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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
M
Magellanic Clouds
Two small, irregular galaxies found just outside our own Milky Way
galaxy. The Magellanic Clouds are visible in the skies of the southern
hemisphere.
Magnetic Field
A condition found in the region around a magnet or an electric current,
characterized by the existence of a detectable magnetic force at every
point in the region and by the existence of magnetic poles.
Magnetic Pole
Either of two limited regions in a magnet at which the magnet's field is
most intense.
Magnetosphere
The area around a planet most affected by its magnetic field. The
boundary of this field is set by the solar wind.
Magnitude
The degree of brightness of a star or other object in the sky according
to a scale on which the brightest star has a magnitude -1.4 and the
faintest visible star has magnitude 6. Sometimes referred to as apparent
magnitude. In this scale, each number is 2.5 times the brightness of the
previous number. Thus a star with a magnitude of 1 is 100 times brighter
than on with a visual magnitude of 6.
Main Belt
The area between Mars and Jupiter where most of the asteroids in our
solar system are found.
Major Planet
A name used to describe any planet that is considerably larger and more
massive than the Earth, and contains large quantities of hydrogen and
helium. Jupiter and Neptune are examples of major planets.
Mare
A term used to describe a large, circular plain. The word mare means
"sea". On the Moon, the maria are the smooth, dark-colored areas.
Mass
A measure of the total amount of material in a body, defined either by
the inertial properties of the body or by its gravitational influence on
other bodies.
Matter
A word used to describe anything that contains mass.
Meridian
An imaginary circle drawn through the North and South poles of the
celestial equator.
Metal
A term used by astronomers to describe all elements except hydrogen and
helium.
Meteor
A small particle of rock or dust that burns away in the Earth's
atmosphere. Meteors are also referred to as shooting stars.
Meteor Shower
An event where a large number of meteors enter the Earth's atmosphere
from the same direction in space at nearly the same time. Most meteor
showers take place when the Earth passes through the debris left behind
by a comet.
Meteorite
An object, usually a chunk or metal or rock, that survives entry through
the atmosphere to reach the Earth's surface. Meteors become meteorites
if they reach the ground.
Meteoroid
A small, rocky object in orbit around the Sun, smaller than an asteroid.
Millibar
A measure of atmospheric pressure equal to 1/1000 of a bar. Standard
sea-level pressure on Earth is about 1013 millibars.
Minor Planet
A term used since the 19th century to describe objects, such as
asteroids, that are in orbit around the Sun but are not planets or
comets. In 2006, the International Astronomical Union reclassified minor
planets as either dwarf planets or small solar system bodies.
Molecular Cloud
An interstellar cloud of molecular hydrogen containing trace amounts of
other molecules such as carbon monoxide and ammonia.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z