As the planets all go around the sun, sometimes they do come fairly close to being lined up. The last time things were close to being lined up was April of 2000, when Mercury, Venus, Mars, Jupiter, and Saturn were all in the same part of the sky, but Neptune, Uranus, and Pluto were elsewhere, and even then the lineup of the inner planets was only approximate.

Nothing at all special occurs when the planets happen to line up. There are no earthquakes or other natural disasters; the planets just keep on moving in their orbits around the sun, same as always. The lineup is just something that happens as they move around in their orbits and has no more significance than you and a few of your friends all accidentally standing in a line while talking, or something like that.

On the other hand, while nothing physically happens out there in space when the planets align, it can be useful for sending space craft out to them. The planets were positioned just right in the late 1970s that the Voyager spacecraft could be launched in such a way that Voyager 2 could travel past Jupiter, Saturn, Uranus and Neptune all in a row, over about 15 years. However the pattern needed for this "sling-shot" trajectory wasn't a straight line of planets, but rather sort of a curving spiral. We were lucky to launch Voyager 2 when we did - a similar "Grand Tour" alignment won't happen again for over a century.

Yes! It most certainly would. Now, since the Moon is locked towards the Earth, it would always be high-tide at the part of the moon pointing towards the Earth, and always low tide 90 degrees away from there, but there would certainly be tides.

In fact, the Earth and Moon cause tides in the rocky parts of each other, not just in the water. It's just that the water moves around much more, so that the tides there are much more visible. The water in the bay moves up and down several feet every day due to the tides. But the shore is moving up and down by about a foot, too! Likewise, the Moon itself is stretched by the tidal pull of the Earth. It turns out that this pull on the rocks tends to slow down the spin of an object. The action of the Earth's tides upon the Moon for billions of years has been to slow down the Moon's spin. That explains why one part of the moon is always pointed at the Earth.

A wave, most broadly speaking, is a pattern of motion. You can get waves of all sorts: sound waves in the air, seismic waves in the ground, light waves in the vacuum, back-and-forth waves in a slinky, and of course water waves in the Bay! Here's an easy experiment for you to try: Go pick up the handset of a corded phone and walk across the room so the cord is stretched out a bit. Then pluck it with your finger. You should see a vibration run all the way down your stretched cord to the far end and maybe even bounce back a bit. That's a wave.

What most waves have in common is that they are oscillations, that is, back-and-forth motion of some sort. Water waves go up and down, a vibrating violin string goes side to side, and sound waves alternate high and low pressure. Now, why is this important? Well, it turns out that this sort of back-and-forth oscillation is very natural physically and turns up all over the place. The most detailed explanation of why this is the case involves partial differential equations and complex exponentials and other ugly stuff I don't really want to get into right now. :-P But skipping past all that, it turns out that anything "springy" (that is, anything which naturally wants to bounce back towards its original position when you push on it) naturally gives rise to waves. So the bouncing of your car as you go down the bad parts of San Pablo is a kind of wave, too!

For instance, consider sound waves. These arise because differences in air pressure act sort of like springs. Imagine if you run a bow across a violin, causing the string to vibrate. The vibrating string pushes the air out of the way as it moves, creating higher pressure regions where the air is compressed. But as soon as the string moves away, the compressed air expands again becoming low pressure. But then other air around rushes back in to fill up the low pressure, and you get a high pressure region again, and this back-and-forth pattern in the air carries the sound across the room.

So in the end, waves are just special patterns of motion. You asked about if they were like energy; while waves are not a form of energy themselves, they can carry energy or information with them as they move. That's because waves keep their pattern as they move through space, and as they move that up-down pattern from one place to another, the energy of that motion is carried along. You've felt this if you've ever been knocked over by a wave at the beach. Likewise, light waves can carry energy from the sun to the Earth, or from distant stars to our telescopes.

Nope. Anti-matter would feel the pull of gravity just like normal matter. Dark energy is instead a sort of anti-gravity force which apparently permeates all of space at an extremely weak level. It comes from the equations for general relativity. Einstein originally predicted this repulsive force in order to explain the fact that the universe was static, neither expanding nor contracting. But then we learned that the universe is in fact expanding, and Einstein declared that fiddling with his equations the way he had was the biggest mistake of his life. Only now it turns out he was probably right after all, ironically enough.

