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The elements of an orbit are the parameters needed to specify that orbit uniquely, given a model of two ideal masses obeying the Newtonian laws of motion and the inverse-square law of gravitational attraction. Because there are multiple ways of defining a motion, depending on which set of variable you choose to measure, there are several different ways of defining sets of orbital elements, each of which will define the same orbit.

There are seven degrees of freedom in this model (time, position in 3-space at that time, velocity in 3-space at that time) so all sets of orbital elements have seven parameters. (See also: orbital state vectors).

1 Keplerian elements

The traditionally used set of orbital elements is called the set of Keplerian elements, after Johannes Kepler and his Kepler's laws. The Keplerian elements are:

Epoch (time)
Inclination ()
Longitude of ascending node ()
Argument of periapsis ()
Eccentricity ()
Orbital period ()
Mean longitude ( Mean anomaly () at epoch)

Keplerian elements can be obtained from orbital state vectors using VEC2TLE software or by some direct direct computations.

Other orbital parameters, such as the semi-major axisThe Ellipse is also an elliptical street immediately in front of the White House. In mathematics, an ellipse is a figure corresponding to a circle which has been stretched in one direction. This is an example of a conic section and can be defined as the l, can then be calculated from the Keplerian elements. In many cases, the semi-major axis is used as an orbital element instead of period.

Because the simple Newtonian model of orbital motion of idealized points in free space is not exact, the orbital elements of real objects tend to change over time. For artificial satellites grazing the fringes of the atmosphere, an eighth parameter (" dragAtmospheric drag drag is a form of drag. In astrodynamics depending on the situation atmospheric drag can be regarded as ineffciency requiring expense of additional energy during launch of the space object or as a bonus simplifying return from orbit.") may be added. For the planets and moons it is more usual simply to add a time term to the elements.

2 Two line elements

Keplerian elements parameters can be encoded as text in a number of formats. The most common of them is the NASA/NORAD "two-line elements" (TLE) format, originally designed for use with 80-column punched cards, but still in use because it is the most common format, and works as well as any other.

Reference:

Explanatory Supplement to the Astronomical Almanac. 1992. K. P. Seidelmann, Ed., University Science Books, Mill Valley, California.

3 See also

EphemerisAn ephemeris (plural: ephemerides is a table providing the apparent positions of the sun, the moon, and the planets in the sky at a given moment in time, at a given point on the earth. It also provides data on astronomical phenomena of interest to astrolo
Orbital state vectors
VEC2TLE software

Keplerian Elements tutorial
another tutorial
Spacetrack Report No. 3, a really serious treatment of orbital elements from NORADNORAD is the North American Aerospace Defense Command . It is a joint United States and Canadian organization which provides aerospace warning and aerospace control for North America, and was founded on August 1, 1957 under the name North American Air Def (in pdf format)
Celestrak Two-Line Elements FAQ

AstrodynamicsCelestial Mechanics Astrodynamics is the study and creation of orbits, especially those of artificial satellites. It uses the principles of Celestial Mechanics, which is the application of Newton's Laws of Motion and Law of Universal Gravitation to the de Celestial mechanics

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Topics: Orbital Elements, Keplerian Elements, Two Line Elements...

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