The Six Orbital Elements
Every satellite on the globe behind you is, to a tracker, just six numbers and a timestamp. Five of them fix an ellipse in space — its size, its shape, and the three angles that decide how it is tilted and turned. The sixth says where along that ellipse the satellite is right now.
The six elements, one at a time
Semi-major axis — the size
Half the length of the ellipse at its longest. It alone sets the orbital period: bigger orbit, slower lap, by a fixed mathematical relationship. A TLE does not actually store it — it stores mean motion, the number of orbits per day, which is the same fact stated the other way round.
Eccentricity — the shape
How far from circular the ellipse is. Zero is a perfect circle; the closer to 1, the more stretched. Most working satellites sit very close to 0 because a near-circular orbit holds a steady altitude — Molniya orbits are the famous exception, deliberately lopsided at about 0.74.
Inclination — the tilt
The angle between the orbital plane and the equator. 0° hugs the equator, 90° goes over both poles, and anything past 90° is orbiting against Earth's rotation. This is the element you can see: it is why the ISS at 51.6° reaches most of the inhabited world but never the far north.
RAAN — where the plane is swivelled to
Right ascension of the ascending node: take the tilted plane and spin it around Earth's axis like a coin in a slot. RAAN is how far round it has been spun — measured against a fixed direction in the sky rather than against the ground, because the ground is turning underneath and would make a useless reference.
Argument of perigee — where the low point sits
The plane is now fixed in space, but the ellipse inside it can still be rotated. This angle says where the closest approach falls: over the equator, over the north, somewhere between. On a near-circular orbit it barely matters; on a Molniya it is the entire design, parking the slow, high part of the orbit above the latitudes being served.
True anomaly — where it is right now
The first five barely change. This one changes constantly: it is the satellite's current angle around the ellipse, measured from perigee. A TLE gives you mean anomaly instead — a version that ticks at a constant rate and so is easier to store — and the tracker converts it to the real angle before it can draw anything.
Five of the six describe an orbit that would never change if the Earth were a perfect sphere and space were empty. It is neither, so they do change: the equatorial bulge drags RAAN and the argument of perigee round over weeks, and drag eats away at the semi-major axis. That is why the seventh number matters as much as the six — the epoch, the exact instant the elements were measured. Elements without an epoch are meaningless, and elements from last month are wrong. AstroSat refreshes them every two hours and propagates from there with SGP4, the same model and the same data professionals use.
Frequently asked questions
What are the six orbital elements?
Semi-major axis, eccentricity, inclination, right ascension of the ascending node (RAAN), argument of perigee, and true anomaly. The first two describe the ellipse, the next three describe how it is oriented in space, and the last places the satellite along it.
What is the difference between RAAN and argument of perigee?
They are two rotations applied one after the other. RAAN swivels the whole orbital plane around Earth's axis; the argument of perigee then rotates the ellipse within that plane. Both are angles measured from the ascending node, which is why they get confused.
Are the elements in a TLE the same six?
The same information in a different currency. A TLE gives mean motion rather than the semi-major axis, and mean anomaly rather than true anomaly — both are the same fact restated in a form that is easier to store and to propagate. It also carries a drag term and an epoch, which the six pure elements do not include.
Why are they called Keplerian elements?
Because they describe the ideal two-body orbit Kepler worked out — one object circling another with nothing else acting on it. Real orbits drift from that ideal, so the elements a tracker uses are fitted values that make a specific propagation model reproduce the real path, not exact measurements of a perfect ellipse.
What does inclination tell you about a satellite?
Roughly what it is for. Near 0° means geostationary — weather and broadcast. Near 98° means sun-synchronous — Earth observation. 51.6° is the ISS. It is often the fastest way to guess a satellite's job from its numbers alone.
Where can I see the elements for a real satellite?
Every satellite page on this site is propagated from that object's current element set, and clicking any dot on the globe opens the same live readout.