Terms and Definitions

macula: A dark spot.
magma: Molten rock within the crust of a planet that is capable of intrusion into adjacent crustal rocks or extrusion onto the surface. Igneous rocks are derived from magma through solidification and related processes or through eruption of the magma at the surface.
magnetic field: A region of space near a magnetized body where magnetic forces can be detected.
magnetograph: A special telescope which analyzes the color and polarization of sunlight in order to measure the magnetic field of the Sun.
magnetopause: The boundary of the magnetosphere, lying inside the bow shock.
magnetosphere: The region of space in which a planet's magnetic field dominates that of the solar wind.
magnetotail: The portion of a planetary magnetosphere which is pushed in the direction of the solar wind.
magnitude: The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest visible star has magnitude 6, with the scale rule such that a decrease of one unit represents an increase in apparent brightness by a factor of 2.512; also called apparent magnitude.
mare: Latin word for "sea." Galileo thought the dark featureless areas on the Moon were bodies of water, even though the Moon is essentially devoid of liquid water. The term is still applied to the basalt-filled impact basins common on the face of the Moon visible from Earth.
Marius, Simon 1573-1624: (Aka Mayr) German astronomer. Marius gave Jupiter's "Galilean" moons their names. He and Galileo both claimed to have discovered them in 1610 and likely did so independently. They became involved in a dispute over priority. Marius was also the first to observe the Andromeda Nebula with a telescope and one of the first to observe sunspots.
mensa: A mesa, flat-topped elevation.
mesa: A broad, flattop, erosional hill or mountain, commonly bounded by steep slopes.
meteor: The luminous phenomenon seen when a meteoroid enters the atmosphere, commonly known as a shooting star.
meteorite: A part of a meteoroid that survives through the Earth's atmosphere.
meteoroid: A small rock in space.
millibar: This is 1/1000 of a bar; the standard sea-level pressure is about 1,013 millibars.
minor planets: Another term used for asteroids.
mons: A mountain.