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The night sky - Venus (noob Q)

  1. Jan 26, 2012 #1
    Venus' synodic period is 583.92 days. Does that mean that Venus is 583.92/2 days the evening star and 583.92/2 days the morning star (with some period in between when it's probably to close to the sun to be visible)? What I want to know is some kind of formula so that I know where and when to look for Venus. Say in August I will know that in January Venus was an evening star, actually being quite far away from Sun (I don't know the terminology, just starting to look up) - It's close to moon crescent now. So I know it's been around 180 days between January and August - less than 583.92/2, so I should still look for it near setting sun.
    But now that I looked it up in Stellarium I know that it's going to be morning star in July already, so I'm at loss.
    What I'm trying to learn is a way to recognize those shiny dots without use of computer or phone app (which is great! BUT I feel like a lazy cheater)
  2. jcsd
  3. Jan 26, 2012 #2


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    See: http://www.johnpratt.com/items/astronomy/eve_morn.html

    It's described here that it takes 8 days for Venus to transition from the evening to the morning star, spending about 263 days as each.

    Unfortunately, there's no way of knowing what's what to the casual observer who steps out at sunset and sees a bright point of light setting in the West on a given night. Soon, Jupiter will be low in the West, and it is also quite bright (though less so than Venus, so that's one hint.) The best way to tell would be to observe the position of the bright object for a couple days in a row. Jupiter's position, at the same time each night, will noticeably change while the change in Venus's position will be much less evident. So, when you see a bright object setting in the West at sunset, for a few days in a row with no perceptible change of position, there's a good bet it's Venus.

    EDIT: The other way, is to simply learn which other planets are visible. As I said, right now (in the Northern Hemisphere), Jupiter is visible (probably at an altitude of around 70 degrees or so at sunset). Mars is rising (in the East) later in the evening, well after Venus has set in the West. So, by getting a sense of which planets are visible at a given time, and knowing where they are, you can be certain of what's what. No computer or phone app necessary.
  4. Jan 26, 2012 #3
    Thanks! So it IS ~583.92/2 evening/morning star (with 8 and 50 days of transition), but I missed that Venus being now far away from the Sun and thus it has already travelled 583.92/4 of it's "evening star time"... had I noticed that I could probably calculate some rough estimation whether Venus is evening or morning star in August.

    One more question: Why does Jupiter change it's position more than Venus? It's further away, and its period is longer and its velocity is lower... (but it's synodic period is lower)
    [EDIT] I think I know why: Venus orbit is "inside" of Earth's orbit, and it taks only a fragment of the sky, while Jupiter is outside, and its orbit spans accross the whole sky. This way it has higher "velocity" on the night sky. Is that right?

    Is there some web page with interactive solar system, so that I can visualize it better?
    Last edited: Jan 26, 2012
  5. Jan 26, 2012 #4


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    Yes, that's basically it. Jupiter's speed relative to the sun is not important; its speed relative to Earth is. The orbital speed of the Earth relative to Jupiter is several times that of Earth's orbital speed relative to Venus. Hence, in a given period of time, Jupiter's position in the night sky will change more than Venus's.
    I don't know, but that sure would be cool! Let me know if you find one!
  6. Jan 26, 2012 #5


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    The synodic period is the time required for a planet to return to the same position in the sky relative to the sun. In other words, Venus 'laps' the earth once every ~ 584 days. The formula for synodic period of a planet inside earth orbit is 1/P = 1/E + 1/S where P is the orbital period of the planet, E is the orbital period of earth, and S is the synodic period. For a planet outside earth orbit [e.g., Jupiter] the formula is 1/P = 1/E - 1/S. The orbit period of Venus is about 225 days compared to 365 days for earth. Here is an interactive http://www.solarsystemscope.com/
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