Tag Archives: Blue

Why is the ocean blue?

Credit: Pixabay.

Although this might seem like a trivial inquiry, it is in fact a rather wonderful question because answering it involves the physics of light, which kickstarted a golden age of science in the early 20th century. For instance, it was thanks to research into the properties of light, which also includes giving things their color, that Einstein developed his theories of special and general relativity.

As alluded to, the short answer to why the ocean is blue has to do with the way water absorbs and reflects wavelengths of light.

Why is anything colored?

In order to understand why the ocean is colored blue, it helps to understand why things, in general, have color, and it all has to do with some fundamental physics.

You’ve probably heard that light is made of tiny particles known as photons. White light is composed of photons that have many different wavelengths, and together comprise all the colors of the rainbow. The photons with the shortest wavelengths appear blue in the visible spectrum, while those with the longest wavelengths are red.

The only pure type of light is the one immediately shone by the sun. Afterward, the light will inevitably become altered as it interacts with various matter. Depending on what light interacts with, some photons will be absorbed, while others will bounce back. This latter action is known as ‘scattering’.

The way our eyes work is that we only see things when light bounces off of them and hits our retinas. We can’t see absorbed photons, and this has important consequences for color. For instance, leaves are green because red and blue wavelengths are absorbed by chlorophyll, while green photons bounce back towards our eyes. In the fall, leaves appear bright yellow and red because deciduous plants stop producing chlorophyll for the winter.

Likewise, experiments have shown that when light passes through pure water, red photons are absorbed, as well as short-wavelength light such as violet and ultraviolet. If that’s so, why is a glass of water, well, colorless? First of all, it’s not exactly colorless, since even a glass of water has a slight blue tint.

The ocean being distinctly colored blue can be explained by the fact that the quantity of red light absorbed depends on how much water the light has to pass through. The effect becomes more apparent when dealing with quantities of water at least as big as a swimming pool. Oceans absorb a phenomenal amount of red light, making the entire planet look like a marvelous blue marble even from millions of miles away.

This only works up to a point, though. Hardly any light penetrates deeper than 200 meters (650 feet), and absolutely no light exists at depths greater than 1,000 meters (3,280). This means that the vast majority of the ocean is actually in total darkness.

Not always blue

Shallow waters can sometimes look green due to sediments and tiny plants and marine life.

It’s important to realize that oceans aren’t made of pure water. There are many impurities such as salts or small fragments of tissue from marine creatures. For this reason, the light that bounces off the ocean also has a greenish tint.

What about the sky? It is true that the ocean acts as a mirror, reflecting some of the light from the sky, which is blue. However, its role in coloring the ocean blue isn’t critical. An indoor swimming pool’s water will appear blue even at night under artificial lighting.

The reason why some moving bodies of water, such as rivers and even stationary bodies of water such as ponds, appear to be muddy brown rather than blue is due to the presence of sediments that have been stirred up.

Shallow water is also more likely to appear in other colors, such as lighter shades of blue or even green as a result of light bouncing off floating sediments and life forms such as algae and phytoplankton. In fact, even ocean regions with high concentrations of phytoplankton will appear blue-green to green, since phytoplankton is rich in the green pigment chlorophyll.

Because the ocean’s color is so greatly influenced by the presence of phytoplankton, researchers often analyze satellite images of the ocean to gauge the health of marine ecosystems. Although small, when they band together phytoplankton have a huge impact on the biosphere. They’re not only at the very bottom of the food web, but also provide almost half of the oxygen we breathe by converting CO2 pulled from the atmosphere through photosynthesis. 

How on Earth did we start using “once in a blue moon”?

You’ve heard it, I’ve heard it, but not many people we know have actually seen a blue moon — so what gives?

Am image of the moon captured through a blue filter.
Image credits steviep187 / Flickr.

“Once in a blue moon” refers to events that only happen very rarely, but it’s a tricky idiom. It doesn’t refer to a moon that’s actually blue, although it can appear to be that color under certain conditions and that probably shaped the saying.

A blue moon is a real occurrence and, you might be surprised to hear, isn’t actually that rare or unpredictable. Blue moons are ‘extra’ full moons of the regular gray color that pop up every two or three years due to misalignment in the lunar and solar circle. But the phrase was first used to refer to something being absurd — like someone arguing that the moon is blue.

So let’s take a look at both halves of this idiom and see why they came to represent the quintessential rare occurrence.

The literal blue moon

Image credits Bobby Jones.

The moon can naturally appear blue or light-blue in the sky. It’s a rare event caused by the presence of dust or smoke particles in the atmosphere at night which alter the way light is diffracted in the atmosphere. If these particles are of the right size, they can scatter the red part of the light spectrum, leaving the rest untouched.

