Kelvin (K) scale A system of measuring extreme temperatures used in astronomy: 0 Degree K = -273 degree Celsius, or absolute zero. Kinetic energy The energy a body has by virtue of its speed of motion. Kuiper Belt 3 band of asteroids, planetesimals, and other celestial debris beyond Neptune's orbit; comets may originate there. Land of the midnight Sun The part of the Earth's surface near the pole that is bathed in constant light. The Sun never sets in summer. Latitude Imaginary horizontal lines on Earth used together with longitudes to help locate the position of terrestrial objects. Light rays Energy emission of the wavelength and frequency that is detectable by the human eye. Light-year The distance light travels in 1 year in a vacuum. A measure used to describe distances in outer space; equal to 5.9 trillion miles (9.29 trillion km). Limb Edge of the Sun or any celestial disk. Local group  Cluster of more than 30 galaxies, including the Milky Way and the Andromeda galaxy. Extends to over 5 million light-years of space. Local Super cluster The cluster of clusters to which the Local Group (including the Milky Way Galaxy) belongs. Longitude Imaginary vertical lines on Earth used together with latitude to help locate the position of terrestrial objects. Lucifer A Latin name applied both to Venus as the Morning Star (because it rises before the Sun) and to the devil (as the most glorious of angels before his fall). Luminosity The measure of a star's energy output, similar to the measure of a light bulb. Lunation The lunar month, the period from one New Moon to the next, lasting about 29.5 days. Magellanic Clouds Two satellite galaxies of the Milky Way, dwarf irregulars less than 200,000 light-years away, visible in the Southern Hemisphere to the naked eye. Magnetic field The natural magnetic force of a planet or other celestial object. Magnetic poles The positively and negatively charged Focal points of a magnetic field; on Earth, the magnetic north and south poles, which are close to but not at, the celestial poles. Magnetosphere A large tear-shaped field of electromagnetic particles around Earth. Magnitude Apparent magnitude measures the brightness of a star as it appears in the sky. Absolute magnitude is the brightness a star would have if all stars were the same distance from Earth. Main sequence The term used for mature stars; stars that fall along the diagonal midsection on the Hertzsprung-Russell diagram from hottest and brightest to coolest and faintest. Mantle The hot, rocky matter that surrounds the Earth's core. Mariner 4 A space probe that in 1965 passed within 6,000 miles (9,600 km) of Mars and photographed the planet. Mariner 9 A probe that reached Mars in 1971, orbited the planet, and took many photographs.   Mars 1. The god of war in ancient Roman mythology. The planet Mars is named for him. 2. Fourth Planet from the Sun. 4,117 miles (6,787 km) in diameter. Mars is about 140,000,000 miles from the Sun, and orbits the Sun every 1.88 Earth years. It has a very slight atmosphere and appears to have no life forms, although evidence for water on Mars in the distant past seems quite strong. Mass A quantity, or amount, of matter. Mesosphere The third layer of atmosphere from Earth's surface. Meteor shower The display of lights that occurs as Earth travels through the path of a comet's orbit. Some of the best meteor showers are the Perseids, the Geminids, and the Leonids. Meteorites Meteors that make contact (collide) with the surface of a celestial object's surface. Meteoroid Body orbiting the Sun. They range from the size of a grain of sand to miles across. They become meteors or fireball when they enter the atmosphere. Minor planets The asteroids within the asteroid belt between Mars and Jupiter. Morning Star The name by which Venus has long been known when it appears in the morning sky before sunrise. In fact, any planet can be the morning star. Mercury Innermost planet, only 2,780 miles (4,880 km) in diameter and covered with craters. Mercury orbits the Sun in only 88 days. The side facing the Sun is extremely hot, while the side facing away is extremely cold. Mercury is the 2nd densest planet. Meteor A tiny asteroid or meteoroid that has entered the Earth's atmosphere. Also, the bright streak of light made as the meteoroid enters or moves through the atmosphere. Meteorite A meteoroid when it hits the Earth.  