To know the state of motion of a body, it would
be necessary to know the values of the six parameters
corresponding to its position and its speed. But, during a
general motion, these values change constantly and knowing them
at every moment is a very difficult task.

In the reality, for what concerns constant, stable orbits,
the motion of a body can be completely described knowing the
values of 6 constant parameters, called* orbital elements *that
characterize the orbit. In this hyothesis, knowing the position
and the speed of the object at every instant becomes useless.

First of all, let's consider the orbit of
an object of the Solar System (a planet or a minor body). This
orbit can be an ellipse (as the first of Kepler's laws states),
or an opened orbit such as parabola or hyperbola. Anyhow, this
orbit will always be a conic and it will always lie on a plane
called the *orbital plane*. The* *intersection
between this plane and a* reference plane *(which can be
chosen as the plane where the Earth's orbit lies)* *is
called *nodal line. *This *nodal line* passes
through the *ascending and descending node.*

The first 2 orbital elements, needed to
define size and shape of the orbit on this plane, are the *semimajor
axis a* and the *eccentricity e*.

These two parameters can be defined on the orbital
plane; for an elliptical orbit: - 2a is the length of the major axis (M in the
image on the left); - e, the eccentricity, gives an indication of
how much the ellipse is elongated: it is 0 for a circle,
and tends to 1 for more and more elongated orbits. It can
be calculated with the formula (where m and M are defined
in the image): |

Three more parameters are needed to describe where is
the orbit, and how it is oriented: - i the inclination of the orbital plane, the
angle that this plane forms with a reference plane; - the longitude of node,
the angle between a reference direction and the nodal
line, that goes from the Sun to the ascending node; - the argument of
perihelion, the angle from the nodal line to the
line joining the Sun and the perihelion, the point on the
orbit closest to the Sun. Finally, the position on the orbit of the body of interest can be specified by giving the sixth parameter: - T, the time of passage at the perihelion. |