<P> From a vantage point above the north pole of either the Sun or Earth, Earth would appear to revolve in a counter clockwise direction around the Sun . From the same vantage point, both the Earth and the Sun would appear to rotate also in a counterclockwise direction about their respective axes . </P> <P> Heliocentrism is the scientific model that first placed the Sun at the center of the Solar System and put the planets, including Earth, in its orbit . Historically, heliocentrism is opposed to geocentrism, which placed the Earth at the center . Aristarchus of Samos already proposed a heliocentric model in the 3rd century BC . In the 16th century, Nicolaus Copernicus' De revolutionibus presented a full discussion of a heliocentric model of the universe in much the same way as Ptolemy had presented his geocentric model in the 2nd century . This "Copernican revolution" resolved the issue of planetary retrograde motion by arguing that such motion was only perceived and apparent . "Although Copernicus's groundbreaking book...had been (printed) over a century earlier, (the Dutch mapmaker) Joan Blaeu was the first mapmaker to incorporate his revolutionary heliocentric theory into a map of the world ." </P> <P> Because of Earth's axial tilt (often known as the obliquity of the ecliptic), the inclination of the Sun's trajectory in the sky (as seen by an observer on Earth's surface) varies over the course of the year . For an observer at a northern latitude, when the north pole is tilted toward the Sun the day lasts longer and the Sun appears higher in the sky . This results in warmer average temperatures, as additional solar radiation reaches the surface . When the north pole is tilted away from the Sun, the reverse is true and the weather is generally cooler . Above the Arctic Circle and below the Antarctic Circle, an extreme case is reached in which there is no sunlight at all for part of the year . This is called a polar night . This variation in the weather (because of the direction of the Earth's axial tilt) results in the seasons . </P> <P> By astronomical convention, the four seasons are determined by the solstices--the two points in the Earth's orbit of the maximum tilt of the Earth's axis, towards the Sun or away from the Sun--and the equinoxes--the two points in the Earth's orbit where the Earth's tilted axis and an imaginary line drawn from the Earth to the Sun are exactly perpendicular to one another . The solstices and equinoxes divide the year up into four approximately equal parts . In the northern hemisphere winter solstice occurs on or about December 21; summer solstice is near June 21; spring equinox is around March 20; and autumnal equinox is about September 23 . The effect of the Earth's axial tilt in the southern hemisphere is the opposite of that in the northern hemisphere, thus the seasons of the solstices and equinoxes in the southern hemisphere are the reverse of those in the northern hemisphere (e.g. the northern summer solstice is at the same time as the southern winter solstice). </P>

What is the radius of earth's orbit