Earth

Inner planets  -  The four rocky planets closest to the Sun (Mercury, Venus, Earth, and Mars) are considered inner planets.

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

Photosynthesis  -  The chemical process used by plants to convert water and carbon dioxide into oxygen and sugars.

Plate tectonics  -  The Earth's crust is divided into mobile sections, called plates, which move at the rate of a few centimeters per year. Some of these sections move away from each other, others move into each other. The various interactions between the sections determine to a large degree the topographical structure of the world.

Seismic activity  -  A general term describing the occurrence of earthquakes.

Biosphere  -  The totality of the life forms on Earth.

Aristotle  -  Greek philosopher (384-322 BC) who also wrote on Physics and Astronomy. He was very influential on European scientific thought until the early 17th century. His Physics, which presupposed a universe that revolved around the Earth, was supplanted by the Newtonian theory in the seventeenth century.

Lunar eclipses  -  Lunar eclipses happen when the Earth intersects exactly between the Sun and the Moon, projecting its shadow on the latter.

North Star  -  The daily motion of the sky pivots around the North Star.

Latitude  -  The angle that marks the position of points in the North-South direction along a meridian, starting from the planet's equator.

Eratosthenes  -  Greek philosopher (276-195 BC) who first measured the radius of the Earth. He accomplished the feat by measuring the North-South distance between two locations in Egypt, Assuan and Alexandria, which were situated along the same parallel, and then calculating the percentage of that distance as a measure of the entire Earth.

Copernicus  -  Polish astronomer (1473-1543) who first theorized the heliocentric system in modern times. The heliocentric system is the model in which planets revolve around the Sun. Read more on Copernicus and the Scientific Revolution.

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 on Brahe and the Scientific Revolution.

Kepler  -  German astronomer (1571-1630), his mentor was Tycho. He completed Tycho'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. More on Kepler in relation to the Scientific Revolution.

Galileo  -  Italian astronomer and physicist (1564-1642), who first utilized the telescope for astronomical observations. An incredible experimental scientist, his discoveries did so much to support the heliocentric model of Copernicus that the Church, which for theological reasons supported the geocentric model, excommunicated him. Read the SparkNote on Galileo.

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

Atmospheric pressure  -  The force per unit area exerted by a gas on any given surface.

Density  -  Mass per unit volume.

Dissolved  -  Gas molecules can individually be incorporated in a liquid. For an example, think of carbonated water, in which carbon dioxide remains dissolved in water when kept under pressure, and is released as bubbles when the pressure diminished (i.e. you open the can).

Greenhouse  -  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.

Rifts  -  Regions of the Earth's crust where the crust is itself being created. Following the Earth's natural plate tectonics, the new crust slowly moves away from the fissure, in opposite directions.

Subduction zones  -  Regions of the Earth's crust where the crust is being destroyed. The crust of one plate is pushed under the other, and it gets melted and 'eaten' by the warmer mantle surrounding it.

Plates  -  The Earth's crust is divided into mobile sections, called tectonic plates, which move at the rate of a few centimeters per year.

Faults  -  A stretch of crust that is fractured and subject to seismic activity.

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.

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 convective currents.

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 (Ni) and iron (Fe), while the external portion is made of iron (Fe) and sulphur (S).

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

Continents  -  On Earth, continents float on the underlying crust, because they are made of slightly less dense rock.

Lake Tanganica  -  A large lake in Tanzania, Africa, situated along the African Rift.

Hot spots  -  Small regions of the crust where magma emerging from far down in the mantle create and fuel volcanoes.

Plumes  -  Magma corridors along which molten material rises from the mantle to the crust.

Ocean trenches  -  Oceanic regions where the oceans are deepest. These trenches come to exist at points where two tectonic plates meet and engage in subduction.

Himalayas  -  The highest mountain chain in the world, situated at the border of India, Nepal and China.

Vesuvius  -  Volcano in the vicinity of the modern city of Naples, Italy.

Mt. S. Helens  -  Volcano in the state of Washington.

Mt. Pinatubo  -  Volcano in the Philippines.

Monserrat Is.  -  Island volcano in the Caribbean.

Erosion  -  The wearing of rocks, due to the action of water and wind.

Meteoric craters  -  Craters origination in the impact of extraterrestrial rocks, ranging in size from a few centimeters to many kilometers.

Asteroids  -  Small planetoids, distributed widely throughout the solar system. In our solar system, the largest concentration of asteroids can be found in a belt between Mars and Jupiter.

Life extinctions  -  Short periods in Earth's prehistory when many species of plants and animals went extinct.

Sedimentary layer  -  A layer of rock that was once deposited in the form of sand or silt and later solidified.

Gravimetric  -  A gravimetric measurement finds the minute differences in gravitational attraction that various geological features exert on the measuring instrument. Such measurements helps in determining the distribution of mass and the density of rocks in the vicinity of the instrument.

Seismic waves  -  A earthquake is created by waves of compression and decompression traveling through rocks. These are called seismic waves.

Radioactive decay  -  Nuclei of atoms decay when they are unstable. The decay splits the nuclei or transforms their constituent proton and neutrons into each other, liberating energy in the process.