Friday, August 31, 2007

Possible meteor flurry Saturday morning

Most of the time, we know in advance when meteor showers will happen. Some, like the Perseids in August, put on a good show every year. Others, like the Leonids in November, put on a fairly lazy show every year, but occasionally produce a big storm (like in 2001). And some showers are so pitifully poor that we don't even bother much with them.

Tonight (actually tomorrow morning), though, there is a chance that, for a few minutes, a brief meteor storm will light up the skies. I don't know how good the chance is -- maybe 50/50, maybe lower. But, if you are up early tomorrow morning, you might as well take a look.

The meteor shower is called the Aurigids, as they appear to come from the constellation Auriga, which will be part-way up the sky in the east tomorrow morning, above the constellation Orion (though any meteors would be visible across the sky). Most years, the Aurigids produce nothing, but three times in the past, they have produced short bursts of activity.

The still-emerging field of meteor prediction has found that those previous bursts happened when the Earth crosses the orbit of a comet that last came through in 1911 (and won't come again until about 3900). Saturday morning, at 4:27am Pacific (give or take 20 minutes), the Earth will pass through the meteor stream left by the comet when it passed through the solar system in 82 BC.

If a meteor burst happens, it may produce 1 meteor a minute or as many as one every 5 or 6 seconds! Or, maybe nothing will happen at all.

So, if you live in the western US (the meteor burst will be after sunrise for those in the east), and if you are up before dawn tomorrow, see if you can spot any meteors appearing to come from a spot in the northeastern sky. And, if you do, know that these meteors were shed off a comet over 2100 years ago, when Julius Caesar was about 20 years old.

To read more on the Aurigids, see this page from Sky & Telescope magazine. (This article was also my main information source for my post.)

Thursday, August 30, 2007

Some "fun" research

Yesterday I was waiting on colleagues to send me various files, and so I spent some time working on a "fun" project -- one that is not central to my normal research, but for which I have some data.

My main research is to look for and analyze normal white dwarfs (the white-hot glowing remains of dead stars) in star clusters. So, I look for objects that have the same colors as normal white dwarfs, and then I go and get more detailed data on each of those objects. Most do turn out to be normal white dwarfs, but I often stumble across other extreme objects.

Last year, one of my candidate normal white dwarfs turned out to be an abnormal white dwarf. Instead of a single dead star, slowly cooling off in space (like most white dwarfs), this one is a bit of a vampire. It is ripping material off of an unseen companion star and, using a very strong magnetic field, keeps the material for itself. This type of star, called a "polar" (pronounced POLE-are), is fairly rare, but many are known to exist. An artist's conception of a polar can be seen here

What makes the star I found especially interesting is that it may be in a star cluster. Since all stars in a cluster are the same age, we know how old this star system is. There are also a few other details about the star that we know, like how much metal it has. While stars are mostly made of hydrogen and helium, the metals (like iron) affect how a star lives and shines, and all stars in a star cluster have the same amount of metals.

I don't know how much we will learn from my observations, but it is still fun to be working on something completely different, and for which my observations may be important (or at least useful) to other people.

Wednesday, August 29, 2007

Here they come!

A university is what a college becomes when the faculty loses interest in students.

Today is the first day of fall classes here at the University of Texas. In some ways, life becomes a lot harder. The buses are no longer a comfortable way to get to work, but a more reminiscent of transporting livestock, the elevators are full of students jabbing incessantly at the buttons for floors where the elevators don't stop, the line at the coffee cart is out the door, and getting about campus became much slower today.

Of course, universities are about the students. Without them, we wouldn't have our jobs, and scientific research would be a much harder profession. The main goal of astronomers is education; having 30,000 students around makes that goal much more tangible.

So, I'll just have to re-develop some patience, allow extra time to get to work, and help lost freshmen find their classes. Like any change of seasons, I'll get used to it.

Tuesday, August 28, 2007

Modern technology


Image Credit: Melissa Williams

Just 50 years ago, astrophotography was a hobby reserved for the select few. Taking pictures of astronomical objects and events required special film, expensive cameras, and lots of expertise. How things have changed! Digital cameras allow anyone with a tripod to take decent photos of the moon and stars, and other than the cost of the camera, the hobby is cheap! Those with time and money to spare can buy impressive setups that can rival professional astronomers in terms of quality.

