Sun, Moon, and Earth - Key terms
ABSOLUTE ZERO:
The temperature at which all molecular motion virtually ceases.
ASTRONOMICAL UNIT (AU):
A figure equal to the average distance from Earth's center to the center of the Sun. TheSI figure for an AU, adopted in 1996, is equal to 1.49597870691 × 10 8 km, or approximately 92,955,807 mi.
ATMOSPHERE:
In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%.
ATOM:
The smallest particle of an element, consisting of protons, neutrons, and electrons. An atom can exist either alone or in combination with other atoms in a molecule.
ATOMIC MASS UNIT:
An SI unit (abbreviated amu), equal to 1.66 × 10 −24 g, for measuring the mass of atoms.
ATOMIC NUMBER:
The number of protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table in order of atomic number.
AVERAGE ATOMIC MASS:
A figure used by chemists to specify the mass—in atomic mass units—of the average atom in a large sample.
BIOSPHERE:
A combination of all living things on Earth—plants, mammals, birds, reptiles, amphibians, aquatic life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed. Typically, after decomposing, a formerly living organism becomes part of the geosphere.
CORE:
The center of Earth, which appears to be of molten iron. For terrestrial planets in general, core refers to the center, which in most cases is probably molten metal of some kind.
COSMOLOGY:
The study of the origin, structure, and evolution of the universe.
COSMOS:
The universe.
ELECTROMAGNETIC ENERGY:
A form of energy with electric and magnetic components, which travels in waves.
ELECTROMAGNETIC SPECTRUM:
The complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energylevels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are long-wave and short-wave radio; microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays.
ELECTRON:
A negatively charged particle in an atom, which spins around the nucleus.
ELEMENT:
A substance made up of only one kind of atom. Unlike compounds, elements cannot be broken chemically into other substances.
GEOSPHERE:
The upper part of Earth's continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.
HYDROSPHERE:
The entirety of Earth's water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.
INERTIA:
The tendency of an object in motion to remain in motion and of an object at rest to remain at rest.
ION:
An atom that has lost or gained one or more electrons and thus has a net electric charge.
ISOTOPES:
Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference ofmass. An isotope may be either stable or radioactive.
JOVIAN PLANETS:
The planets between Mars (the last terrestrial planet) and Pluto, all of which are large, low indensity, and composed primarily of gases.
KELVIN SCALE:
Established by William Thomson, Baron Kelvin (1824-1907), the Kelvin scale measures temperature in relation to absolute zero, or 0K.(Note that units in the Kelvin system, known as Kelvins, do not include the word or symbol for "degree.") The Kelvin scale, which is the system usually favored by scientists, is directly related to the Celsius scale; hence Celsius temperatures can be converted to Kelvin by adding 273.15.
LIGHT-YEAR:
A unit of distance used by astronomers for measuring the extremely large expanses of space. Equal to the distance light travels in a year, a light-year is 9.460528405 × 10 12 km, or approximately 5.88 trillion mi.
LITHOSPHERE:
The upper layer of Earth's interior, including the crust and the brittle portion at the top of the mantle.
MANTLE:
The layer, approximately 1,429 mi. (2,300 km) thick, between Earth's crust and its core. In reference to the other terrestrial planets, mantle simply means the area of dense rock between the crust and core.
MASS:
A measure of inertia, indicating the resistance of an object to a change in itsmotion. (By contrast, weight—which people tend to think of as analogous to mass—is a measure of gravitational force, or mass multiplied by the acceleration due to gravity.)
NEUTRON:
A subatomic particle that has no electric charge. Neutrons are found at the nucleus of an atom, alongside protons.
NUCLEAR FUSION:
A nuclear reaction that involves the joining of atomicnuclei.
NUCLEUS:
The center of an atom, a region where protons and neutrons are located and around which electrons spin.
ORGANIC:
At one time, chemists used the term "organic" only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of calcium carbonate (lime-stone) and oxides, such as carbon dioxide.
PERIODIC TABLE OF ELEMENTS:
A chart that shows the elements arranged in order of atomic number, along with chemical symbol and the average atomic mass for that particular element.
PHOTON:
A particle of electromagnetic radiation carrying a specific amount of energy.
PLANETARY SCIENCE:
The branch of the earth sciences, sometimes known as planetology or planetary studies, that focuses on the study of other planetary bodies. This discipline, or set of disciplines, is concerned with the geologic, geophysical, and geochemical properties of other planets but also draws on aspects of astronomy, such as cosmology.
PROTON:
A positively charged particle in an atom.
RADIATION:
The transfer of energy by means of electromagnetic waves, which require no physical medium (for example, water or air) for the transfer. Earth receives the Sun's energy, via the electromagnetic spectrum, by means of radiation.
RADIOACTIVITY:
A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei); beta particles (either electrons or sub atomic particles called positrons); or gamma rays, which occupy the highest energy level in the electromagnetic spectrum.
REFRACTION:
The bending of light as it passes at an angle from one transparent material into a second transparent material. Refraction accounts for the fact that objects under water appear to have a different size and location than they have in air.
SI:
An abbreviation of the French term Système International d'Unités, or "International System of Units." Based on the metric system, SI is the system of measurement units in use by scientists worldwide.
TERRESTRIAL PLANETS:
The four inner planets of the solar system: Mercury, Venus, Earth, and Mars. They are all small, rocky, dense, have relatively small amounts of gaseous elements, and are composed primarily of metals and silicates. Compare with Jovian planets.