What is a Circular Orbit?

Mary McMahon
Mary McMahon

A circular orbit is type of orbit in which a celestial body moves in a circle around another celestial body. While drawings of objects in orbit often depict these objects in a circular orbit for reasons of simplicity, circular orbits are actually quite rare, requiring a sort of perfect storm of circumstances to occur. In our solar system, the Earth comes the closest to having a circular orbit, which is one of the reasons it is habitable, and among the planets, Mercury has the least circular orbit. (Now that Pluto has been downgraded, it no longer holds the “most eccentric orbit” title.)

All of the planets in the Solar System have slightly elliptical orbits, though their eccentricity varies.
All of the planets in the Solar System have slightly elliptical orbits, though their eccentricity varies.

Celestial bodies tend to orbit in an ellipse, with the object they are orbiting around at one of the focal points of the ellipse. The ellipse can be very stretched out and elongated, or closer to a circle, with the term “eccentricity” being used to describe the shape of the ellipse. An orbit with an eccentricity of zero is a circular orbit, while an orbit with an eccentricity of one would be highly elongated. Just for reference, the eccentricity of Earth's orbit is .0167.

In order for a circular orbit to occur, the orbiting object has to achieve the right velocity, and the interaction between the object in orbit and the object it is orbiting around must remain stable. This is fairly rare; satellites launched from Earth, for example, usually have a more elliptical orbit because it is difficult to get them to fall into a perfectly circular orbit.

A number of calculations can be used to determine the eccentricity of an orbit, and to play with variables which could change the shape of the object's orbit. These calculations can be used to analyze data about objects in other solar systems, and in the development of mission plans for satellites and other objects being launched from Earth.

The eccentricity of an object's orbit can have some interesting implications. For Earth, the slight shifts in position relative to the Sun play a role in the seasons, but the fact that the Earth's orbit is close to circular in nature also prevents extremes. If Earth had a more eccentric orbit, the temperature swings between seasons could be too intense for organisms to adapt, making it impossible for life on Earth to occur. Differences in orbit also explain why sometimes various celestial objects come into alignment with each other, and at other times, they do not.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


@GlassAxe- @Geosynchronous orbit and geostationary orbit are orbits that rotate around the Earth at the same speed as the Earth turns. A geosynchronous orbit can move in oscillations relative to the north and south directions, but matches the speed of the Earth's spin. For example, a satellite that is in a circular geosynchronous orbit about the earth would appear to move north and south to an observer standing stationary, but would not appear to move east or west.

A geostationary orbit is a type of geosynchronous orbit, but is completely stationary to the observer. A satellite that follows a geostationary orbit would appear completely stationary to a stationary observer. This type of satellite would essentially hover over the same spot on the earth, moving directly in stride with the planet.

Now the question is how this relates to real life. A communication satellite would use a geostationary orbit because it would allow the antennae on the ground to simply point at the satellite while remaining stationary. If communication satellites were geosynchronous but not geostationary, ground based antennae would need to shift from pointing northward to pointing southward throughout the day. This would lead to a requirement for powered antennae that would significantly increase the cost for our phone bills.


What is geosynchronous orbit?


@georgesplane- The simplistic answer as to why the Earth's orbit is elliptical is twofold. The first reason is because of the gravitational pull of other planets and the moon. The second reason is because of the past bombardment of debris that created the planet and that occasionally still bombard the planet

The reason that the Earth's orbit is so circular really has to do with chance. By chance, all of the planetismal parts that collided to create the Earth averaged together to create a nearly circular orbit. This chance makes the circular orbit that we enjoy on Earth very rare. For this reason, it is hard to find other planets in other solar systems that could possibly support life. Not only do the space collisions that make up a planet need to average to create a circular orbit; the planet must also be a distance from the sun that falls within a biological sweet spot. This sweet spot is the distance where the temperature not too hot or cold for water to form and life to thrive.

If the Earth's orbit were not nearly as circular as it were, the seasons would be much more extreme, making it very hard for complex life forms to adapt and survive. An elliptical orbit would lead to hotter summers and colder winters.

Finally, what the article stated and what you have been taught regarding the seasons are both right. The shape of a planets orbit along with the tilt of the planet dictates the seasons. The varied tilt of the Earth combined with the ellipse of the Earth’s orbit creates the seasons. For example, a planet with a stationary tilt and a circular orbit would have regional climates rather than seasons.


Why is the Earth's orbit elliptical? Why is the earth's orbit the most circular orbit in our solar system? Is it rare to have a circular orbit? What would happen if the earth's orbit was not as circular? In other words, why would the earth be less habitable if it had a more elliptical orbit?

The article stated that the eccentricity of the Earth's orbit plays is responsible for the seasons, but I was taught that the tilt of the earth is responsible for the seasons. Which is right? I thought this article was very informative, but I would like to understand the importance of a planet's orbit and the properties of a circular orbit in better detail. I am not very well versed in physics or calculus, so please respond in somewhat layperson's terms. Thank you wiseGEEKS!

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