Ask An Astronomer 

Answer Newsletter

17 July 2003


Here are the answers to selected questions. . Questions may have been edited for clarity or brevity. Click on a link to move directly to the answer.

  1. Why do some planets have reverse rotation?

    Almost all celestial rotation in our solar system is from West to East, or counterclockwise when looking down from the North pole. All planets orbit the Sun in this direction; the Sun itself, as well as all but two planets rotate in this way. This is not too surprising, since the angular momentum associated with all this rotation is the same as the total angular momentum of the primordial solar nebula from which the sun and all planets formed.

    The two exceptions are Venus and Uranus. Uranus' rotational axis is tilted by slightly more than 90 degrees with respect to its orbital plane, i.e. it is rotating "on its side". Venus is the only planet in the solar system that truly exhibits reverse rotation, spinning in the opposite direction to its orbital motion.

    It is thought that this anomalous rotation was caused by a collision with a very large asteroid, or even a planetoid, early on in the history of our solar system. The collision would have destroyed the asteroid, but the impact could have changed the orientation of the orbital rotation axis.

  2. what is the easiest way to measure the distance between the earth and the sun?

    The easiest way to calculate the distance from the earth to the sun is by using the known distance to one of the other planets. It takes a few steps though.. so bear with me. Take Venus for example; we can use radar signals to find the distance to Venus. Signals sent from earth will bounce off the surface of Venus and then travel back to earth where we can detect them. Because radar travels at the speed of light (a quantity that is well known from laboratory experiments) you can easily calculate the distance between Venus and Earth at any time.

    distance = 0.5*(time for radar to travel both ways)*(speed of light)

    Now that we know the distance to Venus we can calculate the distance from the Earth to the Sun using trigonometry. Both Venus and Earth travel in almost perfect circles around the earth, so this is a fairly easy calculation. It might help to envision the situation if you draw two concentric circles on a piece of paper to represent the orbits of the Venus and Earth. Now imagine that the earth is set at one position in its orbit, as Venus travels along its orbit there are two points where the Venus-Sun line and the Earth-Sun are perpendicular (90 degrees apart). From our perspective on earth this is the point where Venus is the farthest it will get from the Sun. By keeping track of Venus over time we determine exactly when this will happen. When this happens we make the radar measurements mentioned above and we also measure the angle between the Sun and Venus at this point (here's where the trigonometry comes in!). It turns out that the distance from the Sun to the Earth is:

    (distance of Sun-Earth) = (distance of Earth-Venus)*(cosine(angle))

    This is how we get the distance of Sun-Earth to be 92,955,830 miles! This is also known as one astronomical unit (AU)

  3. If the universe is expanding and the speed at which it is expanding is certainly consistently lower than the speed of light, then the light emitted by the birth of the universe must have passed us by a long time ago, right? How is it possible to look back and see the birth of the universe?

    Many people struggle with the idea that looking farther out means looking farther back, at least in the temporal sense. Presumably, you already get this point, or at least part of it. Just to make sure we're on the same page though, along with anyone who might stumble across this, let me put it this way:

    The light emitted by the birth of the universe comes from... well, everywhere. The early universe was a boiling sea of high energy particles. These particles were packed closely enough to keep light trapped basically right where it was sitting. In other words, the universe was opaque.

    However, the universe was expanding, which means the boiling sea of particles was cooling off, as all expanding substances do, and eventually, the whole shebang cooled to the point where light could escape, which it did, from every point in the universe at once.

    So let's imagine you're sitting in a spaceship in the middle of all of this, watching the universe become transparent. (With appropriate radiation shielding, or some sort of super-robot body. Use your imagination.) What you'd see in the first instant is all the now-free light rushing at you from the nearby part of the universe, naturally, the light that's had time to reach you. Wait five minutes longer, and you're still seeing light, just light that had to travel farther to get to you. But all this light was emitted at the same time, so you conclude that the light is five minutes old, and you're seeing something that happened five minutes ago.

    Hopefully, you see where this is going. Wait around a few billion years, and you're still seeing light from that same event, way back when, but which was emitted so far away that it took all of those several billion years just to reach you. So by virtue of the long unfathomable distances between, you get to look back in time.

    The key to your question may be in your understanding of the place in time and space from which the light was emitted. If the light had been emitted immediately, i.e. at the same instant the universe was created as a tiny little speck, then all the light would certainly have raced past us by now. But we're not looking at some cosmic firecracker, the burst of light that was produced by the explosion that started it all, in fact we can't, precisely because the universe was too bright for the light to move around for the first few hundred thousand years. (Well, too energetic, anyway. Hopefully, this makes sense.) By the time the universe was transparent, it was also very big, such that we can look out to long distances at times long ago.