It's easiest to draw a HR diagram for a cluster, since then you can (as you said yourself) just use the star's brightnesses for their luminosities to draw in the vertical axis. But it's certainly possible to draw a H-R diagram for stars not in a cluster, too: It just means that for each and every star, you have to figure out how far away it is (via parallaxes or some other method) and then you can use the inverse square law to calculate what the luminosity is from the observed brightness. Once you've calculated what the luminosities are for all of your stars, you can plot them on an HR diagram just fine.

This is a fair amount of work, and for many stars where we only have a rough guess about their distance it introduces a fair amount of uncertainty into the numbers. So HR diagrams of clusters are still probably the most accurate ones. But for something like the sun, which we know its distance extremely precisely, it's pretty easy to figure out what its absolute luminosity is and then use that.

As you noted, it's hard to always keep an objective perspective when dealing with a highly political issue like this one.It's true, there is disagreement in the scientific community on whether humans are responsible for global warming or not. Most people are finally acknowledging that the warming itself is real, so the question becomes, are we ourselves responsible? And that's a harder one to answer, given the complexity of the Earth's biosphere and atmosphere.

That said, I personally do believe that human actions are in large part responsible for global warming. Changes in the sun may play some role, but I think the large size of temperature change we're seeing lately is too large to be explained by solar effects. Furthermore, increasing numbers of studies are determining direct effects between the climate changes and our emissions. There's an article in today's Washington Post on two recent studies which assert they have shown a direct link between C02 emissions and rising ocean temperatures.

Beyond that, I think we need to take a "better safe than sorry" approach to the issue. If climate change is linked to changes in the sun, then there isn't much we can do about it. But if it's our fault, then it's vitally important that we stop emitting greenhouse gases into the atmosphere as soon as possible, in the hopes of stopping things before too much damage is done. So in my opinion, even if you're not entirely sure that human emissions are responsible for warming, you should act as though you were sure and take the safer route of minimizing emissions until we really understand better what's going on.

The idea is that galaxies are not spread evenly throughout space. Rather, there are large bubbles of space where there are few or no galaxies, and instead all the galaxies are located along the boundaries between the bubbles.

Think of it this way. If you get a whole bunch of big soapy bubbles together (big bubbles, like bath suds, not little fizzy bubbles like on top of soda) then most of the volume has no soap, but instead is the insides of bubbles. The actual soap itself only exists as the boundaries between the bubbles. Likewise, there are no galaxies in most of space, but rather the galaxies all sit in the boundary regions between larger empty regions.

Everywhere we look, we see a very cool, very uniform glow at 3 degrees. This glow must have come from somewhere, right, so let's think about what that source might be.

It can't be from stars - they're far too hot, and besides this light is evenly spread everywhere. It can't be from nebula - those are far too clumpy and irregular to give the almost perfectly even and uniform background we see. It can't be from galaxies, since those just contain stars and nebulae. Hmmm...

Another thing we can learn about the background radiation is that it is coming from very very far away. In effect, we can look at clouds of gas and see that the CMB is coming from the far side of them, though something called the Sunyaev-Zeldovich effect (don't worry about the details of how this works too much)

We're left with the existence of a nearly perfectly uniform 3 degree glow, all over the universe, coming from farther away than the most distant galaxies. It's very important to note that the CMB looks almost exactly the same on opposite sides of the universe - this is a very surprising result, since the light from every galaxy or star is slightly different from every other one, and you expect things from so far apart to be fairly different.

The best explanation we have for the CMB is that it is left over radiation from the superheated early period after the big bang. The universe was hot and glowing then, at a temperature of thousands and thousands of degrees, but the leftover light has been stretched to the red and cooled down by the expansion of the universe since then. The CMB is older than everything else and seems to come from further away than everything, since we see further into the past the more distant we look. The CMB was produced when the universe was much smaller than it was today, which can explain why it looks so uniform over the entire sky. The "opposite sides of the universe" were much closer together then than they are now! In short, it's easy to explain the CMB as a leftover glow from the Big Bang, and pretty hard to explain it any other way.

