# Planetary Orbits

## Planetary Orbits

### Overview (The Solar System)

Planets in the solar system move around the Sun in elliptical orbits. Those whose orbits are closest to the Sun move more rapidly than those that are farther away. These simple, universally accepted observations form the basis of the knowledge of planetary motions. Gravity—the force that causes apples to fall from trees and keeps humans firmly planted on Earth’s surface—plays the central role in the mechanics of planetary motions.

A simple experiment illustrates the energy relationships inherent in orbiting bodies. If a person attaches a string to a small rubber ball and the ball is swung around the person’s head in a horizontal circle, the tension along the string that holds the ball in its “orbit” is analogous to the Sun’s gravity pulling on a bound planet. The English astronomer and mathematician Sir Isaac Newton (1642-1727) explained how the force of gravity affects planetary motion. Newton proved in his laws of motion that once an object is in a straight-line motion, it will continue on that course with no further input of energy (law of inertia) unless its motion is perturbed by an unbalanced force. In the case of planets, this force is provided by the gravitational attraction of the Sun (or more massive planet, in the case of a satellite). Depending upon the Magnitude of the orbiting body’s “kinetic energy” (energy of motion), the body will move in either a Circular orbit or, with greater kinetic energy,...

(The entire section is 1768 words.)

### Applications (The Solar System)

Consequences of Kepler’s and Newton’s laws of planetary motion and gravity had an impact on the scientific world not only during their own time but to this very day. The results of their work continue to be used by astronomers to solve problems. For example, the Flight path of the Apollo astronauts to the Moon and back was calculated using all three of Kepler’s laws. The energy required to propel the Saturn 5 rocket on its way and later to orbit the Moon was calculated using Newton’s laws of gravity. The same can be said for all interplanetary spacecraft, such as Voyagers 1 and 2, which visited and photographed the outer planets, Jupiter, Saturn, Uranus, and Neptune. The two Voyager probes were assisted in their journeys by using the gravitational attraction of these massive planets to accelerate them toward their next target. Calculating gravity assists involves kinetic energy and gravitational relationships developed by Newton.

One of the most useful of Kepler’s laws for planetary astronomers is the third law as modified by Newton. This law allows calculation of the mass of a massive body using data about mean distance and period of one or more of its satellites. It has been used to calculate the masses of all planets that have satellites (which excludes Mercury and Venus). One of the most difficult mass determinations was that for the dwarf planet Pluto and its satellite Charon. These bodies are so far away from...

(The entire section is 660 words.)

### Context (The Solar System)

The history of science closely parallels the development of Astronomy in that the study of heavenly bodies and their relationship to Earth dominated philosophical and religious thinking for millennia. One of the first scientists to study religious thinking and astronomical phenomena seriously was the Greek philosopher Aristotle (384-322 b.c.e.). Unlike most of his contemporaries, Aristotle used some observations to prove his speculations. His major contribution to planetary motion studies was his belief that the natural state of matter is to seek the center of the Earth, which is why objects always fall when released above the Earth. Although erroneous, this and related ideas laid the groundwork for later studies by Galileo and Newton on the effects of gravity. Aristotle also believed, as did many others, that Earth was at the center of the universe. That the Sun and planets revolved around Earth in perfectly circular orbits was advocated first by his great mentor,Plato. Later,Aristarchus (c. 270 b.c.e.), a Greek astronomer, adopted the idea that the Sun is at the center of the known universe. That idea was forgotten until revived nearly two thousand years later by Nicolaus Copernicus, whose “heliocentric” model, published shortly after his death in a volume titled De revolutionibus orbium coelestium (1543; On the Revolutions of the Heavenly Spheres, 1952; better known as De revolutionibus), describes a system...

(The entire section is 609 words.)

### Further Reading (The Solar System)

Arny, Thomas T. Explorations: An Introduction to Astronomy. 3d ed. New York: McGraw-Hill, 2003. A general astronomy text for the nonscientist. Includes an interactive CD-ROM and is updated with a Web site.

Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew Chaikin, eds. The New Solar System. 4th ed. Cambridge, Mass.: Sky, 1999. Filled with color diagrams and photographs, a popular work on solar-system astronomy and planetary exploration through the Mars Pathfinder and Galileo missions. Accessible to the astronomy enthusiast. Provokes excitement in the general reader, who gains an explanation of the need for greater understanding of the universe around us.

Consolmagno, Guy. Worlds Apart: A Textbook in Planetary Sciences. Englewood Cliffs, N.J.: Prentice Hall, 1994. A text accessible to college-level science majors and general readers alike. Presents explanations using low-level mathematics and also involves integral calculus where required. Demonstrates how the area of planetary science progresses by questioning previous understanding in the light of new observations.

Halliday, David, Robert Resnick, and Jearl Walker. Fundamentals of Physics, Extended. 9th ed. New York: Wiley, 2007. This textbook has taught millions of college students the fundamentals of physics. Its sections on Newton’s laws of motion are particularly strong, as is the chapter on gravitation, which...

(The entire section is 573 words.)