    Which would be the whole story, if the universe had stopped expanding once it turned transparent. It did not, however, so the answer is a little more complicated (though if you made sense of the above explanation, you should be in good shape). It turns out that your second assumption--that the universe expands more slowly than light speed--is wrong.

    Which might sound like a joke at first, but I assure you I'm serious. Objects are constrained to move at velocities less than light speed, but space itself may stretch as fast as it wishes, over great distances. Of course, if the space between two points is stretching faster than light, then those two points are prevented from ever trading information, and might as well belong to entirely separate universes.

    So our universe effectively has some maximum volume, namely, that volume which is expanding away from us more slowly than light speed. It just happens that this volume still encompasses light that was emitted in the earliest years of the universes existence. Presumably, this light will eventually disappear over the edge from our perspective, never to be seen again.

  4. How can astronomers say that the universe is expanding without knowing where the boundaries of the universe are?

    Before we jump in, let's first correct a bit of trivia in your question. While dark matter has implications for cosmology--the way the universe will behave/has behaved on very long timescales--the evidence of its existence is independent of the concept of an expanding universe.

    Now, as for your question itself. There is something of a boundary to the universe, the Cosmic Microwave Background. It represents a point in the universe's history at which the universe was opaque, therefore we can't see anything beyond it. But we can't exactly measure how much it recedes every year, since it's a few billion light-years away, a distance that's not going to change by a measurable amount over the course of our lives.

    Still, you haven't found a fatal flaw in cosmology, because there are always other tools at a scientist's disposal. I invoke the power of Doppler shift (or redshift, if you prefer). While we can't necessarily measure giant distances with high precision, we can usually get a reasonably precise measurement of the spectrum of an object. You may be used to thinking of a spectrum in terms of prisms and rainbows and such, and it's not far off. However, the spectra astronomers use tend to be much more detailed, to the point where we can make out individual spectral lines.

    Without going into overly boring detail, a spectral line is a particular wavelength of light (a very specific shade of red, for instance) that is preferentially emitted by an individual atom. You can learn more: here, along with a bunch of other quantum physics stuff, if you're interested.

    Back to Doppler shift, when an object is in motion, the spectral lines emitted by the atoms move around to different wavelengths. It's similar to the effect that causes a train's whistle or an ambulance's siren to change pitch as it goes flying past you. If an object is moving towards you, the light gets shifted toward shorter wavelengths; if it's moving away, the light is shifted toward longer wavelengths. These effects are called blueshift and redshift, respectively. (Though, to be sure, if light is already blue, it will get blueshifted toward violet or ultraviolet, and if light is already red, it will get redshifted toward infrared. It gets confusing if you put too much thought into it, so just think about it in terms of what effect it has on yellow light.)

    Knowing this, Edwin Hubble (the guy the telescope is named after) went out and measured the Doppler shift of several nearby galaxies. If you assume the universe is not expanding, you'd expect to see a more or less even distribution of blueshifts and redshifts. Surprisingly enough, he found that almost all of the galaxies he measured were redshifted, meaning every galaxy is traveling away from us. Even more evidence has been amassed today, and we know that galaxies very far away are extremely redshifted. So we know beyond a shadow of a doubt that everything in the universe seems to be moving away from everything else. And if this is the case, the universe must be expanding, as it's hard to imagine a way to make things get farther apart without actually increasing the extent of space they cover.

    To get mathematical for a moment, prior to Hubble's measurements, Albert Einstein had formulated his theory of General Relativity, coming up with an equation that described the entire universe. He was unsatisfied, however, when he realized that his equation predicted that the universe must be either expanding or contracting. He was able to create a solution in which the universe could be stable, but upon the announcement of Hubble's discovery, he changed his mind, calling the alteration the biggest mistake of his career.

    So, one of the greatest minds of all time predicted a universe that was expanding, and one of the first guys to go looking found he was right, as has everyone who's gone looking since. And hopefully, that's all the answer you need.


Thanks to Alex McDaniel, David Lai, Shawfeng Dong, Gabe Prochter, Ian Dobbs-Dixon, Jay Strader, Justin Harker, Karrie Gilbert, Kyle Lanclos, Laura Langland-Shula, Lynne Raschke, Marla Geha, Michael Kuhlen, Nick Konidaris and Scott Seagroves

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Last modified: Fri Jul 18 14:48:07 PDT 2003