Earthquakes are one of most powerful, and sometimes most devastating, forces in nature. Scientists have devised a magnitude system to describe how the power of an earthquake increases. The great Tohuku earthquake in Japan in March 2011, for example, was a magnitude 9. The earthquake that rattled Washington D.C. and the U.S. east coast in August 2011 was a magnitude 5.8. What’s the difference? What does the earthquake magnitude system really mean? The animation below – from oceanographer Nathan Becker at the Pacific Tsunami Warning Center – can help you understand it.

Each earthquake magnitude is 33 times more powerful than the one before. So each jump in magnitude means a lot!

A magnitude-8.0 earthquake is 33 times stronger than a magnitude-7.0 earthquake.

A magnitude-9.0 earthquake is 1,089 (33 x 33) times more powerful than a 7.0.

Bottom line: Each jump in earthquake magnitude represents 33 times more energy release than the magnitude before. So each jump in magnitude means a lot! This animation oceanographer Nathan Becker at the Pacific Tsunami Warning Center can help you picture it.

Will Japan’s big quake in 2011 lead to more earthquake predictability?

You must find a way to protect your eyes if you plan to watch either the annular solar eclipse on May 20-21, 2012 or the transit of Venus on June 5-6, 2012 – or both events. You have options.

• Solar eclipse glasses from commercial manufacturers.
• Welder’s glass, #14 or darker.
• A home-rigged indirect viewing method.
• A local astronomy club that is hosting a viewing.

Whatever you do, never look at the sun directly without a safe filter in place to protect your eyes. That is, unless the sun is totally eclipsed by the moon – which is not happening in May or June 2012. Here are some more things you should not use. Do NOT use sunglasses, polaroid filters, smoked glass, exposed color film, x-ray film, or photographic neutral density filters.

Watching a solar eclipse through special filters. You could use these to watch the May 20-21 annular or ring eclipse, and the June 5-6 transit of Venus.

Find eclipse times for your location on the globe: Annular eclipse of sun – China to Texas – on May 20-21

Solar eclipse glasses. It’s too late to order these online now for the May 20-21 solar eclipse. But you might find them at a local nature center, or museum.  And you can definitely get them in time for the June 5-6 transit of Venus. Solar eclipse glasses are super easy to use, and they’re sort of cool-looking.  Order some now for the Venus transit or next eclipse.  Search on the words “solar eclipse glasses [your country].” I ordered some from Rainbow Symphony.  Or try a great organization, Astronomers Without Borders (whose motto is “One people, one sky”), in which case the proceeds will go to benefit astronomy programs worldwide. I found that they had a minimum order of 100, which is why I went elsewhere. To order from Astronomy Without Borders, look here: Astronomy Without Borders solar eclipse glasses.

Welder’s glass. Be sure it is #14 or darker. The great thing about welder’s glass is that it allows you to view the eclipse directly. Plus welder’s glass is a bit more durable than commercial eclipse glasses. If you’re like me, you’ll forget where you put the commercial eclipse glasses by the next eclipse. With the welder’s glass, you can always add it to your rock collection.  Search for a local “welding supply” company.

To determine if you can see the transit of Venus, look here: Last transit of Venus in 21st century on June 5-6, 2012

Indirect viewing. Creating a pinhole camera is probably your best last-minute option, because it lets families and friends view together. We recommend this article by masters of do-it-yourself science at Exploratorium in San Francisco. Their article on how to view solar eclipses safely teaches you to make an easy pinhole projector. With it, you can shine the sun’s image onto a flat surface and impress your friends and neighbors while giving everyone (including yourself) an unforgettable experience.

Astronomy club local viewing. Find astronomy clubs or events in your location

Bottom line: You basically have several options for safely viewing a solar eclipse: commercial eclipse glasses, #14 or darker welder’s glass, indirect viewing through a pinhole camera, or finding a local viewing event. Never look at the sun directly without some protection in place for your eyes!

Hear from an expert: Eye safety during solar eclipses

More about eclipse filters

Sometimes you look up on a clear day or night and see a huge circle of light around the sun or moon. This ring or circle is called a halo by scientists. We get many messages throughout each year from people who’ve just spotted a ring around the sun or moon. Solar and lunar haloes are pretty common, but they’re so mysterious-looking that people often express amazement upon seeing them. Yesterday – May 16, 2012 – EarthSky’s Facebook page received a rash of photos of a beautiful sun halo seen in Washington state.

