# Math Help - [SOLVED] Measuring distances on Earth

1. ## [SOLVED] Measuring distances on Earth

Hi,
I know that r=Rcos alpha is used for finding the radius of a circle at say 30deg N. And R=6400km

Is the angle alpha measured from:
the side of the sphere;
where the greenwich meridian and the equator intersect;
from the true centre of the earth.

??
Thanks...

2. Originally Posted by Neverquit
Hi,
I know that r=Rcos alpha is used for finding the radius of a circle at say 30deg N. And R=6400km

Is the angle alpha measured from:
the side of the sphere;
where the greenwich meridian and the equator intersect;
from the true centre of the earth.

??
Thanks...
Hi Neverquit,

The circle at $30^oN$ is a line of latitude.
It can be measured from the side of the sphere (to the centre) or centre as shown
in the attachment.