Johannes Kepler (1571-1630) discovered the laws of orbital motion, now called Kepler's laws. A set of 6 parameters, together with the exact time for which they are valid, is required to describe an orbit that obeys these laws. While there are various ways to choose the parameters, the standard set, used by AMSAT (and not coincidentally, the ones published by NASA), follows:

Epoch Time

Inclination

RAAN

Eccentricity

Argument of Perigee

Mean Anomaly

Mean Motion

Real orbits are not ideal: over time they decay due to atmospheric drag, or are gradually

Decay Rate

For bookkeeping purposes, some descriptive information about the satellite is also included in the published "element sets":

Catalog Number

Epoch Revolution

An alternate specification for the size of the orbit, sometimes found in published data, is:

Semi-Major Axis

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