Between mile post 200 and mile post 210 Interstate 5 in Oregon runs due North South and importantly, is completely flat.

The line below indicates this 10 mile segment.

If you had a GPS unit in your car at Milepost 200 that latitude would be 44.222 degrees

At mile post 210 that latitude would be 44.367 degrees.

What is the radius of the Earth?

• Angular seperation is 44.367 - 44.222 = 0.145 degrees
• there are 360 degrees in a circle
• therefore this represents .145/360 = 1/2485 of a circle
• therefore 10 miles represents 1/2485 of the circumference of the earth
• so the circumference of the earth is 10 x 2485 = 24850 miles in the North South direction. This is within 10 miles of the actual value.

Note that the equatorial radius of the earth is slightly bigger than the polar radius becuase the earth is spinning and this tends to squash it a bit (this effect is much larger on Jupiter and Saturn).

Because of this the figure of the earth is most properly described as an oblate spheroid.