The above picture is a series of four photographs of this morning's total lunar eclipse; you can see the moon progressively moving into Earth's shadow. What setup was used to take these pictures? A cell phone camera. We have indeed come a long way.

Although it appears above that the moon completely disappeared, it was still visible, although early reports suggest it was a very dark eclipse. This is partly because the moon went deep into Earth's shadow, and partly because of bad weather in a lot of the Earth that was seeing sunset and sunrise during the eclipse.

If you would like to see more pictures of this morning's eclipse (especially if you were like me and just too tired to roll out of bed), check out Sky & Telescope's Total Lunar Eclipse photo page (as of 11:30am CDT, nothing had been posted yet, but I am sure pictures will be coming).

Here's a beautiful photo and account of this morning's eclipse posted elsewhere on Sky and Telescope by Gary Seronik.

Monday, August 27, 2007

The Passing of Ralph Alpher

Image Credit: American Institute of Physics

Two weeks ago, physicist Ralph Alpher passed away at the age of 86. "WHO?" I hear echoing from across the blogosphere. Only one of the relatively unknown yet important contributors to astronomy of the 1900s, that's all.

In the late 1940s, Alpher was a PhD student working with physicist George Gamow. They were working on calculations surrounding a new idea about the creation of the Universe. It had been known for some time that all the galaxies in the Universe appeared to be moving away form one another, as if they had exploded out of a single point eons ago. Alpher and Gamow worked on a mathematical treatment of what the Universe would have been like during such an explosion. They determined it would have been hot enough to cause nuclear reactions to occur (and they calculated the relative amounts of elements these reactions would have created), and that the fireball should still be visible as a faint echo only a few degrees above absolute zero today.

Their paper was published in the prestigious journal Nature on April 1, 1948. Because it was appearing on April Fool's Day, George Gamow added the name of famed physicist Hans Bethe to the paper, making the authors "Alpher, Bethe and Gamow," a pun on the first three letters of the Greek alphabet: alpha, beta, and gamma. Bethe had done no work for the paper, but was amused at the joke. The idea that the Universe began in a huge explosion was scoffed at, the explosion being derided as just a "big bang."

Yet the paper did something that no other work on the subject had done -- it not only explained why galaxies were flying apart, but it offered concrete predictions on other observables -- how much of the Universe was made of hydrogen and helium, and that faint echo. This is a crucial lesson in science for everybody -- it is not just enough for a theory to explain why things appear to be the way they are. The theory has to make specific predictions on things that have not yet been seen. If those predictions hold true, the theory is strengthened; if they aren't right, the theory is incorrect.

In 1965, two engineers, Arno Penzias and Robert Wilson, were tracking down sources of noise in a radio receiver they were building, but one noise just wouldn't go away. It was noise that seemed to have a temperature of a few degrees above absolute zero. Alpher's prediction (which, in the meantime, he had fleshed out further with Robert Hernan) had been proven correct, and the Big Bang theory instantly became far and away the strongest scientific theory on the creation of the Universe.

George Gamow became famous for his work; Hans Bethe won the Nobel Prize for Physics (not for this work, but for many other vital contributions). Two Nobel Prizes have been awarded for the Big Bang Theory -- to Penzias and Wilson for discovering Alpher's predicted echo, and this year to John Mather and George Smoot for further work on understanding this background. All four deserved this recognition. Unfortunately, Alpher's work went unrecognized by the Nobel committee. But, in 2005, Alpher was awarded the National Medal of Science, nearly 50 years after his crucial work on the Big Bang Theory.

Saturday, August 25, 2007

Total eclipse of the moon on Tuesday!

Image Credit: Laurent Laveder

For those of you in the Americas who are normally up with the chickens, or those of you with insomnia, Tuesday morning is your chance to see a total eclipse of the moon. Starting at about 4:51am EDT (1:51am Pacific), the moon will begin to slip into the shadow of the Earth, and by 5:52am EDT (2:52 am Pacific), the moon will be totally eclipsed, and thus it will remain until 4:23am Pacific; a partial eclipse will last for about another hour. Those on the East Coast of the US will see the moon set while in total eclipse.

