• Support PF! Buy your school textbooks, materials and every day products Here!

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

Riccardo K

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. Homework 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.
 

Bandersnatch

Science Advisor
2,840
1,695
This can be visualised geometrically.

Take the two bits you mentioned, or rather the one bit you mentioned twice:
Venus and Mercury as the inner planets of our Solar System
Mercury and Venus orbit the Sun with orbits smaller than Earth’s
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?
 

Riccardo K

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
 

Bandersnatch

Science Advisor
2,840
1,695
Not sure what to do next
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?
 

Riccardo K

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
 

Riccardo K

Is the extra planet Venus, as it would go clock wise instead of all the other planets?
 

Bandersnatch

Science Advisor
2,840
1,695
Is the extra planet Venus, as it would go clock wise instead of all the other planets?
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.
 

Riccardo K

Bandersnatch

Science Advisor
2,840
1,695
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?
 

Riccardo K

Bandersnatch

Science Advisor
2,840
1,695
It's all good. Try to answer the question above.
 

Riccardo K

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
 

Bandersnatch

Science Advisor
2,840
1,695
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?
 

Riccardo K

Bandersnatch

Science Advisor
2,840
1,695
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
 

Riccardo K

180 Degrees
 

Bandersnatch

Science Advisor
2,840
1,695
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?
 

Riccardo K

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?
 

Bandersnatch

Science Advisor
2,840
1,695
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?
 

Riccardo K

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
 

Riccardo K

Just checked. Apparently just saying that M and V are inner planets should do. Thanks for all the help given. I think I understood a bit more now.
 

ehild

Homework Helper
15,366
1,778
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
What do you mean that you drew the sun as a big circle ? Represent the Sun and all other planets with dots. Represent the orbits with circles. The circle representing the orbit of Venus is smaller than that of the Earth, and the orbit of Mars and the outer planets like Jupiter are bigger than that of the Earth. On the figure, the blue dot is Venus, the brawn one is Earth, the red one is Mars, and the black one might be Jupiter (not in scale).At position A, Venus looks farthest from the Sun, and it looks closer at positions B and C.
upload_2017-7-29_14-15-18.png
 
Last edited:

Riccardo K

I don't really get it
 

Riccardo K

I understand that the rules of this section of the forums say, that if I don't arrive at the answer then you are not allowed to tell me. Thanks for all the help given, but I don't think I will understand this further not having studied Astronomy at school yet.
 

Related Threads for: Why does Venus always appear close to the Sun in the sky?

Replies
0
Views
959
Replies
2
Views
1K
Replies
3
Views
6K
  • Last Post
Replies
3
Views
1K
Replies
5
Views
10K
  • Last Post
Replies
3
Views
2K

Recent Insights

Top