Gibor: Another (simpler) answer to this question is that because the radiation has the spectrum of a perfect blackbody, it must have been produced by a THERMAL process. One requirement is that it must have been produced in an opaque region (remember our discussion of blackbodies). Since it is everywhere, we can conclude that the whole Universe was opaque at the time the radiation acquired its spectral shape. This condition is natural in the Big Bang theory, but otherwise rather unfathomable.

Yes, we use light to carry information all the time. Radio waves are of course a form of light, and are used to carry music, video, and data all over the place. And we don't just use radio waves, either - Infrared radiation lets you command your television without getting up from the couch or synch up your Palm Pilot with a friend's. And yes, as you guessed, visible light is sent down fiber optic cables carrying large amounts of data across the country at the speed of, well, light. The fiber itself merely serves as a guide - it prevents the light from spreading out as it travels, allowing signals to propagate further than they could otherwise, and it allows the light's path to be carefully controlled from point A to point B.

The idea of using light to convey information across long distances isn't even a modern one - think of Paul Revere and the warning delivered by lights that night: one if by land, two if by sea. Sure, it's a far cry from a high-speed optical network, at least in terms of the technology, but the basic principle is the same - it's just a bit of information transmitted in binary through patterns of light.

(I wonder... can I get extra TA bonus points for slipping some history class material into an astronomy course? ;-)

Is it dark matter?....Or is it Antigravity?....Is it both? What does dark matter do in the Universe? If it is antigravity, how come antigravity does not work here on earth?

We believe it's a form of anti-gravity, called various names like "dark energy" or "the cosmological constant", among others. This is very new science, so we're not really sure how it works yet. Dark matter, in contrast, has regular gravity and acts to pull things back and slow down the expansion. It serves to hold together galaxies and clusters, acting as a sort of invisible glue which you can only tell is there based on its gravity.

As for antigravity on Earth, well, we only just discovered this force two or three years ago. Give 'em some more time and we'll see what can be done.... (Somewhat more seriously, we believe the antigravity force is actually very weak, and only has such a large effect because there is such a tremendously large amount of space filled with it. But in any small region the force is quite weak. This may make it not useful for us for lifting things, but my guess is it's really way too early to say.)

You're absolutely correct. Sound waves are vibrations of matter - the air, usually, though sound can travel through pretty much any material object. In fact, it usually travels better through solids than through the air, which is why you can usually hear what's on the other side of a door or wall if you put your ear against it.

Without any matter to travel through, you can't have any sound waves. That's why it's usually said that "there's no sound in space" and all those loud explosions when space craft blow up in the movies are all wrong. :-) On the other hand, as we've learned, sometimes there *is* matter in space - in nebulae and dust clouds. There you can in fact have sound waves, though they are very diffuse and low frequency - too low for you to hear with your own ears, certainly.

Light, in contrast, is a vibration of electric and magnetic fields. (You can also look at it as a particle, and both interpretations are correct; that's part of the weirdness of quantum mechanics) But looking at it as a wave, since it is a wave of fields rather than a wave of matter, it turns out it can propagate even through a vacuum! However, even though we know this now, this was by no means obvious to scientists a hundred years ago, and many of them believed there had to be some sort of mysterious ether filling all of space so that light could vibrate in it. It was only in 1895 that an experiment by two physicists named Michelson and Morley proved that there was no such thing as ether, and that light really did have to be able to travel through nothing at all.

Gibor: This is a question which I feel poses a false conflict. There is no reason why one can't both attack hunger and poverty, and look towards the stars. The amount of money which is wasted on ... (fill in your own favorite here; I like B-2 bombers, national missle defense, hair spray, tobacco, drugs, guns, whatever!) is vastly more than what is spent on Astronomy or space exploration. If you want to devote more money to good causes, look there first! I think it would be an enormous pity if people felt that no money should be spent on ... (science, exploration, cultural enrichment, art - fill in your favorite non-profitable or non-charitable activity here) until all hunger, poverty, war, and disease were eliminated (don't hold your breath).

Marshall: I agree with basically everything Gibor said; there's no reason you cannot try to do both things, and allocate some fraction of your money to each of those tasks based on some criteria.

But I think another point which is worth making is that money spent on space is not money spent in space. That is to say, the amounts which are spent on astronomy and space science don't just vanish into a black hole, but rather turn into the salaries for researchers, engineers, technicians, and us grad students! Then all of those people in turn spend their money on things, and the economy keeps rolling along. I can't help but think it would be a loss to the planet to suddenly have all these brilliant scientists without jobs. Indeed, international funding for telescope construction is now a major source of income for Chile, and there are similar beneficial effects in Arizona and Hawaii in the US.

Beyond that, you should look into what is known as "spin-offs" of space technologies. All sorts of stuff developed for space has turned out to have much wider uses. Everything from protective suits for fire-fighters, to medical imaging devices, to more dolphin-safe fish nets, improved solar power, UV-blocking sunglasses, and more have benefited from NASA inventions. Heck, one of the big initial pushes that led to the miniaturization of electronics and the computer revolution was the need to build powerful computers that were nonetheless still small enough to fit into a cramped spacecraft and go to the moon. Adaptive Optics technology (just like that used to improve telescope images) has now made it possible to give people "super-vision" (i.e about 20/7 vision - 3 times better than normal 20/20!) and while this is still just in the laboratory testing stage now, I bet it will become a commercial product within our lifetimes.

I have a hundred-page book that does nothing but list various NASA inventions that have gone on to improve society as a whole; I'd be happy to let anyone take a look at it. Any time you try to solve problems harder than any which have solved before, you're bound to make new discoveries and new inventions, and who knows where -those- will lead?

Venus rotates in about 240 days, very slowly. In contrast, its winds go around the planet once every four days, so they're tremendously faster than the rotation. The winds transport heat, which is why it's the same temperature everywhere on Venus dispite the really slow rotation (without the winds, things would get much colder during the long night.)

Gravitational and magnetic fields are actually quite different. Gravity acts on all matter, and is always attractive. That is, it always acts to pull things together. Magnetic fields, in contrast, only affect certain things, primarily charged particles such as those in the solar wind. (They also affect certain metals, called ferromagnetic metals, but that's not as important for our purposes out in space, although it does explain how your refrigerator magnets work...) Magnetic fields also don't usually act to attract particles - rather, they tend to deflect them sideways, as in the Earth or Jupiter's magnetospheres deflecting the solar wind around the planet. Sometimes they can trap magnetic particles, such as in the Van Allen radiation belts around the Earth. Finally, magnetic fields are generally (though not always!) much smaller than gravitational fields.

Tycho Brahe's very careful observations of Mars. Tycho, Kepler's mentor, built the best observatory of the time and was able to get better measurements of planet's positions in the sky as a result. Kepler spent years trying to figure out the right equations for modeling the orbits of the planets. Mars in particular he simply could not get to work using a circular orbit, no matter how he tried. Eventually he gave up on using circles and tried an ellipse, which he was able to make work after all.

It turns out that only a very very few of Tycho's data points didn't work with the circular orbit. Something like 99% of them worked fine, so Kepler *could* have decided the other 1% were just errors on Tycho's part and thrown the data away. To his credit, he did not. The moral of the story is, be very careful before deciding that a measurement is just plain wrong, even if it surprises you. Sometimes you discover something new!

Jason:Leap years are necessary because there aren't exactly 365 days in a year. When the earth returns to the same point in its orbit after one year, it's not the same time of day as it was the last time. Because of this, if we measure the year as 365 days instead of by the position of the earth in its orbit, we'll "lose" about one day every four years and eventually summer will be in March, etc. Leap years were invented to correct for this problem with the advent of the Gregorian calendar.

The earth's orbit around the sun is quite regular, so every year is the same length: 365.255 days. There are small changes to the orbit due to the other planets and other complications, but they are negligable.

Gibor: The speed of the Earth in its orbit does change during the year, because it is slightly elliptical. We are moving faster when we are closer to the Sun (closest in January). Thus, if one uses a sundial to tell time, a correction must be made depending on the time of year. This is sometimes called "the equation of time". You can see how it works on the sundial which is just south of the Campanile (the bust of Lincoln is looking at it). But these correction remain the same from year to year.

You're right, it would be very odd if the moon had just happened to form with the same rotation and revolution periods. Luckily, there is an explanation: The moon started out spinning much faster, but over time was slowed down and became 'locked' towards the Earth as it is today.