If you live in Hawaii, the eclipse will be visible around midnight on Monday night/Tuesday morning, and if you are in Australia, New Zealand, or far eastern Russia (I don't think I have many readers there, but just in case...), the eclipse will put on a nice show Tuesday night. Most of Asia, Africa, and Europe will miss the show :(

A lunar eclipse happens when the moon's orbit takes it into Earth's shadow. The picture above, a composite of pictures taken during the March 3rd, 2007 eclipse, shows the round shape of Earth's shadow nicely. Most orbits (most full moons), the moon passes just above or below Earth's shadow, and so there is no eclipse. But, once in a while, we get lucky.

During a total lunar eclipse, the moon turns a reddish color (sometimes dark red, sometimes almost black, sometimes a bright copper). This is because Earth's atmosphere bends some sunlight into what would otherwise be a dark shadow. In essence, the moon gets to see every sunset and every sunrise simultaneously! Some day, perhaps our astronauts will get to view the Earth surrounded by a bright ring of fire as they view a total eclipse of the sun by the Earth. But for now, we can only imagine.

Next March, those of us in the Americas will get to see yet another total eclipse of the moon (three in one year, which is rare!); the next total lunar eclipse will not be until December 2010. And it is only ten more years (Aug 21, 2017) until the continental USA gets its first total solar eclipse since 1979, a drought of 38 years.

For more details on this lunar eclipse, check out Sky & Telescope or NASA's Eclipse pages.

Tuesday, August 21, 2007

30 years of voyaging


Image credit: NASA

Thirty years ago yesterday, NASA launched the Voyager 2 space probe on a journey to visit the outer solar system. Two weeks later, its twin, Voyager 1, was launched on the same voyage. (I don't know why Voyager 2 was launched before Voyager 1.) The two spacecraft jointly explored Jupiter and Saturn; Voyager 1 was sent on a special mission to Jupiter's inner moons, and so headed away from Saturn in the wrong direction to tour any other planets. But Voyager 2 continued on, becoming the only probe to visit Uranus and Neptune. I remember watching images from Neptune coming in live on CNN back in 1989.

Since that time, both Voyagers have remained alive and alert, heading outward at over ten miles per second. Voyager 1 is now 100 times further from the sun than the Earth, nearly ten billion miles away; Voyager 2 is a little closer, a mere eight billion miles away. Both spacecraft will leave the Solar System forever and head out into the Milky Way galaxy for eternity, though it will be tens of thousands of years before either Voyager comes within two light-years of another star. Given that we haven't found all of the mountain-sized asteroids in our own solar system yet, it seems highly unlikely that any civilization that might be around these other stars would even notice the passing of a human-sized lump of metal that long-since stopped operation.

In the meantime, the Voyagers are working on one final science goal before their electricity generators die (around 2020). They are looking for the boundary between the sun's wind and interstellar space -- sort of like the boundary between Earth's atmosphere and space. This boundary, called the heliopause, means that the Voyagers will have essentially left all influence of the Sun (other than gravity strong enough to keep some comets and Kuiper Belt Objects around), and the Voyagers' true interstellar voyage will have begun.

Monday, August 20, 2007

Hurricanes and space

Credit: NASA/STS-118

One question astronomers are asked all the time is how our science contributes to the well-being of humankind. This is a fair question -- after all, if we are using your tax dollars, you have a right to ask for some return. Astronomers (myself included) love to wax poetic about the pushing back of the boundaries of the unknown, of discovering our place in the Universe, and other such big ideas. Yes, these are true, but often people really want to know, "What have you done for me lately?"

There are many concrete examples of space science and technology being used in everyday life. One of the next benefits of astronomy you will begin to see in the coming years will be at your eye doctor's office, where the laser technology developed to help astronomers see more clearly into the heavens by removing the distortions of the atmosphere is being adapted to allow opticians, optometrists and ophthalmologists peer clearly into your eye. This will allow for amazingly precise eyeglasses and contact lens prescriptions and allows the doctors to detect eye diseases and problems earlier than before.

An even greater impact is being wielded by satellites that look not out into deep space, but back at the Earth. These satellites allow us to study the Earth in many ways, from the impacts of deforestation across the globe to precise measurements of ocean water heights and temperatures to constant surveillance of the weather. The technologies that permit these vital observations are developed in tandem with technologies needed to make precise astronomical observations.

