The purpose of this page is to provide a compendium of astronomical terms and concepts. All of the definitions that appear in the sidebars on the right of every page are included here along with a scattering of other related terminology. This is not meant to be used as an encyclopedia; for example, you will not find a one-sentence summary about the planet Mars on this page. Terms and concepts, but generally not objects, are included here.
A.U.: See "Astronomical Unit."
Absolute Magnitude: See "Magnitude, Absolute."
Accrete: To gather material. Usually it is implied that this is through the gravity of objects pulling material together.
Albedo: Reflectivity on a scale of 0 to 1 with 0 being no reflectivity and 1 being a perfect mirror.
Amor: See "Asteroid, Amor."
Ångström (Å): A unit of measure, equal to 10-10 m.
Apoapsis: The largest distance between two objects in an orbit.
Apollo: See "Asteroid, Apollo."
Apparent Magnitude: See "Magnitude, Apparent."
Arcmin: Short for "arcminute."
Arcminute: A degree (°) in a circle can be divided into 60 minutes, or arcminutes (arcmin).
Arcsec: Short for "arcsecond."
Arcsecond: A degree (°) in a circle can be divided into 60 minutes, or arcminutes (arcmin), and an arcmin can be divided into 60 seconds, or arcseconds (arcsec).
Asteroid, Amor: Asteroids that come close to Earth but do not cross its orbit. They generally lie near Mars.
Asteroid, Apollo: Asteroids with a semi-major axis greater than 1 AU but perihelion less than 1.017 AU.
Asteroid, Aten: Asteroids with a semi-major axis less than 1 AU.
Asteroid, Centaur: Asteroids that lie between the orbit of Jupiter and Neptune.
Asteroid, Trojan: Asteroids that share a planet's orbit, lying at either the L4 or L5 point.
Asteroid, Vulcanoid: Asteroids that lie interior to Mercury's orbit. No members have ever been discovered, so this is a hypothetical group.
Asteroid Belt: A region of space between the orbits of Mars and Jupiter where most of the solar system's asteroids lie.
Astrometry: The precise measurement of objects' positions in the sky.
Astronomical Unit: Abbreviated "A.U." It is the average distance between the sun and Earth, approximately 149.6 million km (93 million miles).
Aten: See "Asteroid, Aten."
Blueshift: When light has been shifted towards shorter, bluer wavelengths due to an object moving towards from the observer.
Captured Moon: A moon that did not form in orbit of its now-parent body, but it was gravitationally captured later on.
Centaur: See "Asteroid, Centaur."
Chandrasekhar Limit: Equivalent to 1.4 solar masses. If a star is above this when it runs out of fuel for fusion, it will collapse past a white dwarf and into a neutron star.
Conjunction, Inferior: When a planet comes between the Sun and Earth in its orbit; only inferior planets can do this.
Conjunction, Superior: When the Sun comes between Earth and a planet. Also known as a planet's opposition.
Dark Matter: Material that is not believed to be composed of "normal" matter (protons, neutrons, electrons, and neutrinos). We can observe its gravitational effects, but we have yet to determine what it is.
Dating, Absolute: Being able to measure the age of something by definite, intrinsic techniques such as radioactive decay, tree rings, or birth records.
Dating, Relative: Using a proxy technique calibrated to a known absolute-dated system to estimate the age of something. Such as saying that people with gray hair are over 60.
Distance Modulus: The mathematical relationship between the absolute magnitude, apparent magnitude, and distance of an object.
Dwarf Planet: See "Planet, Dwarf."
Eccentricity: A measure of how elliptical a circle is. An eccentricity of 0 is a perfect circle. An eccentricity of 1 is a straight line. Values between 0 and 1 are a measure of how "flattened" the circle is to make an ellipse.
Ecliptic: Apparent path of the sun in Earth's sky. Most planets in the solar system lie near this path.
Emission Spectrum: The spectrum of light from an object, such as a star.
Empirical Relationship: Something that is "noticed" in data as a trend but has no known physical basis. For example, Kepler's Three Laws were empirical when he formulated them, but a century later Newton discovered the physical mechanism behind them.
Exoplanet: A planet outside of our solar system, usually orbiting a star other than our own.
Force: (1) Classical Dynamics - The mass of an object times its acceleration (m·a) as given by Newton's Second Law. (2) Particle Physics - A way of communicating something, such as an electron communicating its presence to a proton, or a planet communicating its presence to a star.
Fusion: The process of turning less massive atoms into larger atoms by combining them.
Galactic Halo: Objects that are not within the disk of a galaxy, but rather form an extended "atmosphere." Globular clusters are a good example of objects that lie in the Galactic Halo.
Gamma Ray: A high-energy photon.
Gravity Assist: Using a planet's (or other large object's) gravity to increase the speed and/or alter the direction of a spacecraft. This reduces the fuel requirements and hence the cost.
Greatest Elongation: When an inferior planet appears to be farthest away from the Sun as seen from Earth; the planet will appear to be in a "half" phase.
Heliopause: The interface between the heliosphere and the interstellar medium (dust, gas, plasma, and magnetic fields).
