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The last “blue moon,” seen during a partial lunar eclipse in December 2009. Image via Wikimedia Commons/Codybird

This Friday, look to the night sky and you’ll see what’s referred to as a “blue moon”—the last time you’ll get the chance to glimpse this phenomenon until 2015. Those expecting to see a moon that’s actually an unusual color, though, will be disappointed. The term simply refers to the unusual occurrence of a second full moon within one calendar month, and since we already had a full moon on August 2, this will be a blue moon, the first seen in the United States since December 2009.

Every 29.53 days, the moon undergoes a complete phase cycle, as the portion of its surface that’s illuminated by the sun shifts from entirely within our line of sight (a full moon) to entirely hidden on the “far” side of the moon, away from the earth (a new moon, which is completely dark). Since 29.53 days is relatively close to the lengths of the months in our calendar, most months only have one full moon. Our calendar, however, is based off our motion around the sun, not the phases of the moon, so the periods don’t match up exactly.

As a result, every 2.7 years, two full moons are squeezed into one month. It’s the same way that, if you get a paycheck every two weeks, you’ll occasionally end up getting three paychecks within one month, since two 14-day pay periods (28 days) don’t match exactly with the calendar month. That’s what’s happening on Friday—the moon will reach its second full phase of August at precisely 9:58 a.m. EDT on the the 31st—and will happen next in July 2015.

If the moon won’t actually be blue Friday, then why the colorful name? Although it’s frequently cited as a piece of old folklore, Philip Hiscock, a professor of folklore at Memorial University in Canada, writes in Space & Telescope that this isn’t the case. Hiscock writes that it’s “a truly modern piece of folklore, masquerading as something old.”

Originally, in the early 1900s in places such as the Maine Farmer’s Almanac, the term “blue moon” was used to refer to a related phenomenon, when four full moons occurred within a given season, instead of the typical three. However, in 1946, amateur astronomer James High Pruett incorrectly interpreted the term in an article he wrote in Sky & Telescope using the meaning we know today. The mistake was repeated several times—notably, in 1980 on the NPR show Star Date—and eventually the new definition stuck, along with a common misattribution to traditional folklore, which “appeals to our modern sensibilities, including our desire to have plausible origins,” Hiscock writes. Since then, the term has been appropriated for everything from a novel to a butterfly to the widely popular Belgian white-style beer.

On very rare occasions, the moon actually can appear blue, if particulate matter of the right size is suspended in the atmosphere and interacts with light reflecting off the moon. “If there’s been a recent forest fire or volcanic eruption that pumped significant smoke or ash into the upper atmosphere, it is possible for the moon to take on a bluish hue,” says Space.com. Specifically, if the ash or other particles are roughly 1 micron wide (1 millionth of a meter), they will scatter red wavelengths of light, allowing other colors to pass through and reach the earth. This can cause the moon to appear blue or greenish and has happened several times in recent history, such as during the June 1991 eruption of Mount Pinatubo in the Philippines, which caused the phenomenon to occur in several places around the world.

Perhaps this accounts for the most commonly used meaning of the term, “once in a blue moon,” which refers to something that happens on a very infrequent basis. Unlike the blue moon that you can see Friday night and can count on like clockwork every 2.7 years, seeing a moon that’s actually in blue color will be more difficult. To do so, you might have to be patient for quite a while—and wait around for a massive volcanic explosion.

One of the hundreds of trees lost to Friday night’s derecho (courtesy of flickr user woodleywonderworks).

The Washington, DC area has seen its fair share of destructive storms–we get hurricanes, tornadoes and even the rare snowpocalypse. But on Friday night we got hit with another type of storm–one that I’d never heard of–called a derecho (pronounced ”deh-REY-cho”).

The storm swept through the area late Friday evening, bringing an incredible amount of thunder and lightning, winds up to 80 mph and sheets of rain. By morning, hundreds of trees had been blown down, millions were left without power and several people were dead. Netflix, Pinterest and Instagram had all been taken down by Amazon server outages caused by the storm. The Smithsonian Folklife Festival had to shut down for a day to clean up the mess. We were all left wondering, “what in the world had happened?”

Friday’s derecho originated near Chicago and raced southeast towards Washington, DC (courtesy of NOAA)

The stifling heat wave that we’d been suffering through, which had stretched from the Midwest through the mid-Atlantic to the Southeastern United States and brought temperatures in excess of 100 degrees Fahrenheit, was to blame for the fast-moving band of thunderstorms. The Capitol Weather Gang explains:

As this stifling air bubbled northward, clashing with the weather front draped from near Chicago to just north of D.C., thunderstorms erupted. They grew in coverage and intensity as they raced southeast, powered by the roaring upper level winds and fueled by the record-setting heat and oppressive humidity in their path.