Right now, Hurricane Dean is crossing the Caribbean Sea and threatening the Yucatan Peninsula and northern Mexico. Because of the multitude of weather satellites, meteorologists knew that Dean was a serious threat to be a monster hurricane threatening these areas days ago, when Dean was a minor hurricane just entering the Caribbean Sea from the Atlantic. Saturday night, when I was flying back to Austin from my visit to Tucson, Arizona, my flight from Dallas to Austin was delayed quite a bit. Our plane was coming into Dallas from Cancun, and the delay was due in large part to many tourists cutting vacations short and trying to fly out of harm's way. While Dean is causing major travel headaches because of this, the extra few days of warning we have had thanks to the bevy of satellites watching the storm and the general weather in the region has allowed thousands of people to move to safety.

These same warnings were available for New Orleans many days prior to the arrival of Hurricane Katrina two years ago. Although federal, state and local authorities were slow to react (and then even slower in the horrific aftermath), the advanced warnings and evacuations likely saved tens of thousands of lives. Had warnings been heeded earlier, perhaps many more could have been saved. It wasn't a lack of information that was the problem there...

So, astronomical science does have a beneficial impact for humans. The tax money spent on astronomy is relatively small (in 2006, a total of about 6 billion dollars, not including manned space flight. For comparison, this is 0.24% of the entire federal budget, or one quarter of a penny for every dollar you pay in federal taxes. It's also about 10% the size of the Department of Education budget or about 20% the size of the Housing and Urban Development budget for that year, and 1.5% the size of that year's Department of Defense funding, not including the costs of the ongoing wars in Iraq and Afghanistan). I'd like to think that the return on that investment is worthwhile, but, of course, I'm biased.

Friday, August 17, 2007

A new comet? No, an old star.

Photo Credit: JPL-Caltech/C. Martin (Caltech)/M. Seibert (OCIW)

A news story about the above ultraviolet photograph of the star Mira has been making the rounds the last few days. It is a neat discovery, although the physics behind what is causing the light in the picture is reasonably well understood.

The star Mira is a red giant star, a star nearing the end of its life, and is found in the constellation Cetus, the whale. Don't ask me to point this constellation out -- it's big but pretty obscure. Mira varies in brightness from a medium-brightness star to naked-eye invisibility every 330 days (roughly, the time varies a little). It represents a star in the final stages of its life -- its nuclear core is out of hydrogen fuel, but there is still hydrogen around the star's center that can burn and keep the star shining. The sun will go through a phase like this in a mere 5.5 billion years or so.

When stars are red giants, they tend to start losing a lot of matter. Winds from the star's surface blow matter off into space. The sun does this now, but at a tiny rate -- about one million tons a second (compared to the sun, that's tiny!), or about one earth-mass of material every 100 million years. By comparison, Mira is losing mass at a rate of one earth-mass every ten years.

Mira is moving through space very fast, about 80 miles per second. But space is not completely empty; it has very tiny amounts of gas in it. So, as Mira rockets through space, shedding lots of material, that material collides with the slow-moving gas in space and slows down, so Mira's material gets left behind as the star plows ahead. Think of it like a poorly-covered garbage truck going down the freeway, with bits of trash flying out along the way. Yes, most of the garbage will make it to the dump, but a tiny stream of debris marks out the path the truck took.

What is interesting about Mira's comet-like tail is that it hadn't been seen before. The tail is only visible in light from hydrogen atoms, which glow brightest in ultraviolet light. Thanks to ozone, ultraviolet light doesn't reach Earth's surface, which is good for us, but bad for astronomy. So, it took the launch of an ultraviolet telescope called "GALEX" in order to discover this tail.

Thursday, August 16, 2007

Visiting colleagues in Tucson

I needed to return to Tucson in order to visit colleagues and finish up some work, so I decided that this might be a good time. Classes have not started yet, so I don't have to fight crowds of students (or so I thought), and, it being summer, travel to Tucson is cheap. Let's face it -- why go to the desert in the middle of summer?