Meteoroid A lump of rock or metal drifting through space. Meteoroids can be as big as asteroids or as small as specks of dust. Milky Way Our spiral galaxy, containing at least 100 billion stars, including the Sun. Above average in size. Also, the band of light visible in the night sky which is our galaxy. Mimas Satellite of Saturn, 243 miles (391 km) in diameter; orbits the planet in only 23 hours. One of the craters on Mimas's surface is huge (a quarter of the satellite's own diameter). Miranda Satellite of Uranus, 300 miles (480 km) in diameter; orbits the planet in 1.4 days.  Missing mass Invisible matter believed to be present in the universe because of its gravitational effect. Amounts to approximately 10 times the mass of the stars and nebulae that are visible. Moon A large natural object that orbits around the Earth, 2,160 miles (3,476 km) in diameter. The moon orbits Earth in 27.3 days. The surface is covered with craters. The same face of the Moon always points towards the Earth. However, about 60% of the Moon's surface is visible from the Earth in total. Natural satellites Another name for the moons that orbit planets. Nebula A dense area, or cloud, of interstellar medium (gas and dust between stars). Neptune Usually, the eighth planet from the Sun, 31,000 miles (49,500 km) in diameter. It has a surface of methane gas and winds of over 1,200 miles/h (2,000 km/h). Neptune orbits the Sun in approximately 165 Earth years. Nereid Satellite of Neptune, about 190 miles (300 km) in diameter; takes 360 days to orbit the planet, in a very eccentric orbit. Nereid was almost certainly a passing body captured by Neptune's gravity. Neutrinos Subatomic particles that are emitted in large numbers during thermonuclear fusion in a star. A neutrino interacts so weakly with matter that it has a 50% chance of passing through a light-year of Lead! Neutrons Atomic particles with no electrical charge and a slight mass, found in all atomic nuclei, with the exception of hydrogen. Neutron star Solid atomic matter remaining after the center of a super giant star collapses in a supernova explosion. A teaspoon of a neutron star weighs about 1 billion tons! Nonbaryonic matter Theoretical matter not based on the building blocks of protons, electrons and neutrons. Nova The bright flare of a white dwarf in a binary star system when it accumulates enough matter from its neighboring star. Nuclear fusion The collision and combination of atoms to produce heavier ones, such as Hydrogen atoms fusing into Helium. Fusion reactions release tremendous amounts of energy according to the formula E = M x C x C , where M is the difference between the masses of the hydrogen atoms and the helium atom, and C is the speed of light. Nucleus This word has many meanings, including the center of a cell, galaxy, or atom, and the rocky core of a comet. Oberon Satellite of Uranus, 950 miles (1,524 km) in diameter; orbits in 13.5 days. It has an icy surface with many craters, but is also crossed by valleys several hundred miles long where the crust has split open. Objective lens The lens at the opening of an optical telescope that gathers light. Occultation Occurs when the Moon or a planet passes in front of a star, blocking it from view.  Occultation Occurs when the Moon or a planet passes in front of a star, blocking it from view.  Olympus Mons A huge, extinct volcano on Mars. Omega In cosmology, the term for the unknown, but critical amount of mass density that theoretically will determine the outcome of the universe. Omega Centauri One of over a hundred global star clusters that surround the center of the Milky Way galaxy. Probably formed at the same time as the galaxy. Omega Centauri contains at least a million stars and lies about 17,000 light-years away. Oort Cloud An outlying area of the solar system from which comets are thought to originate. Open clusters Loose groups of stars appearing in a galaxy's nucleus and spiral arms. Open universe model The theory that states that the universe will continue to expand forever. Orbit The path that one celestial object follows as it circles, or revolves, around another. Orbital plane The imaginary flat surface (plane) along which a planet, or other celestial body, orbits another. Orbital tilt The angle of a planet's orbit plane in reference to Earth's orbital plane; also known as inclination. Outer planets Planets whose orbits lie beyond the asteroid belt: Jupiter, Saturn, Uranus, Neptune, and Pluto. Oxygen The gas in Earth's atmosphere that makes human and animal life possible. Simple life forms changed carbon dioxide to oxygen and carbon compounds as life evolved on Earth. Ozone layer That part of our atmosphere that shields us from the Sun's dangerous ultraviolet rays.