Home » All Calculators » Aerospace and Aerodynamics » Orbit Eccentricity Calculator

Orbit Eccentricity Calculator

Photo of author
Published on

The orbit eccentricity calculator is a simple yet powerful tool designed to measure how much an orbit deviates from being a perfect circle. In the vast expanse of space, celestial bodies like planets, asteroids, and comets follow paths around larger bodies, primarily stars or planets. These paths, or orbits, are often elliptical, and the degree to which they stretch out is known as their eccentricity. This calculator helps astronomers, students, and space enthusiasts quantify that stretch, ranging from 0 (a perfect circle) to values close to 1 (highly elliptical orbits).

Purpose and Functionality

Orbit eccentricity is a fundamental concept in celestial mechanics and astrophysics, providing insights into the dynamics of celestial bodies. It affects various aspects of an orbit, such as the length of seasons on a planet or the temperature variations it might experience. By inputting specific parameters of an orbit into the calculator, one can quickly determine its eccentricity using several methods based on available data:

  1. Periapsis and Apoapsis Method: This method requires knowing the closest and farthest points of the orbiting body from the central body.
  2. Semi-Major and Semi-Minor Axes Method: This involves the major and minor radii of the orbit’s ellipse.
  3. Total Energy and Angular Momentum Method: This approach uses the physical dynamics of the orbiting body.
  4. Semi-Major Axis and True Anomaly Method: This calculates eccentricity based on the orbit’s shape at a specific point.


Let’s break down the formulas used to calculate orbit eccentricity into simpler terms. Orbit eccentricity tells us how stretched out an orbit is compared to a perfect circle.

1. Using Periapsis and Apoapsis

  • What You Need: The closest and farthest points of an object’s orbit around the central body.
  • Simple Words: Eccentricity = (Farthest distance – Closest distance) / (Farthest distance + Closest distance)

2. Using Semi-Major Axis and Semi-Minor Axis

  • What You Need: The longest and shortest radii of the orbit’s ellipse.
  • Simple Words: Eccentricity = Square root of [1 – (Square of shortest radius / Square of longest radius)]

3. Using Total Energy and Angular Momentum

  • What You Need: The orbit’s total energy, angular momentum, the mass of the orbiting body, and the gravitational parameter (a number that involves the mass of the central body and the gravitational constant).
  • Simple Words: Eccentricity = Square root of [1 + (2 x Energy x Square of angular momentum) / (Gravitational parameter squared x Cube of mass)]

4. Using the Semi-Major Axis and the Distance at a Given True Anomaly

  • What You Need: The longest radius of the orbit’s ellipse, the distance from the central body at a specific point in the orbit, and the gravitational parameter.
  • Simple Words: Eccentricity = Square root of [1 – (Distance / Longest radius) x (2 – (Distance x Gravitational parameter) / Longest radius)]

Step-by-Step Examples

Example 1: Periapsis and Apoapsis Method

  • Suppose a comet orbits the Sun with a closest approach (periapsis) of 0.5 AU (Astronomical Units) and a farthest point (apoapsis) of 1.5 AU.
  • Using the formula e=ra​+rpra​−rp​​, where ra​ is apoapsis and rp​ is periapsis, we find =1.5−0.51.5+0.5=0.5e=1.5+0.51.5−0.5​=0.5.
  • The eccentricity, 0.5, indicates a moderately elliptical orbit.

Example 2: Semi-Major and Semi-Minor Axes Method

  • For an asteroid orbit around Earth with a semi-major axis of 4,000 km and a semi-minor axis of 3,000 km.
  • Using the formula e=
  • =1−(b2/a2​)
  • ​, we calculate =1−(3000/4000)=0.75
  • The eccentricity, 0.75, suggests a highly elliptical orbit.

Relevant Information Table

MethodInputs RequiredEccentricity Formula
Periapsis and ApoapsisClosest and farthest distances from the central bodye=​ra​−rp/ra​+rp​​
Semi-Major and Semi-Minor AxesLengths of the orbit’s semi-major and semi-minor axese=1−(b2​/a2)​
Total Energy and Angular MomentumOrbiting body’s total energy and angular momentum3e=1+2EL2​​/μ2m3
Semi-Major Axis and True AnomalySemi-major axis and distance at a given true anomalye=1−r/a​(2−​/a)​


The orbit eccentricity calculator is a versatile and invaluable tool for anyone interested in the movements and characteristics of celestial bodies. By providing a straightforward way to calculate the eccentricity of orbits, it offers insights into the nature and dynamics of planetary systems, comets, and asteroids. Whether for educational purposes, research, or sheer curiosity about the cosmos, understanding orbit eccentricity deepens our comprehension of the universe’s intricate mechanics.

Leave a Comment