The coverage and availability of this heat energy was vast, sustaining the storms on their 600 mile northwest to southeast traverse. The storms continually ingested the hot, humid air and expelled it in violent downdrafts – crashing into the ground at high speeds and spreading out, sometimes accelerating further.

Though unfamiliar to those of us here on the East Coast, derechos occur more commonly in the Corn Belt, which runs from Mississippi into the Ohio Valley, but even there they are relatively infrequent. They can wreak their havoc at any time of the year but are most likely to occur during May, June and July. Derechos get their starts in curved bands of thunderstorms called “bow echoes,” which are perhaps better known for their ability to spawn tornadoes. But instead of rotating cells of winds, derechos blow and travel in straight lines.

Derechos have a long history here in the United States. The term “derecho” was coined by University of Iowa physics professor Gustavus Hinrichs in an 1888 paper in the American Meteorological Journal in which he illustrated the path of such a storm that had crossed over Iowa on July 31, 1877. The storm’s straight path across the state gave Hinrichs the inspiration for the storm’s name–”derecho” means “straight” in Spanish. But path alone isn’t quite enough for a storm to qualify as a derecho; wind speeds must also reach a minimum of 57 mph.

Given that derechos are associated with warmer weather, could they become more common as the United States heats up due to climate change? Tom Kines, senior meteorologist at AccuWeather.com, told the Guardian: “If indeed we are seeing global warming, then it will certainly increase the risk of something like this happening again.”

The Turing test, a means of determining whether a computer possesses intelligence, requires it to trick a human into thinking it's chatting with another person

How can we decide whether a computer program has intelligence? In 1950, British mathematician Alan Turing, one of the founding fathers of computer science, proposed an elegantly simple answer: If a computer can fool a human into thinking he or she is conversing with another human rather than a machine, then the computer can be said to be a true example of artificial intelligence.

As we get ready to celebrate the 100th anniversary of Turing’s birth on Saturday, we’re still chewing on the Turing test. He predicted that by the year 2000, we’d have computers that could fool human judges as much as 30 percent of the time. We have yet to build a computer program that can pass the Turing test this well in controlled experiments, but programmers around the globe are hard at work developing programs that are getting better and better at the task. Many of these developers convene annually at the Loebner Prize Competition, an annual challenge in which the some of the world’s most sophisticated AI programs to try to pass themselves off as human in conversation.

Strike up a conversation with some of these chatbots to see just how human they might seem:

Rosette won the 2011 Loebner Prize. It was built by Bruce Wilcox, who also won the previous year’s award with the program’s predecessor, Suzette. Wilcox’s wife Sue, a writer, wrote a detailed backstory for Rosette, including information on her family, her hometown and even her likes and dislikes.

Cleverbot is a web application that learns from the conversations it has with users. It was launched on the web in 1997 and has since engaged in more than 65 million conversations. At the 2011 Techniche Festival in India, it was judged to be 59.3 percent human, leading many to claim it had successfully passed the Turing test.

Elbot, created by programmer Fred Roberts, won the 2008 Loebner Prize, convincing 3 of the 12 human judges that it was a human. In its spare time, it says, “I love to read telephone books, instructions, dictionaries, encyclopedias and newspapers.”

A.L.I.C.E. (which stands for Artificial Linguistic Internet Computer Entity) is one of the programming world’s classic chatbots, and won the Loebner Prize in 2000, 2001 and 2004. Although it has been outstripped by more recent programs, you can still chat with it and see how it revolutionized the field more than a decade ago.

Opinions vary on what caused the Hindenburg to explode so suddenly.

On May 6, 1937—75 years ago this week—the Hindenburg airship was about the complete its 35th trip across the Atlantic, having departed from Frankfurt, Germany and nearly arrived at Lakehurst, New Jersey. Then, suddenly, after thousands of miles of uneventful travel, the great zeppelin caught fire while less than 300 feet from the ground. Within a minute of the first signs of trouble, the entire ship was incinerated, and the burning wreckage crashed to the ground. Thirty-five of the 97 people on board perished in the disaster.

Then the finger-pointing began. From the very start, observers disagreed about what exactly sparked the explosion and what caused it to burn so quickly. In the years since, scientists, engineers and others have used science to weigh in on the debate and attempt to solve the mystery of the Hindenburg.

