Well, this is a question that the Greeks worked on 2000 years ago! Basically, the earth is spherical. So if we look down on a sphere like a basketball from the top, we see that a in cross section at the equator is a circle.

The circumference of a circle is:

2* pi*R

Where R is the radius of the circle. In this case the radius is that of the Earth which is about 6380 kilometers. Hence the circumference of the Earth (at the equator) is about 2 x 3.14 x 6380 = 40,000 Kilometers.

As there is 1.6 km in a mile, so in miles the circumference is about 25,000.

Its about 450 miles to San Francisco and its about 3000 miles to New York City from Santa Barbara, California.