February’s birthstone is the amethyst. Amethysts contain the second most abundant mineral found in Earth’s crust – quartz. Quartz is often found lining the insides of geodes. So it’s no wonder that geodes sometimes contain amethysts, too. Like quartz, amethysts are a transparent form of silicon dioxide (SiO2). An amethyst’s color can range from a faint mauve to a rich purple. It’s not clear why they’re purple. Some scientists believe the purple color arises from the amethysts’ iron oxide content, while others attribute the color to manganese or hydrocarbons.

Amethysts are very sensitive to heat. When heated to 400 or 500 degrees Celsius, an amethyst’s color changes to a brownish-yellow or red. Under some circumstances, the stones can turn green when heated. Heat may even transform an amethyst into a naturally-rare mineral called citrine. And even without heating, the violet color of an amethyst may fade over time.

Commercial sources of amethyst are Brazil and Uruguay; while in the U.S., most amethyst is found in Arizona and North Carolina.

The amethyst has a rich history of lore and legend. It can be traced back as far as 25,000 years ago in France, where it was used as a decorative stone by prehistoric humans. It has also been found among the remains of Neolithic man.

It’s said that the signet ring worn by Cleopatra was an amethyst, engraved with the figure of Mithras, a Persian deity symbolizing the Divine Idea, Source of Light and Life. It is also said to be the stone of Saint Valentine, who wore an amethyst engraved with the figure of his assistant, Cupid. Saint Valentine’s Day is still observed in February.

The word amethyst comes from the Greek word “amethystos” meaning “not drunk,” and was believed to prevent its wearers from intoxication. The following is a story from Greco-Roman mythology, as quoted from Birthstones by Willard Heaps:

“Bacchus, the god of wine in classical mythology, was offended by Diana the huntress. Determined on revenge, he declared that the first person he met as he went through the forest would be eaten by his tigers. As it happened, the first person to cross his path was the beautiful maiden Amethyst on her way to worship at the shrine of Diana. In terror, she called upon the goddess to save her, and before his eyes, Bacchus observed the maiden changed to a pure white, sparkling image of stone. Realizing his guilt and repenting his cruelty, Bacchus poured grape wine over her, thus giving the stone the exquisite violet hue of the amethyst. The carryover to non-intoxication was quite logical, and in ancient Rome, amethyst cups were used for wine, so drinkers would have no fear of overindulgence.”

The early Egyptians believed that the amethyst possessed good powers, and placed the stones in the tombs of pharaohs. During the Middle Ages, it was used as medication, believed to dispel sleep, sharpen intellect, and protect the wearer from sorcery. It was also believed to bring victory in battle. In Arabian mythology, the amethyst was supposed to protect the wearer from bad dreams and gout.

February’s birthstone, the amethyst, was the stone of royalty, representing power. See the birthstones for the rest 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 Credits: Ra’ike and Wikimedia

Venus is so much brighter than any other planet viewed in the sky. Why is it so bright?

Photo credit: James Jordan

Astronomers use the term “albedo” to describe how bright a planet is. When light strikes a planet, some is absorbed by the planet’s surface or atmosphere – and some is reflected. Albedo is a comparison between how much light strikes an object – and how much is reflected.

Venus has the highest albedo of any planet in our solar system. Venus is so bright partly because it reflects over 70 percent of sunlight striking it. It owes its reflective ability to the fact that it’s blanketed with clouds. Sunlight bouncing from these clouds is what makes Venus so bright.

When the moon is close to full, it can look a lot brighter than Venus, but the moon reflects only about 10 percent of the light that hits it. The moon has a low albedo of around .1, meaning that it reflects about 10% of the light that it receives. Venus, on the other hand, is the most reflective object in our solar system, with an albedo of close to .7. The moon has a low albedo because it’s made of volcanic rock. It appears bright to us only because it’s so nearby.

The clouds in the atmosphere of Venus contain droplets of sulfuric acid – one of the eye-stinging ingredients in our urban smog – as well as acidic crystals suspended in a mixture of gases. Light bounces easily off the smooth surfaces of these spheres and crystals. That’s one reason the clouds of Venus are so good at reflecting light.

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

At this writing (January 24, 2012), the sun is in an active part of its 11-year cycle of activity. Many dark sunspots are visible to those using telescopes and solar filters. Space observatories are detecting brilliant solar flares – intense bursts of radiation and our solar system’s largest explosive events – lasting minutes to hours on the sun’s surface. Occasional, powerful coronal mass ejections, or CMEs – giant bubbles of gas and magnetic fields from the sun, containing up to a billion tons of charged particles that can travel up to several million miles per hour – are sometimes released into the interplanetary medium. This solar material streams out through space, and sometimes strikes Earth. Is this dangerous? Should we be worried?

