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## Main Question or Discussion Point

So I've calculated the polar coordinates of a planet, with the sun at the origin and the x-axis being the striped line going from the sun towards point P.

Now I have to convert these polar coordinates to heliocentric ecliptic coordinates. To do this, I have to convert to cartesian coordinates first and then rotate the plane of reference so that the x-axis will point towards [tex]\Upsilon[/tex]. This is the answer:

Converting to cartesian coordinates is easy, but then I'm lost. Could anyone tell me how exactly I go from [tex]x = r \cdot \cos{v}[/tex] to [tex](6)[/tex]?

Now I have to convert these polar coordinates to heliocentric ecliptic coordinates. To do this, I have to convert to cartesian coordinates first and then rotate the plane of reference so that the x-axis will point towards [tex]\Upsilon[/tex]. This is the answer:

Converting to cartesian coordinates is easy, but then I'm lost. Could anyone tell me how exactly I go from [tex]x = r \cdot \cos{v}[/tex] to [tex](6)[/tex]?