Sun halo seen in Washington state on May 16, 2012. Image via EarthSky Facebook friend Sean Abbasi

Notice in the photos that the sky looks fairly clear. After all, you can see the sun. And yet halos are a sign of high thin cirrus clouds drifting 20,000 feet or more above our heads.

Sun halo on May 16, 2012 via EarthSky Facebook friend Nonya Justagirl

These clouds contain millions of tiny ice crystals. The halos you see are glints of light from these ice crystals, which have to be oriented and positioned just so with respect to your eye, in order for the halo to appear.

Sun halo seen in Washington state on May 16, 2012. Image via EarthSky Facebook friend Jana Kitty Daze.

That’s why, like rainbows, halos around the sun – or moon – are personal. Everyone sees their own particular halo, made by their own particular ice crystals, which are different from the ice crystals making the halo of the person standing next to you.

Lunar halo via master sky photographer Dan Bush.

Because moonlight isn’t very bright, lunar halos are mostly colorless, but you might notice more red on the inside and more blue on the outside of the halo. These colors are more noticeable in halos around the sun. If you do see a halo around the moon or sun, notice that the inner edge is sharp, while the outer edge is more diffuse. Also, notice that the sky surrounding the halo is darker than the rest of the sky.

By the way, there’s an old weather saying: “ring around the moon means rain soon.” There’s truth to this saying, because high cirrus clouds often come before a storm.

May’s birthstone is the emerald.

The emerald belongs to the beryl family of minerals that include aquamarine (one of March’s birthstones), heliodor, and morganite. Beryl, or beryllium aluminum silicate in chemical jargon, is a six-sided symmetrical crystal. Beryl contains beryllium, aluminum, silicon, and oxygen.

Emeralds vary in color from light to deep green. It’s commonly thought that an emerald’s color derives from the presence of chromium and/or vanadium replacing some of the aluminum in the mineral’s structure. The stone can, however, lose its color when heated strongly.

Several famous historical artifacts were made of emeralds. Among them is the Crown of the Andes, said to be made from emeralds worn by Atahualpa, the last Inca (king) of Peru. The crown is set with about 450 emeralds, collectively weighing 10 ounces (1523 carats).

Emeralds are most frequently found inside a form of shale – a fine-grained sedimentary rock. Emerald-bearing shale has undergone recrystallization caused by changes in the physical environment such as pressure and temperature. Colombia produces the largest and highest quality emeralds. They were also discovered, and subsequently mined, in the Ural Mountains of Russia around 1830. In the United States, emeralds can be found in North Carolina. Around the world, they also occur in Zambia, Brazil, Pakistan, Norway, Austria, India, Madagascar, and Australia.

Synthetic manufacture of emeralds was achieved by German chemists shortly before World War II. But growing synthetic stones of fine quality began in the United States in 1946. There are also excellent imitation emeralds on the market made of colored cut glass.

The emerald’s name is indirectly derived from the Greek word “smaragdos,” a term applied to several kinds of green stones. The history of emeralds can be traced back to antiquity. They were worn by royalty in Babylon and Egypt. Tools dating back to 1300 B.C., during the reign of Rameses II, have been found in emerald mines in Egypt. Queen Cleopatra’s emeralds were believed to originate from mines in Southern Egypt, near the Red Sea.

When the conquistadors first arrived in South America from Spain, they saw indigenous rulers wearing emeralds. They took large quantities of emeralds from the Peruvians during the invasion, but the source of the emeralds was not discovered. Then in 1537, the Spaniards found Chivor in Colombia, now the location of an important emerald mine. They also took over the Muzo mine following the defeat of the Muzo Indians. Mining operations at Muzo have continued almost uninterrupted since the Spanish invasion. It is perhaps the most famous emerald mine in Colombia and is said to produce the world’s best emeralds.

