How do I solve for the angle, as a function?

AI Thread Summary
The discussion focuses on deriving an angle as a function of time, specifically the angle from Earth to Mars with respect to the x-axis. Participants emphasize the need to express the positions of Earth and Mars as vectors and utilize vector operations, including the dot product, to find the angle. There is confusion regarding how to handle parametric equations and the changing nature of the vectors over time. A suggestion is made to visualize the problem by drawing a diagram of the Sun, Earth, and Mars to better understand the relationships and angles involved. Ultimately, the goal is to derive a formula that accurately represents the angle of Mars as viewed from Earth.
Phantoful
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Homework Statement


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EIFxygQ.png


Homework Equations


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The two equations in the image, as well as (maybe) standard vector operations like dot product, cross product.

The Attempt at a Solution


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So, I've tried to do what my instructor suggested and put everything in terms of Earth (Using RE and TE, but I'm not sure what to do from there as it doesn't seem that I'm progressing towards the solution. I've also tried to graph these functions, but I can't wrap my head around the fact that both the x and y vectors also have parameter t (time), so I'm not sure if graphing is the way to solve it. The main goal I've tried was to solve for theta, which doesn't seem possible because of the equations for x and y being split...
 

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Can you express the desired angle in terms of the two vectors, ##\vec R_M, \vec R_E##? I.e., as a vector equation?
 
haruspex said:
Can you express the desired angle in terms of the two vectors, ##\vec R_M, \vec R_E##?
I would think I'd need to do the dot product using |A|*|B|cosθ and then calculate theta, but would it even be possible to find the magnitude? The vectors are changing over time and I don't know how to account for that.

Edit: Sorry, re-read your question. Would I have to convert these 'parametric equations' to an x-y equation, and then graph?
 
Phantoful said:
the dot product using |A|*|B|cosθ
Dot product, yes, but not using that formula to find it. How do you perform a dot product on two vectors that are expressed in the same orthonormal basis?
 
haruspex said:
Dot product, yes, but not using that formula to find it. How do you perform a dot product on two vectors that are expressed in the same orthonormal basis?
I can also add after multiplying the x's and the y's, but that would be a scalar, is that what I want? So I'm adding cos(2π(t/TM))*cos(2π(t/TE))+sin(2π(t/TM))*sin(2π(t/TE)), or is it the (u⋅v/v⋅v)v equation?
 
Phantoful said:
I can also add after multiplying the x's and the y's, but that would be a scalar, is that what I want? So I'm adding cos(2π(t/TM))*cos(2π(t/TE))+sin(2π(t/TM))*sin(2π(t/TE)), or is it the (u⋅v/v⋅v)v equation?
Sorry, I think I've led you off in the wrong direction. Let's start again.

Draw a diagram of the Sun, Earth and Mars at some instant in the coordinate system given. Say we label the points S, E, M, and the distant star that fixes the X axis call P.
You can easily relate 2π(t/TE) and 2π(t/TM) to angles in the diagram, right?
What angle are you asked to find?
 
haruspex said:
Sorry, I think I've led you off in the wrong direction. Let's start again.

Draw a diagram of the Sun, Earth and Mars at some instant in the coordinate system given. Say we label the points S, E, M, and the distant star that fixes the X axis call P.
You can easily relate 2π(t/TE) and 2π(t/TM) to angles in the diagram, right?
What angle are you asked to find?
I'm not actually sure of 'what' angle I need to find, which is my big problem, because I can't visualize this well so I'm depending on the mathematics. The question says "Derive a formula for which we see Mars from Earth. This should be an angle with respect to the
Inline5.gif
-axis." I can't easily relate the equations to the diagram, because I don't have a clue what these should look like.
 

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This question will be the end of me...
 
Phantoful said:
I'm not actually sure of 'what' angle I need to find, which is my big problem, because I can't visualize this well so I'm depending on the mathematics. The question says "Derive a formula for which we see Mars from Earth. This should be an angle with respect to the View attachment 219525-axis." I can't easily relate the equations to the diagram, because I don't have a clue what these should look like.
Did you draw the diagram as I described?
What we are after is the angle that the line from Earth to Mars makes to the x axis.
We have vector representations for the positions of Earth and Mars. What vector represents the position of Mars relative to Earth?
 
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