Actually, there are bars at the center of most spiral-shaped galaxies.  But you won’t find any drunken Wookies or neon Budweiser signs, only vast sweeps of dust and stars.

A barred spiral galaxy

Galaxies are enormous, turning cities of stars. There may be a hundred billion galaxies in the universe, separated by vast stretches of mostly empty space. Galaxies come in several basic shapes; the most common are a simple elliptical or a spiral, like a pinwheel.

Our home galaxy, the Milky Way, is a collection of at least 100 billion stars (one of them the Sun), arrayed in a rotating spiral.  As Earth and its siblings planets orbit the Sun, the Sun traces an almost circular orbit around the galactic center.  Traveling along a spiral arm at about 490,000 mph, it takes the Sun some 220 million years to circle once around the Milky Way’s core.  (The last time the Sun was near its current position, dinosaurs roamed North America.)

Such distance are measured in light-years, the distance light can travel in a vacuum in one year (about 6 trillion miles).  Luckily, our solar system cruises through the galaxy’s outskirts, about 26,000 light-years from the core — where lurks a massive black hole.

At least two-thirds of all spiral galaxies have bar-shapes running through their centers, created by dust and millions of stars in very peculiar orbits.  Instead of traveling in near-circles, the stars are orbiting the galactic center in long, bar-shaped loops.

But it wasn’t always so.  Since light from distant galaxies can take billions of years to reach Earth, looking out into space is looking back into time.  And scientists say that the spiral galaxies of 7 billion years ago were much less likely to have bars.  Since the universe is about 13 billion years old, it seems like bars may be the signature of a mature spiral–a grown-up galaxy in the prime of its life.

How do bars form?  The enormous gravitational pull of the galaxy’s center keeps stars in orbit.  Computer models show that over time, as orbits are disturbed by stars’ gravitational attraction to other passing stars, circular orbits can become more elongated.  As orbits stretch out, stars travel toward and then away from the galactic center.  Gradually, over many millions of years, enough stars are locked into long, narrow orbits to form a visible bar across the galaxy.

But bars aren’t just a scenic galactic feature.  The orbiting stars’ gravity pulls more gas, the raw material for new stars, into the inner galaxy.  This may explain the ring of glowing gas around the center of many galaxies, studded with newborn suns.

On the less-cheery side, the oldest stars in a bar tend to follow the most elongated paths, carrying them closest to the galaxy’s center.  In our Milky Way, such bar-crawling old stars may fall into our galaxy’s black hole, disappearing with a burst of x-rays.

View a photo of a barred galaxy at http://apod.nasa.gov/apod/ap050112.html.  See an artist’s conception of the Milky Way at http://apod.nasa.gov/apod/ap050825.html.

It’s coming: this Thursday, March 20^th, at 1:48 a.m. EDT. You may sleep through the Vernal Equinox, but when you wake up, it will be the first day of Spring in the Northern Hemisphere.

The Vernal or Spring Equinox is the midpoint between the Winter Solstice in December, when the hours of daylight begin increasing, and the Summer Solstice in June, when the days begin to grow shorter again. Each day, the sun stays in the sky about two minutes longer, rising a little earlier and setting a little later. Until, near June 20th, we’re enjoying the longest days of the year.

At the moment we call the Spring Equinox, an observer on the Equator will see the sun’s center pass directly overhead. (He’ll notice the same thing during the Fall or Autumnal Equinox, in September.) Meanwhile, at the North Pole, an observer will see the Sun hugging the horizon: Spring Equinox is opening day for six months of daylight at the top of the world.

The equinoxes are a result of the Earth’s tilted journey around the Sun. Our planet rides through space sideways, tipped at about a 23-degreee angle. So the North Pole is leaning toward the Sun for half the year, away from the Sun for the rest.

But on March 20^th (and again on September 22nd, 2008), the North and South Poles will be about equally distant from the sun. Since the Sun is shining directly on the equator rather than at an angle, the Earth is bathed, half and half, in darkness and light.

In fact, the word “equinox” comes from Latin words meaning “equal night.” But the first day and night of spring aren’t exactly 12 hours each. The Sun as seen from Earth is a disc, and the day begins and ends when the top edge of the disc rises or sinks below the horizon. But the Earth’s pesky atmosphere bends (refracts) incoming sunlight, making the Sun appear to rise sooner, and set later, than it actually does. So on the Equinox, day actually lasts a few minutes longer than night. Several days before the Equinox, the day/night division is almost exact.

