Aphelion  -  The farthest point from the Sun in a solar orbit.

Aristarchus  -  Greek astronomer (310-230 BC), who first proposed a model of the solar system with the Sun at the center. This idea was largely forgotten until Copernicus's times.

Asteroid belt  -  The region between Mars and Jupiter where most asteroids in our solar system are found.

Asteroids  -  Small planetoids distributed widely throughout the solar system. Their biggest concentration is found between Mars and Jupiter.

Bar  -  Unit of atmospheric pressure measurements. One bar is the pressure measured on Earth at sea level.

Tycho Brahe  -  Danish astronomer (1546-1601) who built the largest naked eye astronomical observatory in modern Europe. He made countless observations of the position of Mars and other celestial bodies. Read more about Brahe.

Carbonates  -  Rocks containing the chemical group CO₃.

Comets  -  Cosmic 'dirty snow balls'; orbiting remnants of the formation of our solar system. When they get close to the Sun they vaporize and produce a tail of gas and dust often visible to the naked eye.

Condense  -  Change from vapor, i.e. gas, into a liquid.

Convection  -  A type of motion found in a gas or liquid when there is a temperature difference between separate regions. For instance, in boiling pot of water, the water closer to the flame becomes warmer and, correspondingly, becomes less dense. The hot water therefore rises to the surface, pushing the warmer and cooler water into contact and then pushing the cooler water down. This mechanism exchanges heat between warmer and cooler regions.

Copernicus  -  Polish astronomer (1473-1543) who first theorized the heliocentric system in modern times. The heliocentric system is the model where planets revolve around the Sun. Read more about Copernicus.

Core  -  The core of a planet is its central, spherical portion. It can be divided in two regions: the inner region is made of a mixture of nickel, iron, and sulphur, while the external portion is made of silicates. The composition of Jupiter's core resembles the composition of Earth as a whole.

Crust  -  The external, solid portion of the planet, literally its 'skin.' Like the mantle, it is made of silicate minerals. The crust's silicates are less dense than are those of the mantle.

Density  -  Mass per unit volume.

Diatomic  -  Describes a molecule consisting of two atoms.

Differentiation  -  The process by which heavier rocks sink toward the center of a gas giant planet, allowing lighter rocks to end up forming its crust.

Dissociated  -  A molecule is dissociated when it is broken into its atomic constituents. C.f photodissociation.

Doppler effect  -  Change of the wavelength of a wave as the emitting source and the observer move in relation to one another.

Eccentric  -  An ellipse is eccentric if it is quite elongated. Eccentricity is the ratio of the distance from the center to a focus of an ellipse to the length of its major axis.

Energy flux  -  The outflow of energy coming from a source.

Escape velocity  -  The velocity required for an object to escape the gravitational pull of another object (such as a planet or star).

Equinox  -  The two equinoxes occur when the length of day and night is equal everywhere on a planet.

Freezing point  -  The temperature at which a liquid or a vapor freezes.

Fronts  -  The boundaries between air masses of different temperature and pressure, where clouds and weather systems often develop.

Galileo Galilei  -  Italian astronomer and physicist (1564-1642), who first utilized the telescope for astronomical observations. His discoveries lent credibility to the heliocentric model of Copernicus and made it easier for Newton to write his laws of gravity and motion. See Galileo's SparkNote biography.

Geologically active  -  Showing signs of volcanism and/or having a young surface, which is an index of internal forces currently active.

Gravitational energy  -  Energy associated with the action of the gravitational force.

Greenhouse effect  -  The visible light of the Sun passes through the atmosphere and heats the ground. The heated ground radiates that energy back into space in the form of infrared light, invisible to human eye. Greenhouse gases, like water vapor and carbon dioxide, trap infrared radiation, making it more difficult for the air and ground to cool off.

Heavenly spheres  -  The heavens were thought to be concentric spheres revolving around the Earth. Each sphere would move differently, hence accounting for the period motions of the celestial objects.

Hubble Space Telescope  -  A large orbiting telescope above the Earth's atmosphere. Its mirror measures 2.5 m.

Hydrocarbon  -  Compound of hydrogen and carbon.

Hydrostatic equilibrium  -  The equilibrium between the competing forces of gravity and the internal pressure on a large body like a moon, a planet or a star.

Infrared  -  Light of a wavelength longer than that of red light, and therefore invisible to human eye.

Johannes Kepler  -  German astronomer (1571-1630), his mentor was Tycho Brahe. He completed Brahe's observations of the position of Mars and was finally able to pin down its orbit. He realized that planets follow elliptical, rather than circular, orbits around the Sun. He was an ardent proponent of the heliocentric model. Read more on Kepler in relation to the Scientific Revolution

Mantle  -  The mantle is the portion of a planet beneath the crust and above the core. It is mostly made of silicates, though these minerals are more dense on average than those in the crust. The mantle exists mostly in a semi-solid viscous phase, allowing very slow convection currents.

Meteoric crater  -  Crater due to the impact of extraterrestrial rocks, ranging in size from a few centimeters to kilometers.

Meteorites  -  Extraterrestrial rocks that fall through a planet's atmosphere and reach its surface.

Molecular liquid  -  In a molecular liquid, molecules slide against each other without leaving much void space between them. In a metallic liquid the molecules of hydrogen break apart and the electrons of the hydrogen atoms are stripped from their nuclei. In a liquid metal, such as the one described, the electrons are free to wander around the liquid.

Nebula  -  The cloud of gas and dust associated with the formation of the primordial solar system.

Isaac Newton  -  British physicist (1643-1727) and mathematician, he invented modern mechanics and wrote the law of gravity, which, for most applications, is still used today in its original form. SparkNote on Newton.

Organic chemistry  -  Term used to indicate the chemistry of biological entities. Several organic molecules found outside Earth are not of biological origin, however.

Oxidation  -  The chemical reaction between metals and oxygen in which new compounds are formed.

Perihelion  -  The point closest to the Sun in a solar orbit.

Phase  -  Liquid, solid, and gas are three typical phases of matter.

Photodissociation  -  The breaking up of a molecule due to the action of light.

Planetesimal  -  The current theory of the formation of planets involves the successive collisions of smaller bodies, called planetesimals. Such collisions would have been quite common in the first 500 million years of our solar system's history.

Plate tectonics  -  The particular tectonic theory that explains the geology of Earth's crust. Earth's crust is divided in mobile sections, moving at the rate of a few centimeters per year. New crust is formed or destroyed at the boundaries between plates. In places such as the San Andreas Fault, two plates slide by each other moving in opposite directions. Continents are carried on top of the tectonic plates.

Prevailing winds  -  The direction from which winds most often blow, determined by the rotation of the planet.

Radiating  -  Warm object radiate energy, i.e. emit it, in the form of infrared light. Infrared radiation is a form of light, invisible to human eye.

O. Roemer  -  Olaus Roemer was the Danish astronomer who calculated an accurate value for the speed of light in 1676.

Seismic  -  Having to do with quakes.

Silicates  -  Minerals containing silicon oxide and metals, for example Fe₂SiO₄.

Sublimation  -  The volatilization of a solid directly into a gas, without apparently passing through the liquid faze.

Surface gravity  -  The acceleration due to gravity at planet's surface.

Tectonics  -  The study of large-scale features of the crust, including its formation and evolution.

Tilt  -  The angle that measures the direction of the axis as compared to the plane of the orbit.

Ultraviolet radiation  -  Light with a wavelength shorter than the violet, invisible to human eye.

Vents  -  Small openings in the crust--found, for instance, deep in Earth 's oceans--emitting hot gases rich in minerals.