Because visible light spans from red (low-energy) to blue (high-energy), this scattering makes everything take on a blue tint. Since the moon is a white-ish gray on a dark background, this effect causes it to look blue.

This type of blue moon is probably what spawned the idiom. It’s very rare and very unpredictable, as its appearance relies directly on phenomena such as massive wildfires or volcanic eruptions. The fact that it’s entirely dependent on local phenomena also means blue moons are only visible from relatively small areas at a time, not globally — which compounds their rarity.

Some events that led to blue moons include forest fires in Canada, and the eruptions of Mount St. Helens in 1980 and the El Chichón volcano in Mexico in 1983. The eruption of Krakatoa in 1883 (one of the largest in history) reportedly caused blue moons for nearly two years.

A pretty exciting implication of the mechanism that spawns blue moons is a purple sun. In 1950, as huge fires swept the bogs of Alberta, Canada billowing with smoke, leading to sightings of blue moons from the US to England the following night. Two days later, reports of an indigo sun peering through the smoky skies also started to surface.

File:Blood moon 73.jpg
A blood moon.
Image credits Andrey73RUS / Wikimedia.

So why don’t all volcanic eruptions and wildfires turn the moon blue? Well, the size of ash or oil/tar particles they generate is very important. These have to be wider than the wavelength of red light, which is 0.7 micrometers, to block these rays. At the same time, very few to no particles of smaller sizes should be present, as these would help scatter other colors and destroy the overall effect.

Naturally-occurring ash tends to be a mix of particles of various sizes, with most being smaller than the above threshold. Since smaller particles preferentially scatter (i.e. remove) light towards the high end of the spectrum (blue), natural ash clouds typically give everything a shade of red. Red or blood moons are thus a much more common occurrence than blue moons.

The figurative blue moon

Traditionally a blue moon is an additional full moon that appears every 2 and a half years or so, according to NASA. In recent times it has also come to denote the second full moon to appear within a single calendar month in popular use.

This stems from the way lunar and solar cycles relate to one another. There are 29.5 days between full moons, the agency goes on to explain, so each year will have roughly 12.3 full moons. Another implication of this is that 28-days-long February can’t ever have a blue moon.

Both uses of the phrase are considered valid today.

Over time, the idiom turned from meaning that something is impossible to “never” — think along the lines of “I’ll help you when the pigs fly”.

“The definition of a Blue Moon [as] ‘the second full moon in a calendar month,’ is a curious bit of modern folklore. How it emerged is a long story involving old almanacs, a mistake in Sky and Telescope magazine, and the board game Trivial Pursuit,” wrote Dr. Tony Phillips for NASA.

One of the almanacs Dr. Phillips mentions is the Maine Farmer’s Almanac, more specifically its August 1937 issue. The publication followed certain conventions about how to name each moon depending on the time of year. The first full moon of spring for example was called the Egg Moon, Easter Moon, or Paschal Moon, and had to fall within the week before Easter. If a particular season had four moons, the extra one was called a Blue Moon to maintain the naming conventions.

The definition of the blue moon as being the second full month in a single month came, according to Space, from a mistaken interpretation of the term which was popularized by a nationally syndicated radio program in 1980.

Rarer than Blue Moons are double Blue Moons — when the same calendar year gets two of these events. They’re much rarer, only occurring about 3-5 times every hundred years or so; the next double blue moons are expected in 2037. As for a single blue moon, the next one is expected on October 31, 2020.


Australian skinks will literally stick their tongues out at predators — and it works!

When in doubt, stick your tongue out at them!


“You asked for it punk!”
Image credits Shane Black.

Skinks in the genus Tiliqua are pretty inconspicuous as far as lizards go. They don’t really like to draw attention to themselves, and they’re decidedly lizard-shaped. New research shows that when their unassuming nature fails to garner the peace of mind they desire (from predators), the skinks fall back to a surprising — and surprisingly effective — last-ditch defense: their tongues.

Their what now?

Bluetongued skinks are fairly widely spread throughout Australia, eastern Indonesia, and Papua New Guinea. They’re omnivorous, mediumly-sized lizards that primarily rely on their camouflage to keep out of sight. When under attack by a determined predator, however, they make an effort to stand out: the skinks open their mouth suddenly, as wide as they can, to reveal a brightly-colored blue tongue. Not to make them self-conscious but these tongues must be a sight to recoil from — because that’s exactly what predators do.