It's been a very useful and busy trip, but I didn't miss out on the student crowds. Today and tomorrow are move-in days on campus, so traffic is snarled with a new crop of freshmen unloading their earthly possessions into small dorm rooms. And the sorority across the street from the astronomy department has chosen today to have their pledges stand outside and shout various cheers all day long, which is quite distracting. But, I did miss out on the tropical storm back in Texas. So my timing isn't that horrible.

Anyway, I'm here another day for collaboration work. There's only so much one can accomplish via email or video conferencing -- a little face time can work wonders on a stalled project.

Tuesday, August 14, 2007

A reader question: The Brightnesses of Stars

I received the following excellent question from a reader today, and thought the question and answer made for nice blogging:

What are the factors that contribute to the brightness or dimness of stars? Is there a particular combination of these factors that makes Sirius the brightest?

This is a very good question. There are three main factors that contribute to the brightnesses of a star as seen from Earth: the temperature of the star, the diameter of the star, and its distance from the Earth. The temperature and diameter of the star depend mostly on the star's mass and where it is in its life cycle. For example, every star with as much matter as the sun will be just as hot and just as large as the sun for most of its life, but when it starts to run out of nuclear fuel, the star will swell up into a giant and cool off until it glows red (a so-called "red giant" star). Stars with more matter than the sun tend to have larger diameters and hotter temperatures, while stars less massive than the sun are smaller and cooler.

But, as I said, a star's distance is also important. The further away a star is, the fainter it appears. Sirius is not the closest star to the sun -- there are many closer stars, most of which are very low mass "red dwarf" stars, which are too faint to see without a telescope. But Sirius is about twice the mass of the sun, so it emits much more light (about 23 times more than the sun). So, even though Sirius is twice as far away as the sun's closest neighbor and near-twin, Alpha Centauri, Sirius appears four times brighter than Alpha Centauri in the sky. But Sirius is far from the hottest or most massive star in the Milky Way galaxy, so it is a combination of its higher mass and proximity to Earth that makes Sirius the brightest star in our sky.

To help avoid confusion, astronomers have developed two related but separate measures of how bright a star is. The first, the "apparent magnitude," is how bright a star appears as seen from Earth. The second, or "absolute magnitude," is how bright a star would appear if it were 36 light-years away from Earth. Absolute magnitudes allow us to compare how much energy a star puts out, which is more useful from the standpoint of understanding the physics at work in a star. Of course, we need to know a star's distance to determine the absolute magnitude, which is a trick in and of itself.

For some more mathematical descriptions of these concepts, try this page or Wikipedia.

Friday, August 10, 2007

Another trip around the sun

This week, my grandmother, my mom and my dad each complete another orbit around the sun (85, 59, and 60, respectively). The earth, which has completed nearly 4.5 billion such trips, just keeps on trucking and is already starting another lap around the sun.

As we look into space and discover new solar systems, we are starting to appreciate the one we have. In many solar systems, a Jupiter-sized planet careens through the zones around the star capable of harboring life. Any planets that once circled in those zones have long-since been tossed into deep space. In other solar systems with faint suns, the planets close enough to be comfortably warm also become tidally locked, meaning the same part of the planet always faces the star. The lit sides of these planets will become unbearably hot, while the nighttime sides will be freezing cold.

There are almost certainly nice, habitable planets circling their suns just as the Earth is circling ours. Maybe, on one of these, some alien humanoid is also celebrating its grandmother's 85th birthday. I'd like to meet them someday (unless they would be holding a ray gun at the time, but that's just common sense).

Thursday, August 09, 2007

Godspeed, Endeavour


Image Source: NASA

Yesterday evening, the space shuttle Endeavour lifted off for a mission to perform more construction on the International Space Station. I missed the launch, as I was riding a bus home at the time, but I wish our astronauts well.

The big story of the mission so far has been that of astronaut Barbara Morgan. Ms. Morgan was selected in the mid 1980's as the backup "Teacher in Space" behind Christa McAuliffe, one of the seven astronauts on the space shuttle Challenger when it was lost during liftoff on January 28, 1986.

I remember the Challenger accident all too well. I was in 6th grade at the time, and was already quite the space enthusiast. All sorts of educational activities were planned during the Challenger flight, starting with the launch, all beamed live into our classroom (and thousands of other classrooms across the country).