There are many myths associated with the emerald. The stone was once believed to prevent epilepsy, stop bleeding, cure dysentery and fever, and protect the wearer from panic. Its magnificent green color was said to rest and relieve the eye. To the ancient Romans, emeralds were dedicated to the goddess Venus because the green emerald symbolized the reproductive forces of nature. Early Christians saw it as a symbol of the resurrection of Christ. In the Middle Ages, emeralds were believed to hold the power to foretell the future.

Find out about the birthstones for the other months of the year.
January birthstone
February birthstone
March birthstone
April birthstone
May birthstone
June birthstone
July birthstone
August birthstone
September birthstone
October birthstone
November birthstone
December birthstone

Image Credit: Rob Lavinsky

It’s May Day 2012 – May 1 on the calendar – and my neighborhood has already had its annual huge May Day celebration, which includes wrapping ribbons around a Maypole. May Day is an ancient spring festival in the Northern Hemisphere. It’s an astronomical holiday, one of the year’s four cross-quarter days, or day that falls more or less midway between an equinox and solstice – in this case the March equinox and June solstice. The other cross-quarter days are Groundhog Day on February 2, Lammas on August 1 and Halloween on October 31. May Day also stems from the Celtic festival of Beltane, which was related to the waxing power of the sun as we move closer to summer. At Beltane, people lit fires through which livestock were driven and around which people danced, moving in the same direction that the sun crosses the sky.

Maypole wrapping at Bryn Mawr College in Pennsylvannia in 2005. May Day festivities are an annual tradition at Bryn Mawr. Image Credit: Mike Goren via Wikimedia Commons

School children rehearsing Maypole festivity, in Gee's Bend, Alabama, 1939. Image Credit: Wikimedia Commons

A group of happy neighbors in Texas, after wrapping a Maypole in 2012. See the wrapped pole being held up? Image Credit: Rick Patrick

Wrapping a Maypole with colorful ribbons is perhaps the best known of all May Day traditions. In the Middle Ages, English villages all had Maypoles, which were actual trees brought in from the woods in the midst of rejoicing and raucous merrymaking. Maypoles came in many sizes, and villages were said to compete with each other to show whose Maypole was tallest. Maypoles were usually set up for the day in small towns, but in London and the larger towns they were erected permanently.

We’re not too far away from a time in the late 20th century when people left homemade May baskets filled with spring flowers and sweets on each others’ doorsteps, usually anonymously. I can remember doing this as a child. Maybe it’s a tradition that can be revived.

Homemade May basket left on neighbor or friend's doorstep anonymously. Nice tradition!

Bottom line: May Day on May 1 is one of four cross-quarter days, midway between an equinox and a sosltice. It stems from the ancient festival of Beltane, which relates to the waxing power of the sun at this time of year. Its most recognized tradition is the Maypole dance, in which people wrap ribbons around a tall pole.

Meteors are usually dust particles left behind by comets – bits of debris that enter Earth’s atmosphere from space. At what altitude do these meteors – sometimes called “shooting stars” or “falling stars” – become incandescent and begin to glow? That can vary.

Image credit: Dominic's pics

Meteors become incandescent – or glow – almost as soon as they hit Earth’s atmosphere. Some meteors such as the Perseids in August burn up in the atmosphere at about 100 kilometers – or 60 miles – above Earth’s surface.

Other meteors such as the Draconids in October fall to 70 kilometers – or about 40 miles – before they heat up enough to glow and vaporize. The difference is that the Draconids are much slower meteors than the Perseids. So the height in the atmosphere at which a meteor begins to glow depends on its arrival speed.

There are a dozen major meteor showers every year – and many more minor ones. Here are some meteor arrival speeds:

Leonids: 71 kilometers per second
Perseids: 61 kilometers per second
Orionids: 67 kilometers per second
Lyrids: 48 kilometers per second
Geminids: 35 kilometers per second
Fall Taurids: 30 kilometers per second
Delta Leonids: 23 kilometers per second
Draconids: 23 kilometers per second

By the way, the length of a meteor’s path across the sky doesn’t depend entirely on the meteor’s arrival speeds. It depends mostly on the angle at which the particle of dust slices through the atmosphere. If the particle arrives at a low angle, it enters the atmosphere more gradually, heats up more slowly, and cuts a longer swath across the sky than if it barrels in at a steep angle.

The size, composition, and density of the dust particle probably also affect the length of the path – but scientists still aren’t sure exactly how.