The behavior is used as a last line of defense to protect the skinks from attack, writes Martin Whiting, the study’s corresponding author, in a press release. The research revealed that the tongues are very reflective in the UV spectrum, and that they are more UV-luminous towards the back. Some of the lizards’ main predators, such as birds, snakes, or monitor lizards, are thought to be able to see UV light, suggesting the skinks might use this light to startle predators into breaking off their attack.

The study focused on the northern bluetongue skink (Tiliqua scincoides intermedia), the largest species of the group. The species sports good camouflage: broad brown bands across their backs to blend them into their surroundings. However, some of its main predators can still spot them, likely due to their ability to perceive UV light — so the team aimed to determine what tactics it uses to deter attackers.

First, they used a portable spectrophotometer to measure the color and intensity across different areas of the tongues of 13 skinks. This revealed that the blue tongues actually reflect UV light. Further data crunching in the lab later revealed that the tongues were almost twice as bright at the rear compared to the tip.

Mean spectra of different regions of the tongue. Associated illustration by Courtney Walcott of a Bluetongue skink performing a full-tongue display.
Image credits A. Badiane et al., 2018, Behavioral Ecology and Sociobiology.


The next part of the study was to identify how this bright tongue benefited the skinks. The team observed that skinks in the wild would open their mouths and stick their tongues out at would-be attackers. To find out more, the team simulated attacks on the lizards using models of their natural predators — the team used a snake, a bird, a goanna (monitor lizard), a fox — and a piece of wood as a control.

Skinks will rely on concealment for as long as they possibly can, the team reports. Should this fail, however, the lizards open their mouths widely at the last moment, revealing their UV-reflective tongues. One particularly amusing paragraph of the study suggests that the more intense attacks elicited a stronger tongue-response: the more risk the skinks felt exposed to, the more tongue they would poke at their enemies. I can relate to their fighting style.

Bluetongue display.

Northern Bluetongue skink performing a ‘full-tongue’ display in response to a simulated attack by a model predator. The face of a true warrior.
Image credits Peter Street / A. Badiane et al., 2018, Behavioral Ecology and Sociobiology.

“The lizards restrict the use of full-tongue displays to the final stages of a predation sequence when they are most at risk, and do so in concert with aggressive defensive behaviours that amplify the display, such as hissing or inflating their bodies,” explains lead author Arnaud Badiane.

“This type of display might be particularly effective against aerial predators, for which an interrupted attack would not be easily resumed due to loss of inertia.”

Finally, the team notes that tongue-displays were most often triggered by the fake bird and fox models, rather than by those of snakes or monitor lizards.

“The timing of their tongue display is crucial,” adds Badiane. “If performed too early, a display may break the lizard’s camouflage and attract unwanted attention by predators and increase predation risk. If performed too late, it may not deter predators.”

If you’re ever caught between a rock and a hard knuckle, stick your tongue out. It likely won’t be as effective as those of the skinks, but maybe you’ll confuse people enough to make your (brave and honorable) escape. Worth a shot.

The paper “Why blue tongue? A potential UV-based deimatic display in a lizard” has been published in the journal Behavioral Ecology and Sociobiology.

Chameleons display fluorescent bones on the skull, study shows

The lizard master of disguise is surely a very special creature, we can all agree. Researchers discovered a new outstanding feature of the chameleon: its bones shine with a blue hue in UV light.

Fluorescent tubercles showing sexual dimorphism under UV light at 365 nm (A–D) and fluorescence in further chameleon genera (E–G). (A) Male Calumma crypticum ZSM 32/2016. (B) Female C. crypticum ZSM 67/2005. (C) Male C. cucullatum ZSM 655/2014. (D) Female C. cucullatum ZSM 654/2014. (E) Brookesia superciliaris, male (only UV light at 365 nm). (F) Bradypodion transvaalense, male (dim light and additional UV light at 395 nm). (G) Furcifer pardalis, male (daylight and additional UV light at 365 nm).

Bioluminescence is not that uncommon among marine creatures and some insects (see fireflies), but most terrestrial animals don’t quite possess this eye-endearing feature. The fact that researchers found biogenic fluorescence in chameleons — an entirely earthbound animal — is surprising.

Male C. globifer (ZSM 141/2016) showing congruent tubercle/fluorescent patterns (from left to right); top row: alive in the field under sunlight, micro-CT scan of head surface (probable edge artefact in cheek region), micro-CT scan of the skull; bottom row: alive in the field under UV light, ethanol-preserved under UV light.

Male C. globifer (ZSM 141/2016) showing congruent tubercle/fluorescent patterns (from left to right); top row: alive in the field under sunlight, micro-CT scan of head surface (probable edge artefact in cheek region), micro-CT scan of the skull; bottom row: alive in the field under UV light, ethanol-preserved under UV light.

