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Elliptical Orbit
An ellipse can be long and skinny. An ellipse can be very round. Scientists need a way to describe how round or "stretched out" an ellipse is. They use a number to describe this, and call it the "eccentricity" of the ellipse. Eccentricity is always between zero and one for an ellipse. If it is close to zero, the ellipse is nearly a circle. If it is close to one, the ellipse is long and skinny. An elliptical orbit can be useful to a communications satellite because it allows the satellite to travel over a specific region for a long portion of its orbit, and it is only out of contact with that region for a short time when it is zipping quickly around the other side of the Earth. In 1600 Kepler became assistant to Tycho
Brahe who was making accurate observations of the planets. After Brahe
died in 1601 Kepler continued the work, calculating planetary orbits to
unprecedented accuracy. |
A
satellite in elliptical orbit follows an oval-shaped path. One part of
the orbit is closest to the centre of Earth (perigee) and the other part
is farthest away (apogee). A satellite in this orbit takes about 12
hours to circle the planet.