Here's why. The moon's motion around the Earth causes the tides down here. (See p. 94-96 in the text for an explanation of how this comes about.) But at the same time, the Earth causes tides on the moon. These tides act as a drag force, trying to slow things down. Over the billions of years that the Earth and moon have been around, the tidal forces have been enough to slow the moon down to the point where its rotation period is the same as its orbital period. We say that it has become "tidally locked".

It turns out that most moons in the solar system are tidally locked towards the planets they go around. This includes all the large moons of Jupiter and Saturn, for instance.

You may ask, why don't the tides slow down the Earth's spin too? The answer is, they do! But the Earth is much larger than the moon, so (a) the tides from the moon on the Earth are smaller than the tides in the other direction, and (b) the Earth is harder to slow down due to its larger size. So the Earth's spin has not slowed down nearly as much as the moon's spin has. Still, it means that when the dinosaurs roamed the Earth, the day was probably only around 23 hours long! The Earth's spin is still slowing down today, at a rate of some few seconds every century or so - very small, but measurable with our best clocks.

You can imagine that billions of years in the future, the tides will have had long enough to slow the Earth down to match the moon. Then the Earth will take about a month to spin around, too, so that one part of the planet always faces the moon. This has already happened with Pluto and its moon Charon - both of which are quite small, so they didn't take too long to become tidally locked. The small planet and its even smaller moon both have one face which points permanently at the other as they go around their 6 day rotation.

It turns out that it would take so long for the Earth to become tidally locked to the moon, that it's longer than the amount of time we expect the sun to keep burning, about 5 billion years. So it will actually probably never come to pass that the Earth will become locked to the moon, before the end of the solar system. We'll talk about this more when we get to the chapters on stars.

Jason: If you're asking why the day is broken up into 24 hours instead of 15 or 10 or 50, it has to do with the ancient Persians, who used a base-12 counting system, which may have its origins in the fact that there are 12 full moons every year. This system remains with us in many units of measurement, from 12 inches in a foot, to 360(=12*30) degrees in a circle. Their system of measuring hours may have been the most widely used because it could be broken up into 6 4-hour blocks, or 3 8-hour blocks, or 4 6-hour blocks, or any number of other convenient divisions.

If you're asking why a day lasts as long as it does, that is, why the earth spins at the rate it does and not faster or slower, the answer is that it hasn't always done it the way it does now! The moon raises tides on the earth, which means that as the earth turns it must change its shape very slightly as the moon pulls strongest on different parts of the surface. Internal friction (viscosity) converts this motion into heat, which makes the earth turn slower. The effect is that the earth is slowly slowing down: at some point in the past, the earth's rotation was, for instance, only 14 (current) hours long.

This happened to the moon in the past, too, until the moon slowed down so much that it became "locked" with the same side always facing the earth. Given a few billion years, the earth's rotation would eventually do the same, and the earth would rotate only something like once a month, always keeping the same side facing the moon. Pluto and Charon are locked in this sort of a dance right now.

Marshall: Well, in the case of some of the planets, we know because we've gone there and checked. We've got rocks from the Moon brought back by Apollo, and we've sent landers to Mars, Venus, Jupiter, and now the asteroid Eros which all have sensors capable of determining the composition of the planet directly (albeit only in a small region right around where the probes landed. They don't tell us anything about anywhere else, but usually we can assume that most of the planet is pretty similar.)

But beyond that, to answer your question directly: No two elements or molecules give off exactly the same kind of light. They're all unique, and the spectrum of a star or planet will usually have thousands of individual lines. However, some atomic lines are very similar to each other, so you need a good detector to tell them apart! For instance, there's a hydrogen line which shows up in light with a wavelength of 486.1 nanometers. But there's a helium line at 492.1, too. So the detector you use to measure the light (these are called "spectroscopes") had better be good enough to tell the difference between those two kinds of light. The human eye isn't a good enough detector for that - they both look blue-green to us. But a good enough electronic detector will be able to tell the difference between those two colors of light, and thus between those two elements.

JohnJOhn: I think you may be getting planets and stars mixed up. We can tell what stars are made of because of the light they emit. But it's a little more than just the color. Often, it's actually the colors they _don't_ emit. These are called absorbtion lines. Today in lecture we talked about the blackbody curve, or Planck curve. This curve represents the amounts of all colors emitted by a _perfect_ blackbody.

