- #1
Whitehole
- 132
- 4
Homework Statement
The familiar Mercator map of the world is obtained by transforming spherical coordinates θ , ϕ to coordinates x , y given by
##x = \frac{W}{2π} φ,
y = -\frac{W}{2π} log (tan (\frac{Θ}{2}))##
Show that ##ds^2 = Ω^2(x,y) (dx^2 + dy^2)## and find ##Ω##
Homework Equations
##g'_{ρσ} = g_{μν} (\frac{∂x^μ}{∂x'^ρ}) (\frac{∂x^ν}{∂x'^σ})##
In spherical coordinates ##ds^2 = dΘ^2 + \sin^2(Θ)dφ^2## with ##r=1##
The Attempt at a Solution
So in this case, I have to find ##g_{xx}## and ##g_{yy}##, they should be the same since ##Ω^2(x,y)## is factored out.
By isolating ##Θ## and ##φ##, I got
##Θ=2\arctan(e^\frac{-2πy}{W})## and ##φ = \frac{2πx}{W}##
##g_{xx} = g_{ΘΘ} \frac{∂Θ}{∂x} + g_{φφ} \frac{∂φ}{∂x} = \frac{2π}{W}\sin^2(2\arctan(e^\frac{-2πy}{W}))##
##g_{yy} = g_{ΘΘ} \frac{∂Θ}{∂y} + g_{φφ} \frac{∂φ}{∂y}##
For this part, ##y = -\frac{W}{2π} log (tan (\frac{Θ}{2}))##, manipulate so that ##tan (\frac{Θ}{2}) = e^\frac{-2πy}{W}##, then differentiate with respect to
y so that ##\frac{∂Θ}{∂y} = \frac{-4πe^\frac{-2πy}{W}}{Wsec^2(\frac{Θ}{2})}##
After some manipulation I ended up
##g_{yy} = g_{ΘΘ} \frac{∂Θ}{∂y} + g_{φφ} \frac{∂φ}{∂y} = \frac{-2π}{Wcosh(\frac{2πy}{W})}##
According to the book I'm reading the answer should be
##Ω = \frac{2π}{Wcosh(\frac{2πy}{W})}##
Can anyone help me find out what is wrong?