Revolution and rotation

Although often confused, there is a distinct and important difference in the concepts of revolution and rotation. Earth rotates on its axis as it revolves around the Sun .

Earth rotates about its axis at approximately 15 angular degrees per hour. Rotation dictates the length of the diurnal cycle (i.e, the day/night cycle), creates "time zones" with differing local noons, and also causes the apparent movement of the Moon , stars, and planets across the "celestial sphere". The rotation of Earth is eastward (from west to east) making the apparent rotation of the celestial sphere from east to west.

The rates of rotation and revolution are functions of a planet's mass and orbital position. For example, the mass of Jupiter is approximately 317.5 times Earth's mass and the rotation time (the time for Jupiter to revolve once about its axis) is approximately nine hours.

Earth takes approximately 365.25 days to complete one revolution around the Sun in a slightly elliptical orbit with the Sun at one focal point of the ellipse. Ranging between the extremes of perihelion (closest approach) in January and aphelion (most distant orbital position) in July, Earth's orbital distance from the Sun ranges from approximately 91.5 to approximately 94 million miles (147–151 million km), respectively. Although these distances seem counterintuitive to residents of the Northern Hemisphere who experience summer in July and winter in January—the seasons are not nearly as greatly affected by distance as they are by changes in solar illumination caused by the fact that Earth's polar axis is inclined 23.5 degrees from the perpendicular to the ecliptic (the plane of the solar system through or near which most of the planet's orbits travel) and because the Earth exhibits parallelism (currently toward Polaris, the North Star) as it revolves about the Sun.

At the extreme of the solar system, Pluto, usually the most distant planet (i.e., at certain times Neptune's orbit actually extends farther than Pluto's orbit) takes approximately 247 Earth years (the time it takes the Earth to revolve about the Sun) to complete one orbital revolution about the Sun.

Rotation, revolution, polar tilt, parallelism, and Earth's oblate spheroid shape combine to produce an unequal distribution of solar energy , the changing of seasons, the changing lengths of day and night, and influence the circulation of the atmosphere and oceans .

In addition to Earth's rotation about the Sun, the solar system is both moving with the Milky Way galaxy and revolving around the galactic core.

See also Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars