distance - Calculating longitude length in miles

Suppose I have geographic coordinates of "Saratoga, California, USA" as

Latitude:   37°15.8298′ N   
Longitude: 122° 1.3806′ W

I know from here that in case of latitude 1° ≈ 69 miles and that longitude varies:

1° longitude = cosine (latitude) * length of degree (miles) at equator.

How many miles is 1° longitude at longitude: 122°1.3806′ W?

Answer

It doesn't matter at what longitude you are. What matters is what latitude you are.

Length of 1 degree of Longitude = cosine (latitude in decimal degrees) * length of degree (miles) at equator.

Convert your latitude into decimal degrees ~ 37.26383

Convert your decimal degrees into radians ~ 0.65038

Take the cosine of the value in radians ~ 0.79585

1 degree of Longitude = ~0.79585 * 69.172 = ~ 55.051 miles

More useful information from the about.com website:

Degrees of latitude are parallel so the distance between each degree remains almost constant but since degrees of longitude are farthest apart at the equator and converge at the poles, their distance varies greatly.

Each degree of latitude is approximately 69 miles (111 kilometers) apart. The range varies (due to the earth's slightly ellipsoid shape) from 68.703 miles (110.567 km) at the equator to 69.407 (111.699 km) at the poles. This is convenient because each minute (1/60th of a degree) is approximately one [nautical] mile.

A degree of longitude is widest at the equator at 69.172 miles (111.321) and gradually shrinks to zero at the poles. At 40° north or south the distance between a degree of longitude is 53 miles (85 km)

_{Note that the original site (about.com) erroneously omitted the "nautical" qualifier.}