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Magnitude - The brightness of celestial objects is measured using the magnitude scale. The magnitude system originates from ancient Greece when stars were placed into 6 categories based on brightness. First magnitude stars were brightest, and sixth magnitude stars the faintest visible with the unaided eye. Of course, once fainter stars were discovered with the invention of the telescope, the magnitude scale had to be extended to higher numbers (fainter objects). Once the scale was standardized in the 19th century, some stars like Sirius ended up with negative magnitudes. The full moon has a magnitude of -12 and the sun is -26. Each magnitude corresponds to a difference of 2.5 in brightness. Thus, a magnitude 1 star is 2.5 times brighter than a magnitude 2 star, and 6.25 times brighter than a magnitude 3 star, etc. The faintest objects visible in a typical backyard telescope are around magnitude 13-15, or several thousand times fainter than the dimmest stars visible to the unaided eye.

Meridian - This is the line that divides the east and west halves of the sky. It runs from due north to due south and through the zenith. When an object is located on the meridian it is at the highest point it can be observed from a given location. An object east of the meridian is still rising while an object to the west is setting.

Messier Objects - In the late 18th century, French astronomer Charles Messier began cataloging deep-sky objects. The objects in his catalog are some of the best and brightest deep-sky targets, making them popular for all observers, but especially beginners. There are 109 Messier objects (M1 through M110, with M102 being a duplicate observation of M101). The list includes all types of deep-sky objects: open star clusters, globular clusters, galaxies, and nebulae.

Meteor - When a rock or piece of dust from space enters Earth's atmosphere, friction with the air causes it to heat up and begin to glow. A piece of space debris in the atmosphere is called as meteor. If the object is large enough, it may fall all the way to the ground, at which point the remnant rock is called a meteorite. While still in space, before entering the atmosphere, the object is called a meteoroid. Meteors are most often debris from comets, but may also be from asteroids or even pieces of the surface of other planets, blasted into space by large impacts. Meteor showers occur when Earth passes through debris left by a passing comet.

Micron - A unit of measure, equal to one millionth of a meter. This is the standard unit of measure for pixels in a CCD chip (which are usually 3-24 microns in size). This is also a unit of measure used for wavelengths of light. A micron is equal to a thousand nanometers, another common unit of measure. Microns are usually used in reference to infrared light, while nanometers are used for visible light.

Mirror Cell - This is the assembly that holds a mirror in place in a telescope. In a Newtonian telescope, for example, the mirror cell is usually made of metal and includes adjustment screws necessary for collimating (aligning) the optics.

Mirror Shift - When focusing a Schmidt-Cassegrain telescope, the image may shift slightly due to the primary mirror tilting slightly as it is moved to focus. This is also known as image shift. Mirror shift can also occur when pointing a telescope (especially a large one) from one side of the sky to another. The weight of the mirror may cause it to tilt slightly affecting the pointing accuracy of a computerized telescope and (in worst case scenarios) affecting collimation. Fortunately, this is rarely a problem.

Multi-Coated Optics - Glass does not transmit all of the light that strikes it. Coatings are used to increase the light transmission in a refractive optical system. Multi-coated optics have multiple layers of coatings on the glass surface. An optical system such as a binocular which has multi-coated optics has multiple coatings on some surfaces. This differs from fully-multi-coated optics which have multiple coatings on every air-to-glass surface.