When you look at a crescent moon shortly after sunset or before sunrise, you can sometimes see not only the bright crescent of the moon, but also the rest of the moon as a dark disc. That pale glow on the unlit part of a crescent moon is light reflected from Earth. It’s called “earthshine.”

What’s happening in the sky tonight: Daily sky charts and more

Photo credit: carolune

To understand earthshine, remember that the moon is globe, just as Earth is, and that the globe of the moon is always half-illuminated by sunlight. When we see a crescent moon in the west after sunset, or in the east before dawn, we’re seeing just a sliver of the moon’s lighted half.

Now think about seeing a full moon from Earth’s surface. Bright moonlight can illuminate an earthly landscape on nights when the moon is full.

Likewise, whenever we see a crescent moon, a nearly full Earth appears in the moon’s night sky. The full Earth illuminates the lunar landscape. And that is earthshine. It’s light from the nearly full Earth shining on the moon.

So next time you see a crescent moon, expand your thinking – to include the Earth under your feet. See the glow on the unlit portion of the moon for what it really is – sunlight reflected from the nearly full Earth shining in the moon’s sky.

Moon with earthshine and planet Venus. Photo credit: fdecomite

Light and raindrops work together to create a ring of colored light opposite the sun.

Gallery: Rainbows around the world

We see part of that ring as the curved arc of a rainbow. Here’s how it works: the sunlight that shines into a raindrop leaves that raindrop at an angle of 138 degrees from the path that the light traveled before it entered the drop. That’s the “rainbow angle,” discovered by Descartes in the year 1637.

If the light left at 180 degrees, it’d head straight back toward the sun. As it is, the light is traveling in a direction somewhat back toward the sun, which is why you always see rainbows when the sun is behind you.

The sunlight emerges from many raindrops at once. The combined effect is a mosaic of light, spread out in an arc in the sky. But that’s not the end of the story. When sky conditions and your vantage point are perfect, the rain and sun work together in this way to create a complete ring of light – a circle rainbow. You’ll never see this from Earth’s surface because your horizon gets in the way.

Imagine standing on a beach, a mountaintop or any open area, watching a beautiful sunset. As the sun sets, it appears redder and redder as it gets closer to the horizon. Sometimes you can see a gradation of color a across the surface of the sun. Why does this happen?

If you saw a sunset on the moon, our local star, the sun, would look white. That’s because the moon doesn’t have an atmosphere.

Image courtesy EarthSky Facebook friend Tim Haynes in the U.K.

But, unlike the moon, Earth does have an atmosphere. And most of us know that white light – like sunlight – is composed of all the colors of the rainbow. Tiny molecules in our atmosphere cause light to scatter. That’s why our sky looks blue: it’s because the atmosphere scatters the bluish component of white sunlight.And it’s why the sun looks reddish when it’s near the horizon.

No matter where you are on Earth, as you look toward a horizon, you're looking through more atmosphere than when you look overhead. Image via The Bad Astronomer

Think about what happens as the sun sinks lower in the sky. Its light has to travel farther and farther through the atmosphere before reaching your eyes, simply because there’s more atmosphere in the direction toward the horizon than overhead.

Sunlight encounters more air molecules when the sun is low in the sky than when the sun is overhead. Even more blue light is scattered away, leaving mostly the reddish component of white sunlight to travel the straighter path to your eyes. So the setting sun looks red.

Bottom line: The red color of a sunset is due to the fact that, when we look toward any horizon, we’re looking through more of Earth’s atmosphere than when we look overhead.

See more great photos of sunsets – or post your own – on EarthSky’s Facebook page. Thanks to all who posted sunset pics! Glorious.

Sunset over Naxos Island, Greece, via EarthSky Facebook friend Manos Makrakis

Sunset Nord-Tronderlag, Norway via EarthSky Facebook friend Geir Walmann

Sunset over Port Said, Egypt via EarthSky Facebook friend Dr. Ahmed Abdel-Azeem

Sunset over North Dakota via EarthSky Facebook friend Janet Marohi

Sunset over Pacific Ocean, seen from Ventura, California via EarthSky Facebook friend Cyn Lee Masters

April’s birthstone is the diamond.

