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Determining Planet or Star?

  1. Jan 20, 2004 #1
    When looking in the sky, how would you determine if your seeing a planet or a star? Thanks.
  2. jcsd
  3. Jan 20, 2004 #2


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    Hello NanoTech, there are probably several clues that different people use, some conscious, and others not so conscious and harder to put in words. Ideally you will get several answers.

    It isnt completely reliable but planets can actually look different.
    Venus is brighter than any star. Jupiter is too, sometimes.
    Mars has a tinge of orange. I think of Jupiter's color as "cold" compared to Mars.
    Some people say planets dont twinkle as much as stars and that would make sense because the angular size of stars is virtually zero---they are effectively point-sources. Though planets dont appear wider to the naked eye, the near ones have some measurable angular width.

    The best clues probably have to do with position. There are really simple things like "venus is always in the same half of the sky as the sun". This makes it easy to recognize because after sundown, since its brighter than any star, it will probably show up in the sky before any star does and it will be in the western half of the sky.

    Another positional clue is the ecliptic stripe in the sky, like a badly aligned equator going not-quite east-west. The "zodiac" constellations are distributed along it. It marks the plane of solar system so planets appear only in this band of sky. You get to know where the ecliptic stripe of sky is, which depends on your latitude
    and a little bit on the time of year. Then anything you see that is not on the ecliptic has to be a star.

    the ultimate postional clue is to have a mental picture of the main constellations. So if you see a bright "star" and it is someplace where no bright star belongs you can say its a planet.

    (but first wait a bit to see if it moves because then its an airplane
    or a satellite)
  4. Jan 20, 2004 #3
    Ok, I overlooked that in my reading about the Zodiac. The reading says it is an 18 degree band centered on the ecliptic. I know that most of the planets do not revolve in a perfect plane. Does that mean some planets are displaced up to 18 degrees south or north of the ecliptic equator?
    Last edited: Jan 20, 2004
  5. Jan 20, 2004 #4


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    that is right. the ecliptic is simply the plane of the earth's orbit. the other planets orbits are each tilted slightly from that

    but (and someone else please correct me if I am wrong) the other planets are very nearly all in the same plane (except that weird one that you never see anyway, Pluto)
    so because the are very nearly in the same plane as the earth they show up approximately along the ecliptic

    celestial equator is different, being the projection of the earth's equator. the ecliptic is skewed from the celestial equator
    cocked at a jaunty 23 degree angle
    I think Im just repeating stuff you know as well as or better than I

    you probably have some ways of distinguishing planets from stars.
    if so please share them.
  6. Jan 20, 2004 #5
    Thanks for the posts Marcus and Jimmy. I'm still trying to get all of the astronomical definitons straight, I think it will take a while.. But I am thinking about getting a telescope and start observing the local celestial objects. I'll check out the Peterson's guide too, any idea how much that costs?
  7. Jan 21, 2004 #6
    I didn't know they had freeware planetary programs online. I have a cheap Gateway Model(1.6 GHz), but I tuned it up with some memory and a video card. And I run Window XP(home edition). Yeah, I have saw a lot of field guides at the library but they were really outdated, from the 70's and 80's. I'm sure the software would be better because of patches and regular upgrades,that might be a better choice for me in the long run-- but at a higher cost I bet..
  8. Jan 21, 2004 #7
    I didn't find the program I was looking for but I did find a better one. It's called WinStars:


    Here's a link to a great program that's completely free. It's not a planetarium program but it's excellent.

    Screen shots:


    It's a 11 Mb download but it's worth it. There are tons of add-ons such as high-res planetary textures, star databases; I could list stuff all night. I'd recommend checking out the forum if you like celestia. It's a very detailed program.

    If you have trouble downloading it from their page, try this link:
    http://www.shatters.net/celestia/files/celestia-win32-1.3.1-1.exe [Broken]

    And a general astronomy site with lots of good links and info:

    Last edited by a moderator: May 1, 2017
  9. Jan 21, 2004 #8


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    If you want to know where anything is, look at:

    http://www.heavens-above.com/planetsummary.asp?lat=27.947&lng=-82.459&alt=1&loc=Tampa&TZ=EST [Broken]

    It says Tampa, but just go back to the home page and enter your own location for the many other things you can access from there; sun, moon, planetarium, minor planets, etc. Very handy and its all on their computer, not yours.

    For the best "planetarium" program with over 19 million objects, with current Hipparcos stats, precice planetarium program, positions of all solar system objects, etc., etc. try out Software Bisque's program called "The Sky", version 5, level 2. It is about $129 retail and takes about 700 meg, but it is worth it.
    Last edited by a moderator: May 1, 2017
  10. Jan 21, 2004 #9


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    Welcome to Physics Forums, NanoTech! :smile:

    Excellent response, marcus (as usual).
  11. Jan 21, 2004 #10


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    One of the most rewarding things you can do with a few summer night is to become familiar with the constellations and the major stars. You do not need any equipment other then a good star map. It does not have to be new because the constellations have not changed in written history. You only need some Patience and a clear night. In some ways it is better not to have TO GOOD of seeing conditions. I have noticed in the few times that I was in the high Cascade mountains on a clear summer night there were so many stars that it was HARDER to pick out the ones I was familiar with. On a slightly hazy night or near a city which creates a artificially bright sky the stars you can see are the bright ones. These are the stars you need to orient yourself. While you will never see Virgo or Cancer (in the proper season,of course) in these conditions you should be able to find Cygnus, Lyra and Aquila easily. The summer Triangle consisting of Vega (in Lyra), Deneb (in Cygnus) and Altair (in Aquila) is a good place to start.
  12. Jan 21, 2004 #11
    I agree. The best way to start is learning the constellations with a good star guide. A pair of binoculars is nice to have, however, especially in the summer. There are lots of open clusters to view in and around the milky way.

    I live in the city so when I go out to a dark site, I have to take a few minutes to get my bearing in the midst of all the other stars that are visible.
  13. Jan 22, 2004 #12
    It's very simple!!
    Stars Twinkle and Planets don't. Also, planets are brighter than stars.

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