Excel - As-the-crow-flies distance function

The 'standard' MS Excel formula shown below calculates the as-the-crow-flies distance (in km), the shortest distance between two geographical locations specified by latitude and longitude.
An alternative is user defined function (UDF) globeDistance. This one is much easier to read and link, and it can also be used in larger VBA procedures.

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MS Excel 'standard' formula

Given the following input fields (in degrees): From latitude (cell A2), From longitude (cell B2), To latitude (cell C2), To longitude (cell D2), the formula is

Dependent on your MS Excel setting you may have to replace the semicolon by a comma.

Alternative user defined function: MS Excel VBA code to be put in a module

Note: in general, a user defined function will execute slower than a standard function, which can become a disadvantage in case of a large amount of formulas.
In MS Excel, press Alt+F11 to open the Visual Basic Editor (Windows). Via top menu: Insert → Module. Copy the MS Excel VBA code below into the module.