... * Math.PI / 180.0

Public Const RadianConversionConst As Decimal = Math.PI / 180.0

Dim Latitude1Radian As Decimal = Latitude1 * RadianConversionConst

Dim Latitude1Radian As SqlDecimal = Latitude1 * Math.PI / 180.0

SET ANSI_NULLS OFF
GO
SET QUOTED_IDENTIFIER OFF
GO
CREATE Function [dbo].[Distance] (
	@Lat1 as decimal(18, 6),
	@Long1 as decimal(18, 6),
	@Lat2 as decimal(18, 6),
	@Long2 as decimal(18, 6))
Returns decimal(18, 6) With SchemaBinding
As
Begin
	Declare @dLat1InRad as float(53)
	Set @dLat1InRad = @Lat1 * ( PI() / 180.0 )

	Declare @dLong1InRad as float(53)
	Set @dLong1InRad = @Long1 * ( PI() / 180.0 )

	Declare @dLat2InRad as float(53)
	Set @dLat2InRad = @Lat2 * ( PI() / 180.0 )

	Declare @dLong2InRad as float(53)
	Set @dLong2InRad = @Long2 * ( PI() / 180.0 )
 
	Declare @dLongitude as float(53)
	Set @dLongitude = @dLong2InRad - @dLong1InRad

	Declare @dLatitude as float(53)
	Set @dLatitude = @dLat2InRad - @dLat1InRad

	/* Intermediate result a. */
	Declare @a as float(53)
	Set @a = SQUARE( SIN( @dLatitude / 2.0 ) ) + COS( @dLat1InRad ) * COS( @dLat2InRad ) * SQUARE( SIN( @dLongitude / 2.0 ) )

	/* Intermediate result c (great circle distance in Radians). */
	Declare @c as real
	Set @c = 2.0 * ATN2( SQRT( @a ), SQRT( 1.0 - @a ) )

	Declare @EarthRadius as decimal(18, 6)
	Set @EarthRadius = 3956.0  /* miles */

	Declare @dDistance as decimal(18, 6)
	Set @dDistance = @EarthRadius * @c

	Return (@dDistance)
End