What is the difference between the drawing of the Eccentric Anomaly and the Mean Anomaly?

1. Mar 23, 2015

Philosophaie

What is the difference between the drawing of the Eccentric Anomaly and the Mean Anomaly.
http://www.philosophaie.com/EA.png [Broken]
One has a circle radius "a" inscribed an ellipse with a focus "F". The True Anomaly makes "f" makes an angle to point "P" then a perpendicular is drawn to a point on the circle where a line is drawn to the center.

What is this angle Mean or Eccentric? How do you draw the other Anomaly?

Last edited by a moderator: May 7, 2017
2. Mar 23, 2015

Philosophaie

Is Mean Anomaly from point "P" on the ellipse to the center?

3. Mar 23, 2015

phyzguy

The mean anomaly is a mathematical construct. Neither the True anomaly (f in the drawing) nor the Eccentric anomaly (E in the drawing) change at a constant rate, because the planet goes more slowly when it is far away from the Sun than when it is close to the sun. So the Mean Anomaly is obtained by taking E and averaging it over the whole orbit so that M is always increasing at a constant rate. It is similar in concept to the "Mean Solar Day", where we take the average length of the day over the whole year and create a day that is always the same length, even though the actual length of the day changes slightly as we go through the year. Since it is a construct, I don't know of a way to picture it on the drawing.