Johannes Kepler<\/a>‘s first law tells us that planets orbit in an ellipse with the Sun at one of the two focal points (foci).<\/p> An ellipse is just a squashed circle with two foci with one longer, or major axis and one shorter or minor axis.<\/p> Eccentricity is how squashed the circle is and is measured by dividing the distance between the foci by the length of the major axis.<\/p> In a perfect circle, the axes are equal in length and the distance between the foci equals zero, so a circle has an eccentricity of 0.<\/p> An ellipse is a closed orbit with an eccentricity between 0 and 0.99.<\/p> An eccentricity of 1 is a parabolic or escape orbit. Anything over 1 is a hyperbolic orbit, which is open to infinity.<\/p> Most planets in our Solar System have a fairly low orbital eccentricity.<\/p> This may be due to the high number of bodies it contains, including the asteroids<\/a>, Kuiper Belt<\/a> and Oort Cloud<\/a>.<\/p> Mercury is the planet with the highest and Venus has the lowest eccentricity.<\/p> Our Moon has the greatest eccentricity of any large moon at 0.059.<\/p> Neptune’s largest moon Triton holds the record for the least eccentric orbit of 0.00002.<\/p> Its retrograde orbit may subject to drag caused by a debris disc surrounding Neptune with a prograde orbit.<\/p> Dwarf planet Sedna has the largest eccentricity of 0.849, the cause of which is hotly debated but may point to evidence of a planet beyond Neptune.<\/p> Earth’s orbital eccentricity is due not only to our distance from the Sun but also the gravitational pull of Jupiter and Saturn.<\/p> It is the reason Earth’s seasons are not of equal length. In the Northern Hemisphere summer is about 4.5 days longer than winter and spring is 3 days longer than autumn.<\/p> About 23% more solar radiation reaches Earth around 3 January when we are closest to the Sun than on 4 July, when we are furthest from it.<\/p> Yet Earth’s orbital eccentricity is decreasing as part of a 100,000 year cycle.<\/p> Comets have very varied orbits. Most periodic comets have eccentricities between 0.2 and 0.7, though some have highly eccentric orbits, like Halley’s Comet which has an eccentricity of 0.967.<\/p> Non-periodic comets have near parabolic orbits, like Comet Hale-Bopp’s at 0.995. Comet C\/2006 PI McNaught has a hyperbolic orbit of 1.00001 but, still being under the influence of the Sun, returns every 100,000 years.<\/p> Comet 2I\/Borisov, discovered in 2019, is not bound to the Sun and has a hyperbolic orbit with an eccentricity of 3.36.<\/p>![\"An](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2024\/02\/orbit.en_-1024x566.png?fit=800%2C442\")
Orbital eccentricity of Solar System planets<\/strong><\/h2>
Solar System body<\/th> Orbital eccentricity<\/th><\/tr><\/thead> Mercury<\/td> 0.205<\/td><\/tr> Venus<\/td> 0.006<\/td><\/tr> Earth<\/td> 0.016<\/td><\/tr> Mars<\/td> 0.093<\/td><\/tr> Jupiter<\/td> 0.048<\/td><\/tr> Saturn<\/td> 0.054<\/td><\/tr> Uranus<\/td> 0.047<\/td><\/tr> Neptune<\/td> 0.008<\/td><\/tr> Pluto<\/td> 0.248<\/td><\/tr> The Moon<\/td> 0.059<\/td><\/tr><\/tbody><\/table> Earth’s orbital eccentricity<\/strong><\/h2>
![\"Earth\u2019s](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2023\/06\/earth-orbit-equinox-solstice-08e679c.jpg\")
Comets’ orbits<\/strong><\/h2>
![\"Comet](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/07\/Comet-Swift-Tuttle-ed3f99b.png\")
Most eccentric orbit<\/strong><\/h2>