//these are the scientific terms and their definitions, in the form of an associative array, and in alphabetical order.
//the definition pops up whenever a term is hovered over.
//each associative term must be in all lower-case letters.
var defs = new Object();
defs['albedo'] = "Albedo: A term describing the fraction of sunlight reflected by a surface; albedo = zero means no reflection at all (a perfectly black surface); albedo = 1 means all light is reflected (a perfectly white surface).";
defs['aphelion'] = "Aphelion: The point at which an object orbiting the Sun is farthest from the Sun.";
defs['asteroid belt'] = "Asteroid belt: The region of our solar system between the orbits of Mars and Jupiter in which asteroids are heavily concentrated.";
defs['au'] = "AU: The average distance (semimajor axis) of the Earth from the Sun, which is about 150 million km.";
defs['binary planet'] = "Binary Planet: A system of two planets that orbit each other about a common center of mass that is not located within the interior of either planet.";
defs['coma'] = "Coma: The dusty atmosphere of a comet created by sublimation of ices in the nucleus when the comet is near the Sun.";
defs['coriolis effect'] = "Coriolis Effect: An effect that causes air or objects moving on a rotating planet to deviate from straight-line trajectories.";
defs['differentiation'] = "Differentiation: The process in which gravity separates materials according to density, with high-density materials sinking and low-density materials rising.";
defs['doppler effect'] = "Doppler effect: The effect that shifts the wavelengths of spectral features in objects that are moving toward or away from the observer.";
defs['doppler technique'] = "Doppler technique: The detection of extrasolar planets through the motion of a star toward and away from the observer caused by gravitational tugs from the planet.";
defs['dust tail'] = "Dust tail: One of the tails seen when a comet passes near the Sun; composed of small solid particles pushed away from the Sun by the radiation pressure of sunlight.";
defs['dwarf planet'] = "Dwarf planet: An object that orbits the Sun and is massive enough for its gravity to have made it nearly round in shape, but that does not qualify as an official planet because it has not cleared its orbital neighborhood.";
defs['eccentricity'] = defs['eccentric'] = "Eccentricity: A measure of how much an ellipse deviates from a perfect circle; defined as the center-to-focus distance divided by the length of the semimajor axis.";
defs['eclipse'] = defs['eclipsed'] = "Eclipse: An event that occurs when one astronomical object casts a shadow on another or crosses our line of sight to the other object.";
defs['ecliptic'] = "Ecliptic: The Sun's apparent annual path among the constellations.";
defs['frost line'] = "Frost Line: The boundary in the solar nebula beyond which ices could condense; only metals and rocks could condense within the frost line.";
defs['greenhouse gases'] = "Greenhouse Gases: Gases, such as carbon dioxide, water vapor and methane that are particularly good absorbers of infrared light but are transparent to visible light.";
defs['greenhouse effect'] = "Greenhouse Effect: The process by which greenhouse gases in an atmosphere make a planet's surface temperature warmer than it would be in the absence of an atmosphere.
"+defs['greenhouse gases'];
defs['habitable zone'] = "Habitable zone: The region around a star in which planets could potentially have surface temperatures at which liquid water could exist.";
defs['heliosphere'] = "Heliosphere: The region containing our Sun and solar system through which the solar wind extends.";
defs['inclination'] = "Inclination: Relative to our solar system, the angle between the ecliptic plane and the orbit of the object.
Ecliptic: The Sun's apparent annual path among the constellations.";
defs['io torus'] = "Io torus: A donut-shaped charged-particle belt around Jupiter that approximately traces Io's orbit.";
defs['kuiper belt'] = "Kuiper Belt: The comet-rich region of our solar system that spans distances of about 30-100 AU from the Sun. Kuiper belt comets have orbits that lie fairly close to the plane of planetary orbits and travel around the Sun in the same direction as the planets.";
defs['kuiper belt object'] = defs['kuiper belt objects'] = "Kuiper Belt Object: Any object orbiting the Sun within the region of the Kuiper belt, although the term is most often used for relatively large objects. For example, Pluto and Eris are considered large Kuiper belt objects.";
defs['lunar maria'] = "Lunar Maria: The regions of the Moon that look smooth from Earth and actually are impact basins.";
defs['magnetosphere'] = "Magnetosphere: The region surrounding a planet in which charged particles are trapped by the planet's magnetic field.";
defs['micron'] = "Micron: A micron, also known as a micrometer, is a unit of measurement that is equal to 10-6 meters.";
defs['oort cloud'] = "Oort cloud: A huge, spherical region centered on the Sun, extending perhaps halfway to the nearest stars, in which trillions of comets orbit the Sun with random inclinations, orbital directions and eccentricities.";
defs['orbital resonance'] = defs['orbital resonances'] = "Orbital Resonance: A term for any situation which one object's orbital period is a simple ratio of another object's period such as ½, ¼, or 2/3. In such cases, 2 objects periodically line up with each other and the extra gravitational attractions at these times can affect the objects' orbits.";
defs['outgassing'] = "Outgassing: The process of releasing gases from a planetary interior, usually through volcanic eruptions.";
defs['perihelion'] = "Perihelion: The point at which an object orbiting the Sun is closest to the Sun.";
defs['planetesimals'] = "Plantesimals: The building blocks of planets, formed by accretion in the solar nebula.
Solar Nebula: The piece of interstellar cloud from which our own solar system formed.";
defs['plasma tail'] = "Plasma tail: One of two tails seen when a comet passes near the Sun; composed of ionized gas blown away from the Sun by the solar wind.";
defs['protoplanetary disk'] = "Protoplanetary disk: A disk of material surrounding a young star (or protostar) that eventually forms planets.";
defs['retrograde'] = "Retrograde Motion: Motion that is backward compared to the norm; e.g., Triton orbits Neptune in a direction opposite the direction of Neptune's rotation.";
defs['solar nebula'] = "Solar nebula: The piece of interstellar cloud from which our own solar system formed.";
defs['solar wind'] = "Solar wind: A stream of charged particles ejected from the Sun.";
defs['spectral class'] = "Spectral type: A way of classifying a star by the lines that appear in its spectrum; it is related to surface temperature. The basic spectral types are designated by a letter (OBAFGKM, with O for the hottest stars and M for the coolest) and are subdivided with numbers from 0 through 9.";
defs['spectroscopy'] = defs['spectroscopic'] = "Spectroscopy: The process of obtaining spectra from astronomical objects.";
defs['sublimate'] = defs['sublimation'] = defs['sublimated'] = "Sublimation: The process by which atoms or molecules escape into the gas phase from a solid.";
defs['synchronous rotation'] = defs['synchronously'] = "Synchronous Rotation: A description of the rotation of an object that always shows the same face to an object that it is orbiting because its rotation period and orbital period are equal.";
defs['tectonics'] = "Tectonics: The disruption of a planet's surface by internal stresses.";
defs['terrestrial-sized planets'] = "Terrestrial-sized planets: Rocky planets similar in overall composition to Earth.";
defs['tidal force'] = defs['force'] = "Tidal force: A force that is caused when the gravity pulling on one side of an object is larger than that on the other side, causing the object to stretch.";
defs['tidal friction'] = defs['friction'] = "Tidal friction: Friction within an object that is caused by a tidal force.";
defs['tidal heating'] = defs['heat'] = "Tidal Heating: A source of internal heating created by tidal friction. It is particularly important for satellites with eccentric orbits such as Io and Europa.";
defs['transit'] = "Transit: An event in which a planet passes in front of a star as seen from Earth.";
defs['troposphere'] = "Troposphere: The lowest atmospheric layer in which convection and weather occur.";