During an era of tension between the United States and Germany’s new Nazi government, suspicious minds quickly alighted on the idea that a crew member or passenger had sabotaged the airship, intentionally starting a fire. However, nothing more than circumstantial evidence was ever put forth to support the idea. Realistic alternatives for the cause of the explosion include a buildup of static electricity, a bolt of lightning or a backfiring engine, but at this point it’s impossible to determine what exactly caused the spark.

A different question is what provided the fuel for the explosion—and this is where the science really gets interesting. Initially, observers assumed that some of the lighter-than-air hydrogen that kept the ship aloft somehow leaked from its enclosed cells, mixing with the oxygen in the air to create an incredibly flammable substance. Photographs taken right after the initial explosion show lines of fire along boundaries between the fuel cells, and crew members stationed in the stern reported seeing the actual cells burn, supporting the idea that leaking hydrogen caused the craft to explode so violently. Many have theorized that, during one of the sharp turns the ship took just before exploding, one of the bracing wires inside snapped, puncturing one of the cells.

Then, in 1996, retired NASA scientist Addison Bain, who had years of experience working with hydrogen, presented a new idea: the incendiary paint hypothesis. As part of his argument that hydrogen can be safely used for transportation and other purposes, Bain claimed that the fire was initially fueled by a special paint used on the zeppelin’s skin. The varnish compound included chemicals such iron oxide, which can be used as rocket fuel.

Bain also pointed out that the hydrogen inside the cells had been given a garlic scent, to help crew members detect a leak, but no one reported smelling garlic at the time of the explosion. He also said that a fire fueled by hydrogen would produce a blue flame, but the fire was a bright red. In his scenario, the mystery spark would have ignited the varnish rather than leaking hydrogen—meaning that a design flaw, rather than the inherent risks of hydrogen, had caused the disaster.

In 2005, a team of researchers led by A.J. Dessler, a physicist at Texas A&M, published a detailed study in which they attempted to determine whether the chemicals in the varnish could possibly account for the fire. Their answer: no way. Their calculations indicate that, if fueled by the paint alone, the airship would have taken roughly 40 hours to burn completely, rather than the 34 seconds it took for it to be consumed. In the lab, they burned replica pieces of the Hindenburg‘s outer covering, which confirmed their theoretical calculations—and indicated that the paint alone could not have fueled the fire.

So, more than 75 years later, we’re still not quite sure what to believe about the Hindenburg disaster. Can the use of hydrogen gas in transportation be safe? Or is a vehicle filled with flammable gas simply an accident waiting to happen? However it was caused, the terrible explosion had one long-lasting effect: It permanently put airship travel on the back burner.

Read about a new exhibit at the Smithsonian’s National Postal Museum about the Hindenburg and read an eyewitness account of the disaster from a grounds crew member.

The Islamic Empire (top) and Baghdad (bottom), circa 770-910 AD. Images courtesy of the journal Weather

How do scientists reconstruct the climate of the past? They often turn to ice cores or growth rings from trees or deep-sea corals. But a new study gleans a wealth of weather intel from a largely untapped source: old documents.

Researchers from Spain scoured manuscripts from 9th- and 10th-century Baghdad, in modern-day Iraq, for references to the weather. Baghdad, where the Tigris and Euphrates Rivers meet, was at that time the new and bustling capital of the vast Islamic Empire, which stretched from India to the Atlantic Ocean. Much was written about the city and why it was chosen as the capital, including its population size, agricultural potential and climate.

In the 10 analyzed texts, most of which give exhaustive political histories of the region, the researchers found 55 meteorological citations, many of which were referring to the same event. The study points out that although the social and religious content of the documents is probably biased, the historians weren’t likely to fabricate an off-hand mention of a drought, hail storm or solar eclipse.

The researchers were shocked by the number of references to cold periods in this notoriously hot and dry region. They identified 14 chilly periods in all: five in winter, two in spring, one in summer and two that denoted cold weather for a whole year. Some of the descriptions specified snowfalls, ice and frozen rivers.

For instance, an entry from December 23, 908, noted when “four fingers of snow accumulated on the roofs,” and another, on November 25, 1007, that the snow reached somewhere between 30 and 50 inches. One particularly odd event was in July 920, when it was too cold for people to sleep on their roofs, as they did on most summer nights. This temperature drop could have been caused by a volcanic eruption the previous year, the researchers speculate.

In any case, it seems safe to say that the weather of that Islamic Golden Age was much more variable than it is today. The only time that snow has hit Baghdad in modern memory was on January 11, 2008, melting as soon as it hit the ground.

Images from Domínguez- Castro et al., ”How useful could Arabic documentary sources be for reconstructing past climate?” appearing in Weather, published by Wiley.