Diamonds are the rich cousins of graphite. Both are crystalline forms of pure carbon. The enormous differences in their properties are a result of the way the carbon atoms are bonded together. In graphite, carbon atoms are arranged in sheets that easily slide past each other, which makes graphite ideal as a lubricant and, of course, pencil lead. Diamond crystals, on the other hand, are a tight-fisted network of carbon atoms securely held in four directions, making it the hardest naturally-occurring substance in the world.

In order to achieve such a compact and strongly-held network of carbon atoms, it is believed that diamonds must have crystallized deep under the Earth’s surface. At these depths the proper conditions for the formation of diamonds exist; at 90 to 120 miles deep, pressures are more than 65,000 times that of the atmosphere at the Earth’s surface, with temperatures exceeding 2,700 degrees Fahrenheit. Such pressures and temperatures reproduced in laboratories have successfully yielded synthetic diamonds.

There are many kinds of diamonds: transparent, translucent, or opaque; ranging from colorless to sooty black, with many colors in between. Mostly transparent diamonds, colorless or tinted, are used as jewelry. Others are used widely in industry. The color of a diamond depends on the kind of impurities embedded inside it. Yellow diamonds, for example, betray minute quantities of nitrogen, while boron imparts a bluish hue. There are other inclusions in diamonds that have great scientific value. Such samples are time capsules that yield valuable information about conditions deep in the Earth’s upper mantle where diamonds formed, as well as clues to the formation and age of the diamond.

Diamonds are found in alluvial deposits – gravel swept by streams, rivers, glaciers, and ocean currents. They are also found in sedimentary rock where gravel deposits and organic material have been compressed into rock. Diamonds can be found in some samples of kimberlite – a type of volcanic rock first identified in Kimberley, South Africa. Diamonds found in kimberlite are thought to be very old, perhaps as much as three billion years old. Tiny flecks of diamond have even been found inside meteorites – bits of rocky space debris that land on Earth.

Diamonds are crystals. Crystals are the ultimate form of symmetry in nature. Their shape reflects the internal orderly arrangement of atoms within the crystal. In diamonds, atoms of carbon are held tightly by covalent bonding, where two neighboring atoms share an electron, endowing the diamond crystal with great strength. But despite that hardness, diamonds can be cut with saws and polished with grinding wheels coated with tiny industrial diamond fragments. In their natural form, diamonds can appear quite unimpressive. They are cut and polished by skilled craftsmen in a pattern that reflects and refracts the light among its facets to reveal the hidden beauty of the stone.

Diamonds’ cold, sparkling fire has held us spell-bound for centuries, inspiring rich, passionate myths of romance, intrigue, power, greed, and magic. Ancient Hindus, finding diamonds washed out of the ground after thunderstorms, believed they were created by bolts of lightning. In our place and time, the diamond is a symbol of enduring love, and often graces engagement rings.

Some diamonds seem to have lived lives of their own. One legendary stone in the diamond hall of fame is the Koh-i-noor (“Mountain of Light”). The Koh-i-noor diamond’s early history is shrouded in time. It is believed to be 5,000 years old, and was featured in the great Sanskrit epic The Mahabharata. Originally owned by the Rajah of Malwa in India, the Koh-i-noor has since been a player in victories and defeats spanning India, Persia, and Afghanistan. It was in the possession of the great Mogul dynasty from 1526 to 1739. Its owners included Shah Jehan, who built the Taj Mahal in memory of his queen Mumtaz. The Persian invader Nadir Shah briefly possessed it until his assassination in 1747. The jewel then fell into the hands of Afghan rulers who eventually surrendered it to the Rajah of Punjab, Ranjit Singh.

Two years after Ranjit Singh’s death in 1839, Punjab became part of India under British rule. The stone was presented to Queen Victoria, who had it cut from its original 187 carats to 108 carats in an attempt to further enhance its beauty. After her death, the diamond became part of the British crown jewels. Queen Elizabeth (later the Queen Mother) wore it in her crown at her 1937 coronation.

Find out about the birthstones for the other months of the year.
January birthstone
February birthstone
March birthstone
May birthstone
June birthstone
July birthstone
August birthstone
September birthstone
October birthstone
November birthstone
December birthstone

Image Credit: Rob Lavinsky