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

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

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