Ask An Astronomer 

Answer Newsletter

21 November 2001


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

  1. When is the best time to see the Northern Lights? Where should I look? How do I know when there will be a display?

    The Aurora are caused by storms on the sun ejecting large amounts of gas toward the Earth. When that gas hits our atmosphere, it glows, creating the northern lights. The more intense a storm, the further south it can be seen. This is why Alaskans can see aurora nearly every night, while the rest of us only get a few chances once in a great while.

    The best way that I know of to be informed about impending aurora is to sign up to an "AstroAlert" email list for solar activity. I belong to this list. When there is a major storm on the sun, an email is sent out containing information on where aurora might be visible. The web site with all of the available mailing lists is Astro Alerts from Sky and Telescope magazine.

    Aurora are fairly faint, and are best seen far from city lights. They also are visible mainly in the northern sky, so you want the northern sky to be free of cities. For truly dark skies, you need to be 30 or 40 miles from the city.

    For some more information on the Aurora, see: and

  2. What would be the after affect of a meteor striking an ice mass?

    Most of the effects of a large meteor impact on ice would be the same as if it struck water. The impact creates enough heat that the ice would vaporize, just as water would. However, there are differences between a meteor striking the North Pole and one striking the South Pole. The northern polar ice cap sits on top of an ocean, and so the effects would be like a meteor striking an ocean. But the southern polar ice cap covers a continent. A meteor large enough to vaporize the ice cap would also be large enough to hit the Antarctic continent's land under the ice, and so the effects would be similar to an asteroid hitting land.

    A nice description of the effects of large asteroids hitting the Earth, and a great story of how scientists determined that an asteroid hit the earth at the time the dinosaurs went extinct is contained in the book "T. Rex and the Crater of Doom" by Walter Alvarez. Also see our asteroid impact page.

  3. I have always been told that looking at the band of the Milky Way in the night sky is viewing our own galaxy. What is our positional orientation when viewing this band, i.e., are we looking at another spiral arm of the Milky Way? Are we looking towards the center or away? Why is it tilted in reference to us?

    The Milky Way galaxy is a type of galaxy known as a "disk" galaxy. This means that the majority of the visible material in the Milky Way is in a round disk, like an LP record or a compact disc. The Milky Way is about 80,000 light years in diameter, and only about 300 light years thick, so it really is very, very flat! We are located in the disk, and the band that we see is looking into the disk -- look at a Frisbee edge on, and you'll see a similar band.

    The Milky Way is also a spiral galaxy, but the spiral arms are in the disk, so we have trouble seeing the arms. The gas in our galaxy tends to be concentrated in the spiral arms. Cool gas emits radio waves, and so radio telescopes have been used to map out the spiral arm structure of the Milky Way.

    The center of the Milky Way is in the direction of the constellation Sagittarius, visible low in the south in the summer.

    The Milky Way appears tilted for two reasons. First, the Earth's axis is tilted with respect to the sun. It is uncertain why this is the case -- perhaps a collision between the young Earth and a large protoplanet knocked the Earth's axis sideways. Second, the sun's rotation axis is tilted with respect to the Milky Way's plane.

    For a cool picture of our galaxy taken in the infrared (so that dust doesn't block our view), see the 2MASS All-Sky Image. Beware -- it's huge (2 Megabytes)!

  4. It is said the sun is the center of our solar system. Why can the earth not be the center ?

    It certainly seems like the Earth is the center of everything from our everyday experience -- it doesn't feel like we are moving! But it is actually proven that the Earth is moving in a circle around the sun.

    The ancient Greeks realized that the Earth could go around the sun, or the sun around the Earth, and it would look the same to our naked eye. The Greeks also thought (correctly!) that the stars were objects like the sun, only far away. If the Earth moved around the sun, then we should be looking at the stars from a slightly different direction when the Earth is on opposite sides of the sun, and so the stars should appear to change position. This is called "parallax." However, the Greeks didn't see any parallax, so they assumed the Earth was stationary. It wasn't until 1838 that a star was observed to shift position extremely slightly during the year due to parallax. Since then, all nearby stars have had their parallax measured. The only explanation for parallax is that the Earth is moving in a circle.

    Second, we can also prove that the Earth is moving in a circle due to the Doppler shift. Just like police can use radar to determine how fast a car is moving, we can use light from a star to determine how fast it is moving. When we look at stars at different times of the year, we find that their speed changes. At one point a star might be moving toward us at 30 kilometers per second (11,000 kilometers per hour!), and six months later the same star is moving away from us at 30 kilometers a second. This changing Doppler shift is observed in every star, and the Earth's motion around the sun explains this observation.

    Last in this list (but certainly not last overall), the effect of "stellar aberration" proves the Earth is moving. Stellar aberration is a complicated phenomenon, too complex to explain here. I refer you all to a decent description by Paul Bickerstaff.

    While it is possible to explain each of these facts individually using contrived theories (for instance, if all the stars in the galaxy moved in circles at a rate of 30 km/s over a period of one year, you could explain the Doppler effect), the necessary theories contradict one another. The motion of the Earth around the sun is a much simpler way to explain these effects! And, once we know the Earth goes around the sun, we can use that information to send spacecraft to different planets -- and the spacecraft arrive safely and on target -- more proof of our picture of the solar system!

    Some more reading on this topic:

  5. Are there mainly huge planets out there (like Jupiter and larger), or is it just that our equipment hasn't got the resolution to detect smaller ones?

    This is a very perceptive question! So far, extrasolar planets (planets around stars other than the sun) have been found by looking for the pull of a planet's gravity on the parent star.

    The force of gravity of a planet on the parent star depends on two factors -- the mass of the planet, and the distance between the planet and the star. The more massive a planet, the stronger it pulls on the star. The closer the planet to the star, the greater its pull.

    Astronomers can measure the speed of the parent star to about 3-5 meters per second (7 to 11 miles per hour). This accuracy gets better with time. Jupiter causes the sun to move at 12 meters per second (27 miles per hour), so it would be barely detectable with today's technology. If Jupiter were much closer to the sun or much more massive, this motion would be much faster. For example, the planet around the star 51 Pegasi is about 1/2 the mass of Jupiter and is about 1/20 the distance from its star that the Earth is from the sun. We see the star 51 Peg moving at about 56 meters per second (130 miles per hour).

    So, the easiest planets to detect are those that are large and close to the parent star. And, in fact, the first planets found were large and close to the parent star!

    There is another reason that the first planets found were close to the parent star. Astronomers like to follow the planets for at least an orbit or two to make sure that their measurements are correct. Jupiter takes 11 years to orbit the sun, but planet searches have only been running for the past 6 years or so.

    Finally, the current means of finding planets will only find large planets, not Earth-sized planets. The Earth's gravity causes the sun to move at a rate of about 0.1 meters per second -- 30 times slower than the best current detection limits! We don't think that we'll be able to detect Earth-sized planets with this method any time soon. However, there are plans to look for Earth-sized planets by looking for the passage of the planet directly in front of the parent star. The planet will block a very tiny portion of the star's light, which we should be able to detect with an ultra-sensitive space telescope. Missions to do this (one planned by NASA is called "Kepler") are being designed, but are still years away from collecting data.

    You can read more about the hunt for other planets at The University of California Planet Search Project Homepage.


Thanks to Marla Geha, Mike Kuhlen, and Greg Novak for helping to answer these questions!

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