Even though day and night won’t be perfectly balanced on March 20th, there is something else that’s neatly aligned: On Thursday, the Sun will rise exactly in the east, and set exactly in the west.

All of the cosmic balancing acts that that seem to go on around the Spring Equinox probably led to the idea that an egg would balance that day, too. But the Earth’s gravity doesn’t shift on the Equinox, and if you can balance an egg on March 20^th, you can do it any day. With a little effort and a level surface, eggs can sit wobble-free on either end. A rough eggshell can make balancing easier.

For more on Equinoxes and eggs, including pictures of eggs balanced on random days of the year, visit www.astrosociety.org/education/publications/tnl/62/equinox.html.

In Charles Dickens’ popular novel “The Pickwick Papers,” first published as a serial in 1836 and 1837, we hear about law clerks working in a very unpleasant office:

“In the ground-floor front of a dingy house, at the very farthest end of Freeman’s Court,  Cornhill, sat the four clerks of Messrs. Dodson & Fogg….catching as favourable glimpses of heaven’s light and heaven’s sun, in the course of their daily labours, as a man might hope to do, were he placed at the bottom of a reasonably deep well; and without the opportunity of perceiving the stars in the day-time, which the latter secluded situation affords.”

In other words, if these poor clerks must work in a “dark, mouldy” room behind a high partition during the sunlit hours, they should at least be able to see stars during the daytime – as someone at the bottom of a real well might. Dickens’ sentiment should resonate with any office worker trapped in a dingy cubicle (or student in a dim classroom) on a sunny afternoon.

More than 2,000 years before, the Greek philosopher Aristotle had also mentioned the idea of seeing daytime stars from a well. However, according to modern astronomers, looking at the sunlit sky from the bottom of a deep hole- like a mineshaft-won’t help us see stars. (Unless, of course, we fall headfirst into the hole.)

Take a noon trip up in the space shuttle, and you’ll see the dark of space, studded with stars. But the Earth’s atmosphere, lit up by our own nearby star, the Sun, drowns out the faint light from distant stars.

According to astronomers, standing at the bottom of a shaft measuring 50 feet deep and 6 feet wide is like looking at the sky through a paper towel tube. Neither helps us see stars in the daytime; the sky is just too bright. However, while looking at the sky through a tube doesn’t increase the contrast between the stars and the sky, there is a reason why the well idea may have taken hold. The darkest, bluest part of the sky is directly overhead. Forced to look overhead, someone in a deep shaft might be a bit more likely to see a star than if he were looking at another part of the sky.

So a well won’t help, but sharp eyes scanning the sky will. Sirius can often be seen just after the Sun rises. The very bright star Canopus can sometimes be seen in daytime, too. We can also see planets in the daytime. From December 2008 to March 2009, look to the upper left of the late afternoon Sun, keeping the disc covered by your hand. You should see Venus. Even the Red Planet, Mars, will be visible in the daytime when it makes its next closest approach to Earth in 2018.

See photos of Venus in daytime at www.galaxypix.com/solarsys/Venus/venus.htm. Read “The Pickwick Papers” online at www.bibliomania.com/0/0/19/40/frameset.html.

It all starts with the Sun. The Sun makes its own glow, by fusing hydrogen atoms into helium atoms, releasing photons of light in the process. Presto: Sunshine. The Moon is lit up by sunlight, reflected back to our eyes. Voila: Moonlight-or Moonshine. But what’s the night side of the Moon lit by, since it’s turned away from the blazing Sun?

The surprising answer: Earthshine.

Walk outdoors on a sunny day into the bright light. Just as sunlight lights up the moon, it lights up the Earth. Likewise, sunlight reflects off the Earth, and some of it is reflected to the Moon. The shadowed side of the Moon is softly illuminated by the light of planet Earth. The Earth is the Moon’s own night light.

So when we look at a crescent moon, our eyes receive both reflected sunlight (from a sliver of the bright side) and Earthlight (from the darkened side). Of course, the Earthlight streaming into our eyes from the Moon is just doubly-reflected sunlight, accounting for its dim glow. One way or another, stars are ultimately responsible for lighting up their solar systems.

