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How to Calculate Orbital Eccentricity With Worksheets

Closed orbits --- orbits that repeat indefinitely --- are in the shape of ellipses. Circles are special cases of ellipses, and some elliptical orbits are so close to circular that it would be very difficult to tell the difference. That's what eccentricity is all about: calculating how close an ellipse is to being a circle. If the eccentricity is zero, the orbit is a circle. If the eccentricity is close to 1, the orbit is flattened and elongated. Using orbital data, it's easy to use a spreadsheet to calculate eccentricity.

Instructions

- 1
  
  Venus orbits the sun. Its closest approach is perihelion and the furthest distance is aphelion.
  
  Find the periapsis and apoapsis of the orbiting body of interest. Periapsis is the object's closest approach to the body it orbits, while apoapsis is the furthest the object gets from the body it orbits. For objects orbiting the Earth, these are the perigee and apogee; for objects orbiting the sun these are perihelion and aphelion.
  
  As an example, for Venus's orbit around the sun, the perihelion is 107.5 million kilometers and the aphelion is 108.9 million kilometers.
- 2
  Enter the values in two cells. Make sure the spreadsheet interprets the numbers as numbers and not text. Often this can be done by starting the entry with an equals sign.
- 3
  In another cell, put the sum of the two values.
  
  For the example problem, the sum will be 216.4 million kilometers.
- 4
  In another cell, put the difference between apoapsis and periapsis.
  
  For Venus, the aphelion - the perihelion = 108.9 - 107.5 = 1.4 million kilometers.
- 5
  
  The orbit of Venus is almost a circle, with just a small difference between its periapsis and apoapsis.
  
  Enter the final calculation in another cell. The eccentricity is equal to the difference between apoapsis and periapsis divided by their sum.
  
  For the example, this is 1.4 million kilometers / 216.4 million kilometers = 0.0065. That's nearly a circle.

Astronomy