# How is the circumference of the earth calculated?

##### 1 Answer

From visible horizon.

#### Explanation:

From visible horizon:

Let h be the height of the flag mast of a ship, from sea level. If d is

the maximum sea-level distance from which the top could be seen.

the circumference of the Earth is nearly

Explication:

If h = 20 meters = 0.02 km and d = 11288 meters,

the radius of the Earth

The circumference

=

Proof:

d=(radius)(angle subtended at the center, in radian)=

Here the proportion

If T is the Top of flag mast, O is the observer, C is the center of the

Earth,

as h/R is very small.

So, the approximation formula is