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Due to perturbations (mainly from planets) the actual trajectory of each minor body (comet, asteroid) is not purely keplerian orbit and is described by its
osculating orbit: gravitational Keplerian orbit about a central body which best approximates the motion of the object at a given instant in time.
Commonly, the osculating orbits are given at a Epoch of osculation. It is in the TT timescale.

An osculating orbit of an asteroid and its position upon this orbit are fully described by the six standard Keplerian orbital elements:
a — semimajor axis (in AU)
e — eccentricity
ω — argument of perihelion (in degrees), equinox 2000.0
Ω — longitude of the ascending node (in degrees), equinox 2000.0
i — inclination (angular distance) of the orbital plane from the ecliptic plane(in degrees), equinox 2000.0
M — the mean anomaly at the given Epoch; descibes the position of object upon the orbit (in degrees)


Commonly, an osculating orbit of a comet at a given Epoch is described by the
T — Perihelion Time given in TT (instead of mean anomaly)
q — perihelion distance (instead of semimajor axis) (in AU)
e — eccentricity
ω — argument of perihelion (in degrees), equinox 2000.0
Ω — longitude of the ascending node (in degrees), equinox 2000.0
i — inclination of the orbital plane from the ecliptic plane (in degrees), equinox 2000.0