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--