Stars are really close to being perfect blackbodies and therefore emit spectrums that look like a Plank curve. However, if you look really close at the spectrum you'll notice missing colors. The missing colors come from light from the star interacting with elements in the atmosphere of the star. Elements like hydrogen and helium absorb only certain wavelengths and are very picky about it. The colors a certain element absorbs (which correspond to different frequencies) make up a spectral "finger print" for that star.

The field of spectroscopy concerns itself with the study of various elemental (hydrogen, nitrogen, helium, etc) lines. If everythign works out right, we'll get to do some spectroscopy in section next week. We'll look at different gasses and see how they all give off very different colors of light. We'll also talk about the difference between absorbtion spectra and emission spectra and what they tell us about stars.

Note: If you were actually talking about planets emitting light, I'd like to see the chapter of the book you are refering to. Otherwise, make sure you didn't really mean star rather than planet.

This is a great question to think about, and in fact lots of scientists are still thinking about it very hard. Truth is, we don't fully know yet why the corona is so hot. The best explanation is that the magnetic field of the sun somehow carries energy out from the deeper inner layers to the corona. However, the mechanism by which energy is transformed from magnetic fields into heat is not yet clear. See this web site for some more information, or just do a Google search on "why solar corona hot" for lots of links.

My guess is that if you were in the corona, you would not really be able to perceive it. Yes, it's several million degrees, but it's also very thin - several billion times less pressure than sea level on the Earth. So even though the atoms are very hot, there are so few of them that you don't notice them. On the other hand, you'd also be sitting right next to the Sun, and I think you'd definitely notice that! Ouch.

The Earth's atmosphere has a similar structure to this, where the highest parts, up where the space shuttle orbits, are technically still a part of the atmosphere with a temperature of 1000 K or so. But the air is so thin that astronauts never notice it directly. The difference is that we know why the upper parts of the atmosphere are so hot - it's because they absorb all the high-energy UV and X radiation from the Sun, protecting all of us down below.

http://hesperia.gsfc.nasa.gov/sftheory/heat.htm

JohnJohn: Electromagnetic radiation is indeed light! We usually only think of the _visible_ part of the electromagnetic spectrum when we refer to light. We say the sun is bright, meaning there is a lot of visible electromagnetic radiation coming from the sun. This is because we have evolved eyes that are very sensitive to visible radiation. However, a car engine can also be very bright, but in this case it would be bright in the infrared part of the spectrum. A snake or a flea would have no problem seeing a car engine on a dark night, or better yet, a warm-blooded animal. The big radio tower on top of Twin Peaks in San Francisco is also very bright, but this time in the radio part of the spectrum. And by now I'm sure you can think of your microwave oven or your doctor's X ray machine as bright. And that would definately be seeing things in a different light! :P

The human eye, I think, has evolved to see visible wavelengths because those are the wavelengths that make it to the ground from the sun. Infrared and ultraviolet light are scattered and emitted by the atmosphere a lot, so there's not as much light in those wavelengths for the human eye to use (during the daytime, anyway, when we're awake).

Bees have a special reason to use UV light, which is that many flowers reflect UV light well, so if you could see in the UV all you'd really see are flowers. Not too useful for humans, but essential for bees.

That's my understanding anyway; I'm not an evolutionary biologist.

JohnJohn:t is possible to observe infrared radiation on the ground. However, it's usually pretty difficult. The instrument has to be cooled with liquid nitrogen, because if the instrument is warm, it gives off infrared radiation. And if you are trying to see faint infrared waves from space when you have infrared radiation from your instrument all around, things get messy. It's like trying hear your friend on the other side of Haas Pavillian with people screaming all around you during a game! So what you do is cool the instrument off so it doesn't interfere (shut the crowd up). Observing in the IR is much easier in space where it's already really really cold. But it's also really really expensive.

Gibor: This answer depends on what part of the infrared you are talking about. The "near" infrared can be observed pretty easily (you can buy "night vision goggles" originally developed for the military that do that for you. Once you get out to wavelengths of a few thousand nm, however, everything is emitting lots of light (that's what room temperature will do), and its a lot harder to make things out in the brightness of all things glowing (including the air).