Heliosphere: Region of the sun's influence where its solar wind dominates over those from other stars and the galaxy as a whole.
Inclination: Usually referred to as "orbital inclination." The Earth's orbit defines a plane from which objects that orbit above or below by a certain amount have a non-zero inclination. An object that orbits at a right angle to the Earth plane has an inclination of 90°. Most planets orbit within a few degrees of Earth's plane and hence have a low inclination. Many comets have higher inclinations.
Inferior Planet: A planet that orbits between Earth and the Sun - Venus or Mercury.
Interstellar Medium: Material that lies between stars. This includes dust, gas, and plasma. Often abbreviated "ISM."
Ionized: When electrons are stripped from atoms.
Kelvins: A temperature scale equivalent to degrees Celsius plus 273.15 degrees. In this manner, 0 K is referred to as "absolute zero" and is the coldest possible temperature, where all atomic motion stops. There is no such thing as "degrees" Kelvin, but rather a temperature is referred to as "Kelvins."
Light Pollution: Any extra light that limits the ability to see objects in the sky at night. City lights, especially, significantly limit what can be visible.
Magnitude: (1) Mathematics - Ususally refers to a power-of-10. For example, the magnitude of the weight of an adult is about 100 kg. The magnitude of the weight of a paperclip is 1 gm. (2) Brightness - Refers to a logarithmic brightness scale where an increase in 2.5 magnitudes is a decrease in brightness by a factor of 10. There are two types of magnitude - see "Magnitude, Absolute" and "Magnitude, Apparent."
Magnitude, Absolute: The true brightness of an object. Often abbreviated as M (upper-case) or MA.
Magnitude, Apparent: Also known as "Visual Magnitude." The brightness of an object as seen from a given location. This is a function of the distance from the object, its true brightness, and anything between the object and observer that may enhance or weaken the amount of light. Often abbreviated as m (lower-case) or Mv.Microlensing: Proposed by Albert Einstein as a consequence of relativity, microlensing is the phenomenon when a high-mass object passes in front of another object, and due to the way the massive object bends space, it acts as a lens to magnify the light from the object behind it.
Near-Earth Asteroid: Apollo, Amor, and Aten asteroids.
Neutrino: An elementary particle that is almost massless and very rarely interacts with other forms of matter.
Neutron: An elementary particle generally in the nucleus of an atom that is a neutral charge.
Opposition: When the Sun comes between Earth and a planet. Also known as a planet's opposition.
Parallax: The apparent movement of an object from one location to another based solely on the observer's moving position. For example, holding out your finger, lining it up with a distant object, and closing one eye and then switching will cause the object to move relative to your finger, an example of parallax.
Parsec: The distance an object must be to move 1 arcsec as seen from Earth over 1 year (parallax). Approximately equal to 3.26 light-years.
Periapsis: The smallest distance between two objects in an orbit.
Photolysis: The breaking up of, or decomposition of, molecules via absorption of light energy.
Photon: A "piece" or "element" of light.
Planck's Constant (h): Equal to 6.626068·10-34 m2·kg/s.
Planet: Current Definition - A planet is an object that (1) orbits the Sun; (2) is large enough to assume a spherical shape due to its own gravity; and (3) has cleared its orbit.
Planet, Dwarf: Fits all planet criteria other than (3). Is not a satellite.
Plutoid: A class of dwarf planets that orbits beyond Neptune.
Proton: An elementary particle in the nucleus of an atom that carries a positive charge.
Radial Velocity: The velocity of a star towards or away from Earth.
Redshift: When light has been shifted towards longer, redder wavelengths due to an object moving away from the observer.
Relative Magnitude: See "Magnitude, Apparent."
Resonance: In orbital mechanics, a resonance is when one object's orbit is an integer multiple of an integer multiple of another object's orbit. For example, if for every time Object A goes around the Sun twice, Object B goes around three times, they are in a 2:3 resonance.
Secondary Craters: Craters that are formed by lower-velocity impacts due to ejected material from a primary crater.
Semi-Major Axis: In an ellipse, the semi-major axis is half of the longest axis of the ellipse.
Solar Mass: The mass of our sun.
Spectrum Emission: The spectrum of light from an object, such as a star.
Spectrum, Transmission: The spectrum of light as it is passed through a transparent or translucent object, such as an atmosphere.
Speed of light (c): Equal to 299,792,458 m/s.
Sublimate: Convert directly from a solid to a gas without going through a liquid phase. Ice often does this in dry environments when exposed to enough light.
Terminator: The line between night and day, where sunrise or sunset is occurring.
Terrestrial Planets: A rocky planet with a relatively thin atmosphere. Mercury, Venus, Earth, and Mars are the four terrestrial planets in our solar system.
Tidally Locked: Due to gravitational interactions, a tidally locked object has a rotation period identical to its orbital period (day = year) which means it always shows the same face to the object it orbits.
Trans-Neptunian Object: Asteroids (or comets) that lie beyond the orbit of Neptune.
Transit: When one object passes in front of another, as viewed from a third object.
Transmission Spectrum: See "Spectrum, Transmission."
Trojans: See "Asteroid, Trojan."
Vulcanoid: See "Asteroid, Vulcanoid."