1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why does Venus always appear close to the Sun in the sky?

  1. Jul 29, 2017 #1
    1. The problem statement, all variables, and given/known data

    Venus is sometimes described as either the “Morning Star” or the “Evening Star”, since it can only be seen near sunrise and sunset, very close to the Sun in the sky. Why does Venus always appear close to the Sun in the sky for an observer on the Earth, and which other planet would you expect to be the same?


    2. Relevant equations
    None

    3. The attempt at a solution
    For the first part of the question what I understood was the Venus never more than 48° to the Sun, but at the same time major planets like Jupiter, Saturn and more are on the other part of the Sun. The other planet would be Mercury as Venus and Mercury as the inner planets of our Solar System, but due to it being very close to the sun and very small, I suppose it is easy to lose track of it in the Sun's Glare. Another thing I discovered was that Copernicus said that Mercury and Venus orbit the Sun with orbits smaller than Earth’s. The problem for me is that I can't piece all of this information together to find an answer to the First Part of the Question. The Second Part I am sure is Mercury.

    Thanks for any help provided.
     
  2. jcsd
  3. Jul 29, 2017 #2

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    This can be visualised geometrically.

    Take the two bits you mentioned, or rather the one bit you mentioned twice:
    and start drawing.

    Draw the Sun and Earth in its orbit (doesn't have to be to scale - just points and circles). Then draw another planet with the largest possible orbit that is still inside Earth's. Connect the three (Sun, Earth, and the planet) to form a triangle.

    The angle at the vertex marked with Earth's position shows the separation of the Sun and the planet on the sky.
    What is the largest possible value of this angle? How does it change if you make the orbit of the planet even smaller?

    Next, draw a planet on a larger orbit than Earth's and do the same thing. What is the largest value of the same angle now?
     
  4. Jul 29, 2017 #3
    Sorry, but I do not understand. I drew the Sun as a Big circle then did a circle that is meant to represent the orbit line and drew a small circle on that line to represent Earth, and then I drew a Line which was really close to the orbit line of Earth but still in between Earth and the Sun and added a small circle on that line. Then I connected the Sun, Earth, and Planet together to form a Triangle. Not sure what to do next
     
  5. Jul 29, 2017 #4

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    Look at the angle in the triangle at the vertex where Earth is. How large can you make this angle by moving the other planet around on its orbit? Can you, for example, make it 90 degrees? 180?
     
  6. Jul 29, 2017 #5
    I should have drawn it with a rule now that I think about it, I don't have one with me right now, but Ill figure something out
     
  7. Jul 29, 2017 #6
    Is the extra planet Venus, as it would go clock wise instead of all the other planets?
     
  8. Jul 29, 2017 #7

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    It doesn't matter. It can be imaginary. Call it Steve or something. You're just trying to show what maximum angle on the sky between the Sun and an inner planet can be.
    (maybe use MS Paint, or Google Draw and show your work here)

    And Venus is not orbiting clockwise. It's rotating clockwise.
     
  9. Jul 29, 2017 #8
  10. Jul 29, 2017 #9

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    Great.
    Do you think it's possible to make the angle at Earth's vertex 180 degrees? 90? 60? 45? If yes, where would the other planet have to be in those cases?
     
  11. Jul 29, 2017 #10
  12. Jul 29, 2017 #11

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    It's all good. Try to answer the question above.
     
  13. Jul 29, 2017 #12
    If you make the vertex over 90 degrees you can no longer see that planet and the question is stating that we can always see it so I think you can not have it at 180. I know that Venus does not go over 48° so the planet should be around here> http://prntscr.com/g1rs1y
     
  14. Jul 29, 2017 #13

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    Yes, very good. In fact, if you made the orbit of the other planet almost exactly like the orbit of the Earth, you could make that angle 60 degrees at maximum.

    Now, if you were to think of a planet on a higher orbit, are there any such restrictions? Can you make the angle between the Sun and an outer planet 90 degrees? 180?
     
  15. Jul 29, 2017 #14
  16. Jul 29, 2017 #15

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    What is the angle between the sun and the outer planet in the picture below? (Earth is the inner planet)
    upload_2017-7-29_13-30-38.png
     
  17. Jul 29, 2017 #16
    180 Degrees
     
  18. Jul 29, 2017 #17

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    Right. So can you now make an argument why some planets can be seen anywhere in the sky, while others are confined to being next to the Sun?
     
  19. Jul 29, 2017 #18
    Would it be because Venus always appears close to the sun in the sky due to it being on the opposite side of the Sun and Earth Being in the middle and Mercury can only be seen as long as it does not a certain angle go over the sun?
     
  20. Jul 29, 2017 #19

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    I'm sorry. I'm having a hard time understanding your response.

    But maybe let me lead you a bit more - M & V are inner planets, as you yourself have stated, right? This is actually the straight answer to the problem question. But you need to be able to substantiate it with some argument.

    So what is it that you have then shown about how far away (in terms of angular separation on the sky) from the Sun they can be seen, and how that differs from outer planets?
     
  21. Jul 29, 2017 #20
    Ok
    Inner Planets have to be less than 90 degrees for them to be seen from Earth, but Outer Planets can be over 90 degrees but still less than 180 to be seen from Earth
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted