# Definition of Elliptical Orbits

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An elliptical orbit is the revolving of one object around another in an oval-shaped path called an ellipse. The planets in the solar system orbit the sun in elliptical orbits. Many satellites orbit the Earth in elliptical orbits as does the moon. In fact, most objects in outer space travel in an elliptical orbit.

## Understanding Ellipses

An ellipse is like an elongated circle, as if it were stretched at the ends. As the size of a circle is measured by the diameter, the size of an ellipse is measured by the major and minor axis. The major axis measures the longest distance across the ellipse while the minor axis measures the shortest. Mathematicians define an ellipse by the foci, essentially the two “centers” of the shape, or in the case of an elliptical orbit, the two points around which the object orbits.

## Why Planets Orbit

Every object with mass exerts a gravitational pull on every other object. Gravity increases with mass, so the more massive an object, the greater the pull of gravity. Therefore, on a planetary scale, the force of gravity is huge. When a planet, such as Earth, moves through space, it is influenced by all the other bodies around it and the most massive body in the solar system is the sun. When the Earth gets caught in the gravitational pull of the sun, its path is diverted, causing it to turn toward the more massive object. If the gravity of the more massive object is enough, the Earth will revolve around it in a path known as an orbit.

## History

Johannes Kepler was the first scientist to accurately describe the elliptical orbits of planets with his first law of planetary motion in 1605. Before Kepler, planets were thought to move in perfect circles around the sun as described by Copernicus in 1543. Kepler devised three laws in all, even inspiring Sir Isaac Newton to develop the law of gravity.

## Highly Elliptical Orbits

The elliptical orbits of the planets in the solar system have very little “eccentricity,” or deviation from circular. Some objects, however, such as comets, have much more eccentricity in their orbit. These orbits are referred to as “highly elliptical orbits,” or HEOs. A comet in an HEO swings close to the sun at a very high velocity before speeding back into space. At the furthest point from the sun, the comet moves very slowly, lingering for a long time. Scientists have used the concept of HEO to put satellites in space that linger over one part of the Earth for a long time. These satellites then speed around the other side of the Earth in a close fly-by. GPS satellites use highly elliptical orbits to maintain total coverage of the Earth at all times.

## Effects of an Elliptical Orbit

It is a common misconception that the Earth is closer to the sun during summer and further away in winter. In the northern hemisphere, the opposite is true. The elliptical orbit of the Earth is very nearly circular and the distance to the sun does not change enough to have a large effect on the seasons. The tilt of the Earth on its axis has a much greater impact than the elliptical orbit and is the cause for the seasons.