Usage
{{Great circle distance/er|latitude}}
Calculates the Earth's radius, in kilometers, at a given latitude. Uses the following formula:
![{\displaystyle R={\sqrt {[(r_{1}^{2}*\cos {B})^{2}+(r_{2}^{2}*\sin {B})^{2}]/[(r_{1}*\cos {B})^{2}+(r_{2}*sin{B})^{2}]}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d3534f888759126d4455edc87ebc9f021fb65d36)
with latitude
, radius
, radius at equator
, and radius at pole 
With no latitude input, it defaults to the mean Earth radius.