Earthshine is also known as “ashen light” and the “da Vinci glow.” Artist and scientist Leonardo da Vinci actually solved the moon mystery in the early 1500s, suggesting that light reflected from the Earth created that “ghostly glow.”

The intensity of Earthshine varies, depending on the Earth’s cloud cover, weather, and the shifting position of land masses as the planet rotates. For example, as the Earth turns and the Sun rises over Asia, its big land mass reflects more light than the Pacific Ocean. This makes the night side of the Moon appear slightly brighter to observers on the night side of Earth.

And measuring the light reflected from the darkened part of the Moon may help scientists studying climate change. About 30 percent of the Sun’s radiation striking Earth ends up reflected back into space. The intensity of the reflected radiation depends on the reflectivity of our planet, otherwise known as albedo. If the average albedo decreases — if more sunlight is absorbed, rather than reflected – the Earth’s temperature rises. So as the planet gets hotter, the night side of the Moon may get darker.

Just as moonlight on Earth seems romantic, so does Earthshine seem to give the Moon a dimly romantic glow. In fact, another name for the crescent moon is “the old Moon in the new Moon’s arms.” Just as we walk around on a full-moon night without a flashlight, observing moon shadows and moonlit gardens of white flowers, so might someone on the night side of the moon cross craters bathed dimly in Earthshine, looking up at the source: a shining blue and white planet in the night sky.

Likewise, someone visiting the moons of other worlds would experience planetshine, too. To see the night side of Saturn’s moon Rhea lit by Saturnshine, visit http://www.saturntoday.com/news/viewsr.html?pid=25230.

It just wouldn’t be fall without a huge Halloween moon glowing orange at the horizon, rising above a spooky landscape of black tree limbs and piled-up leaves.

We call the moons of autumn harvest moons, but the official Harvest Moon rose on September 26th. The Harvest Moon is the full moon closest to the autumnal equinox, which fell on September 23rd this year. The Hravest Moon was probably named by farmers. Long before there were huge, gas-powered harvesters with blinding headlights, there was the bright fall moon, lighting the fields as the work of crop-gathering stretched into the night.

The romantic image of the autumn moon even inspired a famous song: “Shine On Harvest Moon,” written more than 100 years ago, remained popular for much of the 20th century. (For a clip of comedic actors Laurel and Hardy performing the song, visit
www.hamienet.com/midi12542_Shine-on-Harvest-Moon.html. )

The moon is a rocky gray-and-brown ball, lit up by brilliant sunlight. But the color we on Earth see depends on where the moon is located in the sky. No matter what the time of year, as the moon first peeks over the horizon, it may appear yellow, orange, or nearly red. Gradually, as the Earth turns eastward and the moon rises higher in the sky, the color pales to white.

The color deepens and fades because of how the Earth’s atmosphere plays with streaming-in moonlight. The bottom layer of air is the thickest — more full of gas molecules, dust, and pollutants. So as the moon begins to rise, we are looking at it through a heavy blanket of air, extending from us to the horizon.

Moonlight is white light. But white light is made of a hidden rainbow of colors — red, orange, yellow, green, blue, violet. As white moonlight travels through the thick air near the ground, gas molecules and tiny particles scatter bluer light out the sides of the beam into the rest of the sky. What’s left behind is mostly red, orange, and yellow light. So the moon’s face near the horizon is significantly reddened.

But as the moon rises higher, we see it through thinner and thinner air. More moonlight reaches our eyes unscathed, with more blues left in the beam. Since we are able to see more of the entire spectrum of moonlight, we see a whiter moon.

Why do fall moons seem particularly big and orange? In mid-northern latitudes, autumn is the time when the moon’s path across the sky reaches its minimum or shallowest angle. During most of the year, the moon rises about 50 minutes later each night. But near the time of the fall equinox, the time shortens to about 30 minutes. That means more light for farmers harvesting after sunset in the Northern hemisphere. Since the moon traces a curve nearer to the horizon, it also looks bigger and redder, creating the perfect Halloween moon to decorate the autumn sky.

Human beings are fascinated with the idea of a time machine–a way to shake off the bonds of the present and travel into the past or the future. No one has ever made a time machine, and scientists say it may be impossible–the very nature of the universe may prevent such “travel.”

