# How did scientists determine how fast earth's orbit is around the sun?

Feb 22, 2016

The orbital speed of the Earth is easy to calculate using geometry - we can find the circumference (distance) and divide it by a year (time)

#### Explanation:

The Earth completes one full orbit, ${360}^{\circ}$, in one year, which is 365.256 days, or exactly 31,556,926 seconds.

The Earth's orbit is slightly elliptical, but is pretty close to circular. The mean (semi-major) distance from the sun to Earth is 149.6 million km. Remembering our geometry a circle with a radius of 149.6 million km is:

$c = 2 \pi r$
$c = 2 \cdot \pi \cdot 149600000 k m$
$c = 939964522 k m$
or
940 million km.

the orbital speed can be calculated using:
$v = \frac{d}{t}$
$v = \frac{940000000 k m}{31556926 s}$
$v = 29.79 \frac{k m}{s}$