“We could hardly believe our eyes when we illuminated the chameleons in our collection with a UV lamp, and almost all species showed blue, previously invisible patterns on the head, some even over the whole body,” said David Prötzel, lead author of the new study and a Ph.D. student at the Bavarian State Collection of Zoology (ZSM).

German biologists found that the small bone bumps on chameleons’ heads fluoresce under UV light in a blueish shade. These tiny bone structures absorb UV radiation through small “windows” in the skin and then emit a soft blue light. Actually, the windows are just metaphorical, because the thin epidermis layer that covers the projections is transparent.

After seeing their shimmer under UV-lighting, scientists performed microCT scans and matched the small bone tuberosities to the blue colored pattern.

The fact that bones fluoresce under UV conditions was long-known. But using this phenomenon to intentionally fluoresce different body parts surprised the authors, as it was the first time scientists had encountered such a feature.

Okay, okay, but what’s the deal with all this effort to display such a multitude of colors, even fluorescence?

The myth that chameleons use color-change as camouflage has been debunked. A new theory states that these reptiles use skin color-shifting as a way to communicate with their kin. Taking into consideration that most males from the Calumna genus have significantly more fluorescent tubercles than the females, researchers suppose that their goal is to attract mates. Blue, being a rare color in the forest, should be quite eye-catching in this regard.

The well-known panther chameleon (Furcifer pardalis) which is also popular as a pet, shows fluorescent crests on the head. (David Prötzel; ZSM/LMU)

Another interesting observation is the distribution of fluorescence among different genera of chameleons. Researchers discovered that forest-living species are more prone to exhibit glowing tubercles than species which live in open environments.

“As shorter (UV, blue) wavelengths are scattered more strongly than longer wavelengths the UV component under the diffuse irradiation in the forest shade is relatively higher compared to the direct irradiation by the sunlight,” the authors write in the journal Nature.

“Consequently, using UV reflections for communication is apparently more common in closed habitats than in open habitats, as has been shown in chameleons of the genus Bradypodion.”

Synthetic Photosynthesis.

New, cheap artificial photosynthesis scrubs the air and produces fuel

A research team from the University of Central Florida has found a way to trigger photosynthesis in an inexpensive synthetic material. The technology could be used to scrub the air clean and produce ‘solar’ fuel from atmospheric CO2.

Synthetic Photosynthesis.

The team’s photoreactor, with a sliver of the MOF dangling inside.
Image credits Bernard Wilchusky.

Scientists the world over have been trying to re-create the process that plants rely on to feed in a synthetic material for years now, with some success. Photosynthesis-like reactions can be maintained in common materials such titanium dioxide under higher-energy UV light. But, since the lion’s share of energy released by the Sun lies in the violet to red wavelengths, the challenge lies in finding a way to keep it going under visible light. Up to now, we’ve known comparatively few materials that can do so, and they’re very expensive (think platinum or iridium compounds), keeping them far away from commercial applications.

Uribe-Romo, a chemistry professor at the UCF, and his students may bring artificial photosynthesis into the market. The team has found a way to trigger the reaction in an inexpensive synthetic material, offering a cost-effective way to turn atmospheric CO2 into fuel.

The system uses a class of materials known as metal-organic frameworks (MOFs) to break down the CO2 into its two compounds. Uribe-Romo’s MOF was constructed from titanium with a pinch of organic molecules to act as light-harvesting points and power the reaction. These molecules, called N-alkyl-2-aminoterephthalates, can be tailored to absorb specific wavelengths (colors) of light when incorporated in the MOF — the team went for blue.

They tested the system under a photoreactor — a battery of blue LED lights which looks like a tiny tanning bed — constructed to mimic the sun’s blue wavelength. Controlled amounts of CO2 were pumped into the photoreactor, and the MOF scrubbed it out then split it into two reduced carbon compounds — formate and formamide.

“The goal is to continue to fine-tune the approach so we can create greater amounts of reduced carbon so it is more efficient,” Uribe-Romo said.

He says that the next thing the team will be looking into is how to adjust their material so it can sustain the process under other colors of light. If they can pull it off, the system will gain hugely in versatility and could grow to become a significant carbon sink. Stations could be set up near CO2-producing areas, such as power plants, to capture the gas and use it to produce energy which could be fed back into the system. Or it could be fashioned into rooftops that homeowners can install to clean their neighborhood’s air while saving up on power bills.

“That would take new technology and infrastructure to happen,” Uribe-Romo said. “But it may be possible.”

The full paper “Systematic Variation of the Optical Bandgap in Titanium Based Isoreticular Metal-Organic Frameworks for Photocatalytic Reduction of CO2 under Blue Light” has been published in the Journal of Material Chemistry A.