But the sheer size of the universe means that light carries information from the distant past into our present, showing us what the cosmos looked like long, long ago and far, far away. When we look into the night sky–or even at our own Sun–we are seeing the past, not the present.

Here’s how it works. Light, the speediest thing we know of, zips along at 186,000 miles a second in the vacuum of space. Light leaves the surface of a star or planet, travels a great distance, and finally enters our eyes. We see the star or planet as it was–not as it is.

Our moon, for example, is about 240,000 miles away; it takes light about 1.3 seconds to travel from there to here. If something happened on the Moon–if it were hit by a big asteroid–we wouldn’t see the explosion until 1.3 seconds later.

The Sun is much further away than the Moon–about 93 MILLION miles. When light leaves the Sun, it takes 8 minutes to speed through space and reach Earth. If the Sun were to magically vanish, sunlight would continue streaming through our windows for 8 blissful last minutes, and the Sun would shine in its normal place in the sky. Finally, we would see the Sun wink out–480 seconds after it actually happened.

Relax–stars don’t suddenly disappear with a POOF. But over millions of years, stars do eventually become cold, dark cinders. So when we look out into the night sky, especially through a telescope, some of the glowing star s we see are actually dead and dark by now.

Why? After the Sun, the nearest star is 24 TRILLION miles away. Light must travel for 4 YEARS to cover that distance, so we are seeing the star as it appeared 4 years ago. Most stars are much further still; it takes their light thousands, millions, or even billions of years to reach Earth. So some stars that we see shining brightly through our telescopes may have actually burned out millions of years ago.

We may see one star as it appeared in 1803, another as it was in 3100 BC, and the next shining with light that left it 100 million years ago, when dinosaurs roamed Earth. The most distant stars–billions of “light-years” away–are a living snapshot of the early universe. Each star in the sky represents a completely different point in time. The night sky is a map of the past, and our eyes and telescopes are our own personal time machines.

Have you ever been riding in a car in the evening and noticed something huge and yellow behind the trees and buildings in the east–and then realized it was the Moon? Especially in the fall, a pumpkin-colored harvest moon, looming up over the horizon, looks enormous and even spooky. It’s not hard to imagine a broomstick-riding witch flying across.

But picture this Halloween scene when the moon is high in the sky, small and white, and it’s just not the same.

Scientists say that the horizon Moon appears up to twice as big as the overhead Moon to most of us. People have been arguing over why for more than a thousand years. Astronomers, psychologists, and nonscientists all have their theories. In 1989, researchers even published a book of such explanations, called “The Moon Illusion.”

You can prove to yourself that the Moon is actually the same size no matter where it is in the sky, using a key or a ruler. Note the Moon’s width at the horizon, and later, compare it to the Moon’s width overhead.

You may even make the size illusion vanish. Some suggest bending over and looking at the horizon moon upside down, in a kind of lunar yoga. Others recommend looking at the moon through a cardboard tube that blocks out landscape features.

What causes the illusion? In the past, some textbooks stated that the Moon appears larger at the horizon because dense air near the ground refracts (bends) moonlight, causing a magnifying-glass effect. This theory, scientists now say, is not really a contender: While there is refraction, it doesn’t magnify the Moon’s image, and would actually tend to make the Moon appear squashed.

Most agree that the illusion is a matter of perception–a trick of the brain. Some argue that the horizon Moon looks bigger because it is framed by smaller objects like trees, houses and hills, making it huge by comparison. However, that doesn’t explain why the moon looks so big rising over the flat expanse of the ocean. (And it also doesn’t explain why the pretend moon in a planetarium appears to be the same size at the horizon AND overhead.)

Several complicated theories involving the brain’s visual system also try to explain the moon paradox. Here’s a simplified version of one popular explanation: The brain perceives the sky (and Moon) above us as closer than the sky (and Moon) at the horizon. When an object is perceived to be nearer, the brain may compensate by making it look smaller to us. Likewise, an object thought to be farther away will be seen as larger.

(To see how perceived distance makes an object look bigger or smaller, visit the website www.howstuffworks.com/question491.htm.)

For now, The Moon Illusion remains one of nature’s loveliest unsolved mysteries.

