The Physics Factbook™
Edited by Glenn Elert  Written by his students
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Callister, Jeffrey. Brief Review in Earth Science. New York: Prentice Hall, 1990: 38.  avg. radius = (152 + 147 million km)/2 = 149.5 million km C = 2πr = 2[pi](149,500,000 km) = 939,336,203.4 km v = C/T = 939336203.4 km/365.25 day = 2571762.364 km/day = 29.77 km/s 
29.77 km/s 
Farndon, John. Dictionary of the Earth. New York: Dorling Kindersley, 1994: 34.  "Orbited by its companion, the Moon, the Earth travels at more than 65,000 mph (105,000 kph), covering millions of miles each year as it journeys through space."  29.17 km/s 
Asimov, Isaac. Exploring the Earth and the Cosmos. New York: Wightfall, 1982: 243.  "Earth's average speed of revolution about the sun is 29.8 kilometers per second, while Mercury, which is the closest planet to the sun and therefore the fastest, travels at an average speed of 47.9 kilometers per second."  29.80 km/s 
Fraser, Ronald. Once Round The Sun. New York: Macmillan, 1957: 19.  "The earth moves round the sun in an oval track, that has an average radius of 93 million miles, at a speed of 18½ miles a second."  29.77 km/s 
Williams, David. Earth Fact Sheet. NASA Goddard Space Flight Center.  "Mean orbital velocity (km/s) 29.78"  29.78 km/s 
The geocentric model was an early concept of celestial objects and their motions. In this model, the earth was stationary and all celestial objects revolved around it. The modern concept of the motions of celestial objects is the heliocentric model. This is also known as the "suncentered"model. According to this model, the earth and the planets revolve around the sun.
A planet's revolution is its motion around the sun in a path called an orbit. The shape of the earth's orbit is a closed curve called an ellipse. The force of gravitation between the sun and a planet is always pulling the planet toward the sun. The planet does not fall into the sun because of the centrifugal effect of its orbital motion. It is kept steady in its orbit, like any other planet or satellite, by two balancing forces, the gravitational pull of the sun, and the centrifugal force due to its own speed. The speed of the earth varies depending upon its location in relation to the sun. However, this value is almost always the same approximate number. In the earth's orbit around the sun, it travels at an average speed of 29.658 km/s (I took the average of all of the values that I obtained for the velocity). The earth is closest to the sun, which is called the perihelion and has its greatest orbital speed around January 3. It is farthest from the sun, which is called the aphelion and has its least orbital speed around July 4. Since equal areas must be covered in equal intervals of time, the planet's orbit speed has to be the fastest at perihelion and slowest at aphelion.
Ilana Epstein  2000
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