Editing Orbital elements (section)

=== Required parameters ===
In general, eight parameters are necessary to unambiguously define an arbitrary and unperturbed orbit. This is because the problem contains eight [[Degrees of freedom (mechanics)|degrees of freedom]]. These correspond to the three spatial [[dimension]]s which define position (''{{mvar|x}}'', ''{{mvar|y}}'', ''{{mvar|z}}'' in a [[Cartesian coordinate system]]), the velocity in each of these dimensions, the magnitude of [[Gravitational acceleration|acceleration]] (only magnitude is needed, as the direction is always opposite the position vector), and the current time ([[Epoch (astronomy)|epoch]]). The mass or [[standard gravitational parameter]] of the central body can specified instead of the acceleration, as one can be used to find the other given the position vector through the relation <math id="https://en.wikipedia.org/wiki/Gravitational_acceleration">a=\mu/r^2</math>. These parameters can be described as [[orbital state vectors]], but this is often an inconvenient and opaque way to represent an orbit, which is why orbital elements are commonly used instead.

When describing an orbit with orbital elements, typically two are needed to describe the size and shape of the trajectory, three are needed describe the rotation of the orbit, one is needed to describe the speed of motion, and two elements are needed to describe the position of the body around its orbit along with the epoch time at which this occurs. However, if the epoch time is chosen to be the time at which the position-describing element of choice (e.g. mean anomaly) is equal to some constant (usually zero), then said element can be omitted, meaning only seven elements are required in total.

Commonly only 6 variables are specified for a given orbit, as the motion-describing variable can be the mass or standard gravitational parameter of the central body, which is often already known and does not need specifying, and the epoch time can be considered part of the reference frame and not as a distinct element. However, in any case, 8 values will need to be known, regardless of how they are categorized.

Additionally, certain elements can be omitted if they are not required for the desired application (e.g. both epoch elements and the motion element are not needed if only the shape and orientation need to be known).