input out of error with haversine formula

The following will return an input out of error as the acos() function cannot be -1 <= x <= 1.

SELECT * FROM

                (SELECT *, (3959 * acos(cos(radians(37.7438640)) * cos(radians(37.7438640)) * cos(radians(-97.4631299) - 

                radians(-97.4631299)) + sin(radians(37.7438640)) * sin(radians(37.7438640))))

                AS distance

                FROM foo) AS distances

                WHERE distance < 10

                ORDER BY distance

If I break this down the following returns 1:

SELECT (cos(radians(37.7438640)) * cos(radians(37.7438640)) * cos(radians(-97.4631299) - radians(-97.4631299)) + sin(radians(37.7438640)) * sin(radians(37.743864000)));

acos(1) would give me 0.

Thoughts or workaround suggestions?

Thanks.

--Vince--