Ask An Astronomer 

Answer Newsletter

16 February 2002


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. I would like to photograph star trails. How can I take these pictures?

    What you need:

    What to do:

    For more pages on astrophotography, see: Google's Astrophotography Directory.

    (Thanks to Scott Seagroves for this answer!)

  2. Is Jupiter's Great Red Spot a thunderstorm? If yes, how is it different of similar to the ones on Earth?

    The Great Red Spot on Jupiter is one of the most beautiful phenomena in the solar system (see APOD's Great Red Spot image). The spot was first observed by the Italian-French astronomer Jean-Dominique Cassini in 1655, roughly 350 years ago! Its size is 12,000 by 25,000 kilometers, big enough to hold two Earth's.

    The Great Red Spot is indeed thought to be a giant hurricane-like storm system. More exactly it would be classified as an anti-cyclone. Cyclones on Earth are low pressure systems with stormy winds spiraling around the calm center. The Coriolis effect, which is due Earth's rotation, causes cyclones in the northern hemisphere to rotate counter-clockwise, and clockwise in the southern hemisphere. The Great Red Spot, in contrast, is a high pressure system, and is thus called an anti-cyclone. The orientation of the rotation is flipped, and since the Great Red Spot is located on the southern hemisphere of Jupiter it rotates counter-clockwise. Its outer edge completes a rotation about every 6 days, which is much longer than the 10 hours that it takes Jupiter to rotate about its own axis.

    Incidentally, the Great Red Spot is currently in the process of colliding with another great storm on Jupiter. The collision should last for about a month. Most likely the Great Red Spot will emerge unharmed from this scuffle, but the smaller one will most likely be disrupted or even absorbed. The last time another storm collided with the Great Red Spot was in 1975, and it caused the Great Red Spot to fade in color for several years.

  3. Do we have rock samples from Mars here on Earth ? How?

    Although we have sent many spacecraft on a one-way journey to Mars, no missions have made the return trip to bring Mars rocks back to Earth. Yet we have many rock samples that are fairly certain to be from Mars. How can this be? All of these Mars rocks are meteorites- objects from space which enter the Earth's atmosphere and have fallen to the ground (while they are entering the atmosphere they glow brightly and are often call falling stars). By studying what meteorites are made of, we know that most are pieces of asteroids or comets. Once in a while, however, the composition of a meteorite is exactly the same as robotic surveys have found on Mars. This doesn't happen very often- of the 22,000 meteorite discovered on Earth so far, only 24 have been identified as originating from Mars. We think that these Martian rocks have been somehow kick off Mars, maybe during a large impact, have wandered around the solar system and eventually found their way to Earth. Since we have not yet been able to get rock samples directly from Mars, these meteorites are very important for studying the composition of Mars.

    For pictures and more information about Martian meteorites, see JPL's Mars Meteorites page.

  4. What is an accretion disk?

    An accretion disk is simply a disk of material that forms as the material falls onto some massive object. The object in the center is gobbling up (accreting) the disk material. People typically talk about accretion disks around very massive and extremely compact objects, such as black holes or neutron stars.

    There are several artists conceptions of accretion disks around black holes:

  5. Why is there sometimes a ring around the moon? Is it useful for weather forecasting?

    Contrary to what some people think, the ring around the moon is not due to failure of the moon to use the proper laundry detergent. Rather, the ring is caused by the reflection of moonlight off of ice crystals in high, thin clouds. The ring has a radius of 22 degrees, which is set by the properties of water.

    Most often, the ring around the moon (and also around the sun) is caused by high, thin cirrus clouds -- sometimes too thin to be seen by any other means. Cirrus clouds tend to precede weather fronts, meaning that bad weather is likely ahead. So, it is often said that a ring around the moon or sun indicates rain.

    To the best of my knowledge, professional meteorologists do not use the appearance of a ring to forecast the weather.

    I found two cool pictures of the ring phenomenon:


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

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