Orbital Eccentricity

Orbital eccentricity (e) is a fundamental parameter that describes how stretched or elongated an orbit is compared to a perfect circle. This dimensionless value ranges from 0 (a perfect circle) to just under 1 (a highly elongated ellipse), with 1 representing a parabolic escape trajectory.</p><p>Most planets have remarkably circular orbits: Earth's eccentricity is only 0.017, meaning its orbit deviates just 1.7% from a perfect circle. In contrast, Mercury has the most eccentric planetary orbit at 0.206, causing its distance from the Sun to vary by 24 million kilometers between closest and farthest approach. Pluto, now classified as a dwarf planet, has an extreme eccentricity of 0.248, creating such an elongated orbit that it sometimes comes closer to the Sun than Neptune.</p><p>Comets showcase the most dramatic eccentricities. Halley's Comet has an eccentricity of 0.967, following a highly stretched ellipse that takes it from beyond Neptune's orbit to between Mercury and Venus every 76 years.</p><p>Johannes Kepler first described elliptical orbits in his First Law of Planetary Motion (1609), revolutionizing our understanding of celestial mechanics. Eccentricity directly affects orbital velocity—objects move fastest at perihelion (closest approach) and slowest at aphelion (farthest point), explaining seasonal variations in planetary climates and the dramatic brightening of comets as they approach the Sun.