Strongest solar radiation storm in 7 years expected January 24

A solar flare as observed by NASA’s Solar Dynamics Observatory (SDO) on January 23, 2012. Image Credit: SDO

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The answer is no. These solar storms are awesome to contemplate, but they are not dangerous to us on Earth’s surface.

What is the danger of a solar storm in space? Very high-energy particles, such as those carried by CMEs, can cause radiation poisoning to humans and other mammals. They would be dangerous to unshielded astronauts, say, astronauts traveling to the moon. Large doses could be fatal.

But, for us on Earth’s surface, solar storms aren’t dangerous because we’re protected by Earth’s blanket of atmosphere. Earth’s atmosphere and magnetosphere allow adequate protection at ground level.

An illustration of Earth's magnetic field shielding our planet from solar particles. Credit: NASA/GSFC/SVS

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On the other hand … solar storms are dangerous to our technology. When a coronal mass ejection, or CME, strikes Earth’s atmosphere, it causes a temporary disturbance of the Earth’s magnetic field. The solar storm causes a geomagnetic storm.

The most powerful solar storms send their coronal mass ejections, containing charged particles, into space. As the charged particles slam into Earth’s atmosphere, they can disrupt satellites and bathe high-flying airplanes with radiation. They can disrupt telecommunications and navigation systems. They have the potential to black out entire cities.

But, when a solar storm occurs, it takes several days for the charged particles to reach Earth. When a big coronal mass ejection is on its way, satellites can briefly shut their systems off. Earth-based power grids can be reconfigured to provide extra grounding. And so on.

A solar prominence is vast and awesome in size in contrast to our little Earth. But the Earth is so far from the sun that these prominences pose no danger. Image via NASA

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The current solar cycle – called Sunspot Cycle 24 by space physicists – is expected to peak in February 2013, according to NASA. The number of storms on the sun was high in late 2011 and is still high at this writing (early 2012). Still, according to current predictions, this sunspot cycle is the smallest in over 80 years.

There’s every reason to believe that storms on the sun have been happening for billions of years, since the sun and Earth came to be. If that’s so, then all life on Earth evolved under their influence. So as we approach another peak in activity, the sun isn’t doing anything it hasn’t done many millions of times before. The difference is that now we have technology that can be affected by the sun’s activity.

Bottom line: Storms on the sun are a natural occurrence. They are not dangerous to humans on Earth’s surface. They have been happening for billions of years.

Who will see the aurora on January 24, 2012?

People tell me when I go to the beach to look at the sunset to try to see a green flash. What is a green flash?

The green flash is an optical phenomenon that you can see shortly after sunset or before sunrise. It happens when the sun is almost entirely below the horizon, with the barest edge of the sun – the upper edge – still visible. For a second or two, that upper rim of the sun will appear green in color. It’s a brief flash of the color green – the legendary green flash. It’s really quite exciting to see, especially if you’ve been looking for one.

It’s said that once you’ve seen a green flash, you’ll never again go wrong in matters of the heart.

Green flash over clouds, observed from a mountain on January 10, 2010. Image Credit: Amiteshomar via Wikimedia Commons

Green flash over the North Sea, Belgium on October 6, 2010. Image Credit: Hans Hillewaert via Wikimedia Commons

How can you see a green flash? Here’s all you need:

• A clear day with no haze or cloud on the horizon.
• A distant horizon – and a distinct edge to the horizon. You can see the green flash from a mountaintop or high building. But it’s most often seen over the ocean, by people on beaches or in boats.

Important tip: Don’t look at the sun until it is nearly entirely below the horizon. If you do, you will dazzle (or damage) your eyes and ruin your green flash chances for that day.

Since you need to know exactly where to look along the horizon, and since most of us aren’t up before dawn, green flashes are most often seen after sunset. But diligent observers can see them before dawn, too. And, although they’re most often seen over the ocean, you can see green flashes over land, too, if your horizon is far enough away.

What is the green ray? The flash can be like a flame that shoots above the horizon. In that case, it’s called a green ray. I’ve seen lots of green flashes, but never a green ray, although I was once walking on a beach in Mexico and turned away just as my companion saw one. I did not find any photos of flamelike green rays (if you know of one, let me know), but the photo below suggests the beginnings of a ray.

Mock mirage and green flash seen from San Francisco. 2006. Image Credit: Mila Zinkova via Wikimedia Commons

What makes a green flash? The green flash is the result of looking at the sun through a greater and greater thickness of atmosphere as you look lower and lower in the sky. Water vapor in the atmosphere absorbs the yellow and orange colors in white sunlight, and air molecules scatter the violet light. That leaves the red and blue-green light to travel directly toward you. Near the horizon, the sun’s light is highly bent or refracted. It’s as though there are two suns – a red one and a blue-green one – partially covering each other. The red one is always closest to the horizon, so when it sets or before it rises, you see only the blue-green disk – the green flash.

