Street Parking: Metered parking available on Charles St. $1.25 per hour; 2 hour maximum before having to move to a different meter. Valet parking: Available starting at 5 PM daily. Valet charge is $20. Garage Parking: Boston Common Parking Garage- Charles St./Beacon St. corner of garage.
By definition, pi is the ratio of the circumference of a circle to its diameter. Pi is always the same number, no matter which circle you use to compute it. For the sake of usefulness people often need to approximate pi.
pi is the ratio of the circumference of a circle to its radius. So they drew circles, measured the circumference and radius accurately, then divided the first by the second. There's no 'why' or anything about it; it's like asking why Newton decided to invent his gravitational constant to be exactly what it is.