Parameters (numbers) that determine an object's location and motion in its orbit about another object. In the case of solar-system objects such as comets and planets, one must ultimately account for perturbing gravitational effects of numerous other planets in the solar system (not merely the sun), and when such account is made, one has what are called "osculating elements" (which are always changing with time and which therefore must have a stated epoch of validity). Six elements are usually used to determine uniquely the orbit of an object in orbit about the sun, with a seventh element (the epoch, or time, for which the elements are valid) added when planetary perturbations are allowed for; initial ("preliminary") orbit determinations shortly after the discovery of a new comet or minor planet (when very few observations are available) are usually "two-body determinations", meaning that only the object and the sun are taken into account --- with, of course, the earth in terms of observing perspective. The six orbital elements used for comets are usually the following: time of perihelion passage (T) (sometimes taken instead as an angular measure called "mean anomaly", M); perihelion distance (q), usually given in AU; eccentricity (e) of the orbit; and three angles (for which the mean equinox must be specified) --- the argument of perihelion (lower-case Greek letter omega), the longitude of the ascending node (upper-case Greek letter Omega), and the inclination (i) of the orbit with respect to the ecliptic.