anoomalia = [Greek] deviation, roughness
In astronomy, anomaly is used for different kinds of angles that are
important when calculating the position of objects in their orbits.
Below, several different anomalies are explained for the orbit of the
Earth around the Sun (or rather: of the Earth around the barycenter of
the Solar System, but that is almost the same thing), but they are
also used for orbits of other celestial objects.
- The true anomaly is the angle (as seen from the Sun) between the
Earth and the perihelion of the orbit of the Earth. When the true
anomaly is equal to 0 degrees, then the Earth is closest to the Sun
(or: in its perihelion). When the true anomaly is equal to 180
degrees, then the Earth is furthest from the Sun (in the
aphelion).
- The mean anomaly is what the true anomaly would be if the Earth
moved with constant speed along a perfectly circular orbit (with an
eccentricity equal to zero) around the Sun in the same time. Just as
for the true anomaly, the mean anomaly is equal to 0 in the perihelion
and to 180 degrees in the aphelion, but at other points along the
Earth's orbit the true and mean anomalies are not equal to one
another. The mean anomaly is often used for one of the orbital
elements.
- The eccentric anomaly is an angle that is related to both the mean
and the true anomaly. You encounter this angle if you solve Kepler's
Equation to find the true anomaly from the mean anomaly.