Twilight usually refers to the time just after the sun sets in the evening. But it also can mean the time just before the sun rises in the morning. During twilight, although the sun is hidden below the horizon, the sky is still aglow with light, gradually dimming (after sunset) or intensifying (before sunrise).

During twilight, the light for our evening activities comes courtesy of the upper atmosphere.

Circling in a plane at sunset, you can see the sun (and bask in its light) for much longer than if you were on the ground below. It’s the same for the atmosphere. Light rays from below the horizon strike the upper atmosphere. The light is refracted (bent) as it passes through air molecules, as well as scattered every which way. The softly illuminated sky creates the twilight, an hour or so of afterglow before night sets in.

In fact, there are actually four categories of twilight. During the evening, it all starts with sunset itself, ending when the Sun has just dipped below the horizon. That’s “sunset twilight.”

Next comes “civil twilight.” During civil twilight it is still light enough to carry on most outdoor activities, like playing tag on the lawn or watering a garden. Big shapes are still visible during this early twilight, even without street lamps or porch lights lit. You may see a few bright stars or planets in the sky. In the continental U.S., civil twilight lasts for about 30 to 60 minutes, depending on the time of the year (evening twilight is longest in the summer) and the location. It ends when the sun is about 6 degrees below the horizon.

Then there’s “nautical twilight.” During nautical twilight, the sky is dark enough that all the brighter stars are visible. However, someone at sea could still sea the horizon well enough to navigate by star altitudes. By the end of nautical twilight, the Sun has sunk to 12 degrees below the horizon, and the horizon is no longer visible at sea.

Finally, there’s “astronomical twilight.” More and more stars can be seen, but the sky is still too light for an astronomer to do any serious work. When the sun has dipped to 18 degrees below the horizon, twilight is officially over. Official “astronomical darkness” has begun.

Unfortunately for astronomers, astronomical twilight lasts all night long during the summer in place above 49 degrees latitude. And in winter, it’s twilight at noon for people living in far north latitudes.

To figure out when twilight periods begin and end in your area, visit the website www.cmpsolv.com/los/sunset.html . Listen to the song “Twilight Time” at www.celebritydirect.org/platters/listen.htm.

Feel like you’re being followed? While it seems like the Moon is always just over your shoulder on a moonlit night, the Sun is also shadowing you as you drive on a sunny afternoon. And then there are those distant mountains to worry about…

According to astronomers, the reason why the Moon and the Sun seem to be following us is because they are so far away. The Moon, for example, is about 240,000 miles away; the Sun about 93 million miles. And no matter how fast we drive, we just can’t pass them.

When you drive by a stand of trees or a series of telephone poles near the road, you pass them very quickly. So you see roadside objects first ahead of you, then next to you, and finally behind you, receding into the rear-view mirror.

But when you drive (or stroll) by the Moon, it’s a different story. Because the Moon is so far away, the angle you view it from will change very little as you move along. So mile after mile, the Moon will remain in roughly the same spot of sky. And just as you can’t “pass” the Moon, neither can you shake the presence of the Sun, planets, or stars. Even very distant mountain ranges appear nearly stationary as we drive by. And far-away farms and city skylines seem to move by very slowly.

Since we can’t pass the Moon, we can’t pass its reflection, either. When you walk along the beach at night, the river of moonlight reflected off the water moves right along with you. Try to wade out into the moonlight, and you’ll find it remains tantalizingly out of reach, just as a shimmering patch-of-water mirage retreats down the road as you drive toward it.

When you stand on the beach, moonlight bounces off the water and into your eyes at a nearly fixed angle. As long as the Moon is in the same spot of sky and the water level doesn’t rise or fall much, the angle of reflection will remain roughly the same. So if you can see the entire ribbon of moonlight, your eyes are at just the right height to intercept the rays of light bouncing off the water from the horizon to the beach.

Once you wade out into the water, however, you’ve also moved your eyes. The moonlight bouncing from the water at your feet doesn’t strike your eyes; instead, it shoots right past you at a lower height. So the water at your feet looks dark.

Friends on the beach behind you, however, will see you standing right in the moonlight road, and could even snap a picture of it. So to bathe in moonlight, simply sit down in the water, where your eyes can catch the silvery light at the right angle.

Meanwhile, just as every car on the road thinks the Moon is following them, so every walker on the beach sees their own ribbon of moonlight, stretching towards the horizon.