# Orthodromic distance

##### Calculate air distance between Prague and Shanghai.
The grear-circle distance or orthodromic distance is the shortest distance between two points on the sphere. Suppose two points $\left(\varphi_1,\lambda_1\right),\left(\varphi_2,\lambda_2\right)$ on the sphere. Orthodromic distance is [i]R.w [km][/i], where R = 6378 km and[br][center]$\cos w=\sin\varphi_1\sin\varphi_2+\cos\varphi_1\cos\varphi_2\cos\left(\lambda_2-\lambda_1\right)$[/center]
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