Small-body astronomy runs on a handful of terms that get used everywhere and explained nowhere. Here they are in plain language. Anywhere these words appear on the site they are marked with a dotted underline and link back here.
One astronomical unit is the mean distance from the Earth to the Sun: 149,597,871 km, or about 8.3 light-minutes. It is the natural ruler for the solar system. Mars orbits at 1.52 AU, the asteroid belt lies roughly between 2 and 3.3 AU, Jupiter at 5.2 AU and Neptune at 30 AU.
An orbit is an ellipse, and the semi-major axis (written a) is half of its longest diameter. It is the single most useful number about an orbit, because it fixes the orbital period: double the semi-major axis and the year gets about 2.8 times longer. When we sort asteroids into the inner, middle and outer belt, we are sorting them by semi-major axis.
Perihelion is the closest approach to the Sun (written q when used as a distance). It matters for two reasons: it decides whether an object crosses another body's orbit, and it decides how hot the object gets. A comet's activity switches on near perihelion, and the definition of a near-Earth object is simply a perihelion inside 1.3 AU.
Aphelion is the greatest distance from the Sun in an orbit (written Q). Together with perihelion it describes how stretched an orbit is: a body with a perihelion of 1 AU and an aphelion of 5 AU spends most of its time out near the far end, moving slowly, and only a small fraction of each orbit near the Sun.
Eccentricity (written e) measures the departure from circularity. Most main-belt asteroids sit between 0 and 0.2 — nearly circular. Comets from the outer solar system arrive with eccentricities just under 1, on orbits so elongated that they are effectively open-ended. Above 1 the path is a hyperbola and the object is not bound to the Sun at all, which is how the three known interstellar visitors were recognised.
Inclination (written i) is the angle between an object's orbital plane and the plane of Earth's orbit. The planets are all within a few degrees of that plane. Asteroids spread over tens of degrees, which is why the belt is a thick torus rather than a flat ring. An inclination greater than 90° means the object travels around the Sun in the opposite direction to the planets, as Halley's Comet does at 162°.
An object is at opposition when Earth lies directly between it and the Sun. This is the best time to observe: the object is at its closest, fully illuminated from our viewpoint, and above the horizon for most of the night. Asteroid brightnesses on this site are quoted at opposition because that is when they can actually be measured.
The magnitude scale is inherited from antiquity and is deliberately awkward. Smaller numbers mean brighter objects, and the scale is logarithmic: a difference of 5 magnitudes is a factor of 100 in brightness, so one magnitude is a factor of about 2.5. For orientation: the Sun is magnitude −27, the full Moon −13, the brightest stars around 0, the faintest star a dark-adapted eye can see about 6, and a good amateur telescope with a camera reaches 15 to 18. Something at 20th magnitude is roughly a million times fainter than the naked-eye limit.
Apparent brightness depends on how far away something happens to be, so it is useless for comparing objects. Astronomers remove the distance with an absolute magnitude — but the definition is different for asteroids than for stars. For a star it is the brightness it would have at 10 parsecs. For an asteroid, H is the brightness it would have if placed 1 AU from both the Sun and the observer, with the Sun directly behind the observer. Because asteroids shine by reflected sunlight, H combines size and reflectivity: given one you can estimate the other. H = 3.3 is Ceres at 939 km across; H = 22 is roughly 140 m; H = 28 is a boulder a few metres wide.
Albedo is reflectivity, from 0 (perfectly black) to 1 (a perfect mirror). It is what separates a large dark object from a small bright one that looks identical in a telescope. Carbon-rich asteroids reflect about 5% of the light falling on them — darker than fresh asphalt. Stony ones reflect 15–25%. A handful of objects exceed 0.3. Because brightness depends on size and albedo together, measuring one is the only way to pin down the other.
A light curve is simply brightness plotted against time. As an irregular asteroid rotates it presents a changing cross-section to the Sun, so its brightness rises and falls with a period equal to its rotation. Because most asteroids are elongated, a light curve usually shows two maxima and two minima per rotation. The shape of the curve carries information about the object's outline, and sudden extra dips can reveal a moon passing in front.
The rotation period is the asteroid's day, and it is measured from a light curve. Periods range from under two hours to several hundred days. Because an elongated body brightens twice per turn, the true rotation period is usually twice the most obvious period in the data — a trap that has produced a good deal of published confusion.
The phase angle is the angle at the object between the direction to the Sun and the direction to us. At opposition it is near zero and the surface we see is fully illuminated. As the angle grows, shadows lengthen and the object dims faster than distance alone would explain. The relationship between brightness and phase angle — the phase curve — is itself a measurement, and it depends on how rough and porous the surface is.
A resonance occurs when orbital periods are related by a ratio of small whole numbers — an asteroid completing three orbits for each one of Jupiter's, for instance. The same gravitational nudge then repeats at the same point every time instead of averaging away, and over millions of years the effect builds. Usually this clears a gap: the Kirkwood gaps in the asteroid belt sit at precisely these locations. Occasionally it protects instead, which is why Pluto survives an orbit that crosses Neptune's.
Real orbits wobble continuously under the pull of the planets, so there is no single fixed ellipse. Osculating elements are the ellipse that matches the object's position and velocity at one instant — a tangent to the true path, valid at that moment. They are what catalogues publish. Proper elements, which average the wobble away over long periods, are what you need to identify collisional families, and they have to be computed separately.
The minimum orbit intersection distance is the smallest gap between two orbital paths, regardless of where either body happens to be. It is the first screen for impact hazard: if the paths never come within 0.05 AU, no timing of the two bodies can produce a collision. An object can have a small MOID and still never come near Earth, because the two may simply never arrive at the crossing point together.
When a small body crosses the line of sight to a star, the star vanishes for a few seconds. The length of the disappearance, timed from several sites at once, gives a direct chord across the object and so its true size and profile — no assumptions about reflectivity needed. It requires precise timing rather than a large telescope, which makes it one of the few areas where coordinated small instruments outperform a single big one. Rings around the Centaur Chariklo were discovered this way.
When two bodies orbit each other — the Sun and Jupiter, say — there are five positions where a much smaller third body feels the combined pull of both plus the swing of its own orbit and ends up keeping station relative to them. They are numbered L1 to L5. Three are unstable: a small nudge grows, so nothing accumulates there, though L1 and L2 are useful parking spots for spacecraft that carry fuel. L4 and L5 are stable, sitting 60° ahead of and behind the planet on its orbit, and an object displaced from them drifts back rather than away. Over the age of the solar system that difference is decisive: L4 and L5 have collected large populations of asteroids, which are called Trojans after the Jupiter swarms where they were first found.
An ephemeris predicts an object's position from its orbit. For a moving target it is what tells a telescope where to point, and what tells the photometry software which dot in the frame to measure. Its accuracy depends on how well the orbit is known, which is why newly discovered objects need rapid follow-up before the prediction degrades.