We didn't watch the launch of Challenger, though, because of a snowstorm in eastern Pennsylvania that closed schools for the day. I forgot about the launch (which I could have watched on CNN), but spent most of the morning sledding and enjoying the snow day. It wasn't until around lunch that my mom called and said she heard something over the radio about the space shuttle in the Atlantic Ocean that I turned on the news to learn what had happened.

Sometimes I'm amazed that, twenty-one years after the Challenger accident, NASA is still struggling with questions of vision (where are we going with manned space flight?) and culture (whether it be astronauts and alcohol or the design of our next space capsules). It is good to see that, despite the bureaucratic nightmare that is government-controlled spaceflight, the individuals inspired to travel into space (like Barbara Morgan) still are able to live out their dreams and help humans reach for the stars.

Wednesday, August 08, 2007

Catch a falling star

This weekend (actually late Sunday night into early Monday morning) marks the peak of the annual Perseid meteor shower. If you are up late and can get away from city lights, why not go stare at the skies for a while and look for meteors?

On any given night, if you watch the skies long enough, you'll see a handful of meteors, anywhere from a few to several an hour, depending on how dark the skies are. These meteors are not really "falling stars," but are dust floating through the Solar System that happen to hit the Earth's atmosphere, and pieces of space junk (flecks of paint, pieces of insulation, parts of exploded rockets) falling back to Earth.

In a meteor shower, the Earth passes through the trail of debris left by a comet. Most of this debris is just tiny pieces of dust, though there is the occasional pebble-sized rock. These all burn up in the Earth's atmosphere; the smaller pieces make faint meteors, the larger pieces make bright meteors.

In mid-August, the Earth is passing through the trail left behind by Comet Swift-Tuttle, which passes through the inner Solar System every 130 years (last in 1992, next in 2126, so mark your calendars!). During this time, observers on earth can see 60 or more meteors an hour from a dark site, or about one a minute.

Note that last sentence. Sometimes people go out to look at meteor showers and expect to see meteors every second or two. This rarely happens. The Leonid meteors (seen yearly in November) make a "storm" like this every 33 years, last in 2001. But most of the time, a meteor shower rarely makes more than one meteor a minute. So you have to be patient, but know that you are seeing ten times more meteors than you would on a normal night!

The Perseids are a nice meteor shower. It is reliable, appearing about the same every year. They come in the summer, so the nights are warm enough for people to go outside. And they Perseids are visible for several days on either side of the best night, though with fewer meteors. So, if you have time on Saturday night/Sunday morning, you can still go out and look for meteors and likely see a good show.

Like most meteor showers, the Perseids are best viewed after midnight. This is because after midnight, you are on the side of the Earth that is plowing right into the meteors. Before midnight, you only see the rare meteor than can catch up to the Earth.

So, if you have time and clear skies, go out late one night this weekend and try to look for meteors. If you can get away from the city and have clear skies, I guarantee a nice show.

Monday, August 06, 2007

Last week of class

This summer I have been teaching graduate students (students going after PhDs in astronomy) about telescope observing and analyzing the data they collect. This week is the last week of summer classes, and so the course is coming to an end. The students are giving presentations on their projects, so my preparation time this week is small. Of course, this weekend I will pay for the week of no classwork when I have to grade six term papers in three days.

I am still amazed at how much time it can take to properly prepare for a class. I borrowed most of my materials from other people, so I had far less to prepare than if I created the class from scratch. Even so, it still took several hours to prepare for each 1 1/2 hour class period.

Sometimes I see claims by people (usually congresspeople or state legislators) that college professors get paid too much and do too little, only teaching one or two classes each semester. Although I knew that these claims were wrong, I now have first-hand experience to see how wrong they are. To prepare and teach a college level class properly takes many hours outside of class for each hour spent in class. So those claimed "leisure" hours are not spent sipping drinks on the beach at taxpayer expense, but spent in pouring over new materials, developing better visual aids, and gently reminding one's self what we are going to say when standing up in front of dozens of students, many of whom would rather not be there.

Are there lazy professors? Yes; as in any profession, there will always be those who fail to pull their weight. But, as in the business world, even tenured professors have to submit to performance reviews and evaluations. Eventually, those not putting in sufficient effort will be weeded out. But this group of people is a minority.

