Earth's mean orbital speed is the average speed at which the Earth revolves around the sun. This is defined in two different ways, based on the sidereal year and the tropical year.
The sidereal year is the time it takes for the Earth to revolve once around the sun with respect to the distant stars. This is approximately 365.2564 mean solar days, or 3.155815 x 107 seconds (s). The tropical year is the time it takes for the Earth to revolve once around the sun, as measured between two consecutive March equinoxes. This is about 365.2422 mean solar days, or 3.155693 x 107 s.
The circumference of the Earth's orbit is approximately 2 pi (6.283185) times the mean radius of its orbit. This radius is also known as the astronomical unit (AU), and is about 1.4959787 x 1011 meters (m). Therefore, the circumference is about 9.399511 x 1011 m. The Earth's mean orbital speed, in meters per second (m/s), is obtained by dividing this number by the length of the year in seconds. This can result in either of two figures.
Let vs be the Earth's mean orbital speed as defined based on the sidereal year. This speed is:
vs = (9.399511 x 1011) / (3.155815 x 107)
= 2.978473 x 104 m/s
Let vt be the Earth's mean orbital speed as defined based on the tropical year. This is:
vt = (9.399511 x 1011) / (3.155693 x 107)
= 2.978589 x 104 m/s
A rough, general figure for the Earth's mean orbital speed is 30 kilometers per second (km/s), or 18½ miles per second (mi/s).
Also see the Table of Physical Units and Constants.