Where is "Down Under"?

[Azimov]

Can anyone tell me how to calculate the exact location of the opposite side of the Earth from any given point?

That is to say, if I took the co-ords for my own home town, what formula must I use to calculate the co-ords for the exact point on the opposite side of the Earth?

I guess the answer is probably laughably simple but I'm hopeless at maths (and cartography it seems!).

Thanks!

[Flurfl]

Subtract your longitude from 180 this will give your longitude in the opposite hemisphere (ie: E vs W)

Use the value of your latitude in the opposite hemisphere (ie: S vs N)

So if London is at 51 32'N and 0 5'W then the opposite point is 51 32'S and 180 - 0 5' = 179 5'E

[Personica]

Also your longitude can be calculated as exactly 12 hours (not withstanding daylight savings) difference.