So, the next time you here someone saying that those professors in their ivory towers should try working for a change, know that, in fact, they are working; often 60 hour work weeks, trying to educate students to the best of their abilities while still accomplishing some research work. It's quite challenging to juggle these things, and I'm happy that I have come to appreciate it more now than ever.

Thursday, August 02, 2007

Phases of the moon

One of the first tough things that we ask most astronomy students to understand is the phases of the moon. And most students never really grasp it. It's not easy -- understanding the moon's phases involves picturing a three-dimensional dance of the Sun, the Earth, and the moon. And most people visualize this wrong.

I was happy to get an email from a cousin asking about the phases of the moon. Yes, he had the basic idea wrong, but he was thinking about it, and he noticed some problems with his logic. And that made me very happy. So many of us are just content to believe what we are told without thinking any further about it.

So, here is his question and my answer. There are many websites that talk about the phases of the moon, many with spiffy Java animations, so you can search those out if you'd like. I'm going to stick to an analogy that seems to work for some people.

"The guys and I were trying to figure out how a half moon is possible, since the different phases of the moon are from the Earth's shadow and the last time I checked, the Earth isn't flat. So my question to you is, how is a half moon possible?"
Before we think about the moon, let's imagine something completely different. Suppose you are in a movie, playing the role of an innocent man who is being framed for a murder by a rotten cop. The cop brings you in for interrogation. In the interrogation room, you have the stereotypical setup. You are in a lone chair; there is a bright light on a desk being shone into your eyes, and the dirty cop is pacing around you. He doesn't trust you, so as he walks around your chair, he turns his face so he can always keep a close eye on you. Suppose the cop is almost between the light on you. You'll be squinting into the light, and so not be able to see him well. But even if you could see him well, his face would be in shadow, because his back is to the lamp. The back of his head is in the full glare of the light, but his ugly mug blocks your view of the back of his head. Now the cop walks around to your left. You turn your head to look at him. He's still looking straight at you. The left side of his face will be lit up by the lamp, but the right side of his face will be in shadow -- that big ugly nose, the sunken eye sockets all cast big shadows across half of his face. Half of the back of his head is also lit up, but you can't see that, since you can't see through his face. Now the cop walks behind you. You crane your head to look at him. Now his full face is lit by the lamp, so you can see every hairy mole, every scar from the barroom brawls he's been in. No shadows at all. Of course, the back of his head is in full shadow, but you can't see that either. Finally, the cop walks around to your right. When you turn your head to look at him, you see that now the right side of his face is lit, the left side hidden by those ghastly shadows of his crooked nose. Can you picture that scene? The moon and the Earth work the same way, with the moon being the bad cop, and the Earth is the poor innocent man. And the sun is that awful desk lamp. At new moon, the moon is almost (but not quite exactly) between us and the sun. We can't see it because of the glare of the sun, and even if we could, the moon would be dark, hidden in the shadow of its far side. When the moon is half lit, it has moved to the left or the right of the Earth (it always moves counter-clockwise, as seen from above). Now, half the moon is shaded by the other half of the moon. Finally, at full moon, the moon is directly behind us. We can see the entire moon, as we are looking straight at the sunlit part of the moon. Like the bad cop in the story above, the moon happens to turn at exactly the same speed as its movement around the Earth, so we always see the same face of the moon. We never see the back of it. Why it turns at the same speed can be explained by physics, but it doesn't have any thing to do with the phases we see. And, lastly, on some occasions the moon does come directly between us and the sun -- that's when we see an eclipse of the sun. An eclipse of the moon (such as one that will be visible late this month!) is when the moon passes into Earth's shadow. So it is only during eclipses that the Earth's shadow comes into play. The ancient Greeks used these eclipses of the moon as their proof that the Earth was round. If the Earth was any other shape, then the shadow of the Earth on the moon would be different. But the Earth's shadow during an eclipse is always round. I hope this clears things up a little. Understanding the phases of the moon is all trying to understand the three little spheres that are the Earth, the sun, and the moon, and picturing how they line up with each other. That makes it hard. But I think the story of the interrogation room can help people to visualize what's going on a little bit.