Photographer captures rare green flash from moon

As seen from the Northern Hemisphere, the stars seem brighter in winter. Why? It’s because – as seen from this hemisphere – we’re actually looking toward many, many more stars in summer than in winter.

In summer, our evening sky is facing toward the center of the Milky Way galaxy – some 25,000 to 28,000 light-years away. We don’t see into the exact center because it’s obscured by galactic dust. But, as we peer in summer edgewise into the galaxy’s disk, the hazy quality of the summer sky is really the combined light of billions of stars in the direction of the galaxy’s center.

In winter, we’re looking the opposite way – into the spiral arm of the galaxy in which our sun resides. The winter stars tend to be closer to us – and there really are some gigantic stars located in this direction. We’re looking edgewise into the disk of the galaxy in winter, too. But we’re looking toward the outskirts of the galaxy – so we’re seeing far fewer stars, and we’re looking more deeply into the space beyond our galaxy’s boundaries. That’s why the winter sky has a clearer, sharper quality than the summer sky.

What would happen to an apple on the surface of the planet Mars?

It’s a striking vision: a bright green or red juicy apple against the barren red rocks of Mars. But an apple on the martian surface would shrivel like a raisin in a matter of minutes. Its juices would boil away into vapor almost immediately. With its liquid gone, the apple would essentially become mummified.

Image Credit: USDA

What’s more, Mars is colder than Earth. That dried-out apple on Mars would freeze. Soon, you’d have a freeze-dried mummy of an apple.

But here’s the good news, apple lovers. The apple wouldn’t become rotten. You need bacteria to enable something to rot, and there are no bacteria on Mars.

On the other hand, Mars does have a lot of wind. So the apple might be buried by blowing dust. In that case, the martian soil would corrode the apple – in about a million years.

If the apple didn’t get buried in a martian windstorm, it’d be exposed to intense ultraviolet radiation from the sun. That would turn the apple’s skin black and tarry. But, underneath its blackened skin, the freeze-dried apple would be unchanged.

So you could come back a thousand years later, brush off the dust – or scrape off the tar – and eat the apple. Yummy!

And that’s the fate of an apple left behind by a future astronaut on the surface of the planet Mars.

What makes a rooster decide to crow? The answer from scientists is that it has something to do with an alarm clock.

A rooster crows because he has an internal clock that helps him anticipate sunrise. Like all birds, roosters sing – or crow – in a daily cycle. Almost all animals have daily cycles of activity known as circadian rhythms that roughly follow the cycle of day and night. Roosters anticipate sunrise to get a head start on their daily hunt for food and defense of territory.

But if one rooster in the neighbor has an internal clock that’s set a little early, he can stimulate other roosters to crow early, too. The rooster’s sunrise song is actually a way of establishing his territory. When a rooster crows, he’s sending a signal to other roosters that if they trespass, they’re asking for a fight.

A rooster will often crow from a vantage point above his territory so he can make others more aware of his presence and so that his songs travel farther. Even though roosters are the most famous crooners of the chicken world, hens aren’t exactly silent, either. When a hen spots a hawk, she’ll let out a harsh scream to send her chicks into hiding. But if she sees a less-threatening human, she might just cackle.

Photo credit: normalityrelief

We’ve gotten lots of messages from people who have seen rings around the moon and wonder what they are. Sometimes you look up on a clear night and see a huge circle of light around the moon. This circle is called a lunar halo. We get many messages each month from people who’ve seen rings around the moon. They’re pretty common, but they’re so mysterious looking that people often express amazement upon seeing them.

Some names for the February full moon

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

That’s why, like rainbows, halos around the moon – or sun – 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.

The lunar halo photo at right is by 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.

You might have heard the saying “like a moth to a flame” to describe a fatal attraction. But why are moths attracted to flame?

Photo credit: beinggreen

The fact is scientists think moths aren’t so much attracted to the light of a flame or other bright light as they are disoriented by it. Here’s how it works. Like many flying insects, moths are able to find their way partly by using light as a compass. When the source of light is the sun or moon, that light source is very distant, and the incoming light rays that strike the insect arrive just about parallel to each other.

So moths – and many other flying insects – have evolved to expect to receive light at a fixed part of the eye. As long as the moth flies more or less in a straight line, this visual pattern remains unchanged.

Now consider what happens when the light source is a nearby candle. Then the angle at which the light strikes the moth’s eye quickly changes while the moth holds to a straight-line course. The moth tries to do what it has evolved to do under the light of the sun or moon – that is, maintain a constant angle to the source.

And as it does so, it spirals in toward the light.

So the moth seems “attracted” to the light – so much so that it might end up drawn into the flame.