorption by ozone. They consist mostly of blue light if absorption dominates. The closer to sea level that a sunbeam grazes the Earth, the denser the air, the more light is scattered, the less light transmitted, and the further the beam is refracted into the umbra. Sunbeams that pass close to sea level are refracted enough (1.06°) to reach the center of the umbra and are red because they lose more to scattering than to absorption. Sunbeams that pass 12 kilometers above sea level are refracted just enough (0.27°) to reach the fringe of the umbra. It is these sunbeams that have turned blue because air in the stratosphere is so tenuous that little light is scattered, but about 50 percent of the orange light is absorbed in the ozone layer.

Inconstant Eclipses

Because Earth’s atmosphere changes constantly, the moon’s appearance varies from one lunar eclipse to another. For a few years after the eruption of Mount Pinatubo in 1991, the stratosphere was so clogged with particles that the eclipsed moon was almost invisible. Now that the stratosphere has become clean once again, the moon is much brighter during eclipses.

The more ozone along the path of sunbeams reaching the fringe of the umbra, the deeper the blue in the umbra will be. Models show that the blue should be quite pronounced when sunbeams reaching the moon pass through a part of the atmosphere that contains 5 or 6 millimeters (mm) of ozone and should disappear when the atmosphere contains less than about 1.5 mm. Typical ozone totals are 3 mm, but they average about 2.25-2.5 mm in the tropics and 4-6 mm in the Arctic. The lowest values (≈ 1 mm) are found over Antarctica during September and October, when the ozone hole is deepest. If the moon is so located that the beams illuminating it at the edge of the umbra have passed over the tropics or over Antarctica during September or October, it will be scarcely blue or not blue at all. But if the beams have passed over the Arctic when the ozone is greatest, the blue will be much purer and deeper.

The amount and color of light reaching deep into the umbra also depends on the distribution of mountains, clouds, and aerosols along the terminator, because about 90 percent of it has passed through the atmosphere at heights between about 1.5 and 5 km above sea level. When the Earth is oriented so that there are more clouds, haze, and high mountains along the terminator, less light enters the umbra. Because satellites such as MODIS, CALIPSO, and CloudSat are now beginning to provide details of the 3-D global picture of clouds, aerosols, and ozone, we might soon be able to predict just how blue a blue moon will be.

Finally, moonlight that reaches us during eclipses not only must pass through the atmosphere on the way to the moon but must pass through the atmosphere a second time on the way back from the moon. In this second atmospheric penetration, the eclipsed moon’s light and color are further altered, and the blue fringe can be eliminated when the moon is near the horizon.

The Next Blue Moon

It is a bit ironic that in the International Year of Astronomy there will be no total solar or lunar eclipses. In fact, the first partial lunar eclipse comes on the last day of the year, December 31, 2009. And that eclipse will not be visible from anywhere in the United States. The best place to see that brief eclipse, where only a fringe of the moon enters the umbra, will be Karachi, Pakistan. The next total lunar eclipse will not occur until the winter solstice, December 21, 2010, a real astronomical coincidence. And, fortunately for those in the United States, that eclipse will be total and visible (provided the sky is clear) anywhere in the 50 states. Furthermore the blue fringe should be deeper blue than normal because the sunlight reaching the northern part of the umbra will pass over the high northern latitudes of Earth, where ozone content is generally high.

So, if you are looking for blue moons, you will have to be patient—unless there happens to be a major volcanic eruption before December 21, 2010. And if you do happen to catch one of these rare and superlative events, rest assured, it is a rewarding experience that is worth the wait.                

STANLEY DAVID GEDZELMAN is a professor in the Department of Earth and Atmospheric Sciences and the NOAA CREST Center at the City College of the City University of New York. MICHAEL VOLLMER is a professor at Fachbereich Technik, Brandenburg University of Applied Sciences in Brandenburg, Germany.    

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