Magnitude
A measure of the faintness of a star. Magnitude 6.0 is the faintest we can ordinarily see, and the average of the twenty brightest stars defines magnitude 1.0. Each magnitude step is a factor of 2.5 in brightness.
Meridian
An observer's meridian passes through his zenith at the celestial poles. It is his north-south line. Meteor A flash of light produced by a particle in the atmosphere moving so rapidly that it becomes luminous.
Meteorite
A rock that once looked like a meteor. Morning Sky That half of the celestial sphere containing the sun.