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Celestial Sphere An imaginary sphere surrounding the Earth, concentric with the Earth's center. Collimation The act of putting a telescope's optics into perfect alignment. D - Declination (DEC) The angular distance of a celestial body north or south of the celestial equator. It may be said to correspond to latitude on the surface of the Earth. E - Ecliptic The projection of the Earth's orbit on to the celestial sphere. It may also be defined as "the apparent yearly path of the Sun against the stars". Equatorial mount A telescope mounting in which the instrument is set upon an axis which is parallel to the axis of the Earth; the angle of the axis must be equal to the observer's latitude. F - Focal length The distance between a lens (or mirror) and the point at which the image of an object at infinity is brought to focus. The focal lenSLTh divided by the aperture of the mirror or lens is termed the focal ratio. 35 G GoTo Term used to refer to a computerized telescope or to the act of slewing (moving) a computerized telescope J - Jovian Planets Any of the four gas giant planets that are at a greater distance form the sun than the terrestrial planets. K - Kuiper Belt A region beyond the orbit of Neptune extending to about 1000 AU which is a source of many short period comets. L - Light-Year (ly) A light-year is the distance light traverses in a vacuum in one year at the speed of 299,792 km/ sec. With 31,557,600 seconds in a year, the light-year equals a distance of 9.46 X 1 trillion km (5.87 X 1 trillion mi). M - Magnitude Magnitude is a measure of the brightness of a celestial body. The brightest stars are assigned magnitude 1 and those increasingly fainter from 2 down to magnitude 5. The faintest star that can be seen without a telescope is about magnitude 6. Each magnitude step corresponds to a ratio of 2.5 in brightness. Thus a star of magnitude 1 is 2.5 times brighter than a star of magnitude 2, and 100 times brighter than a magnitude 5 star. The brightest star, Sirius, has an apparent magnitude of -1.6, the full moon is -12.7, and the Sun's brightness, expressed on a magnitude scale, is -26.78. The zero point of the apparent magnitude scale is arbitrary. Meridian A reference line in the sky that starts at the North celestial pole and ends at the South celestial pole and passes through the zenith. If you are facing South, the meridian starts from your Southern horizon and passes directly overhead to the North celestial pole. Messier A French astronomer in the late 1700’s who was primarily looking for comets. Comets are hazy diffuse objects and so Messier cataloged objects that were not comets to help his search. This catalog became the Messier Catalog, M1 through M110. N - Nebula Interstellar cloud of gas and dust. Also refers to any celestial object that has a cloudy appearance. North Celestial Pole The point in the Northern hemisphere around which all the stars appear to rotate. This is caused by the fact that the Earth is rotating on an axis that passes through the North and South celestial poles. The star Polaris lies less than a degree from this point and is therefore referred to as the "Pole Star". Nova Although Latin for "new" it denotes a star that suddenly becomes explosively bright at the end of its life cycle. O - Open Cluster One of the groupings of stars that are concentrated along the plane of the Milky Way. Most have an asymmetrical appearance and are loosely assembled. They contain from a dozen to many hundreds of stars. P - Parallax Parallax is the difference in the apparent position of an object against a background when viewed by an observer from two different locations. These positions and the actual position of the object form a triangle from which the apex angle (the parallax) and the distance of the object can be determined if the lenSLTh of the baseline between the observing positions is known and the angular direction of the object from each position at the ends of the baseline has been measured. The traditional method in astronomy of determining the distance to a celestial object is to measure its parallax. Parfocal Refers to a group of eyepieces that all require the same distance from the focal plane of the telescope to be in focus. This means when you focus one parfocal eyepiece all the other parfocal eyepieces, in a particular line of eyepieces, will be in focus. Parsec The distance at which a star would show parallax of one second of arc. It is equal to 3.26 light-years, 206,265 astronomical units, or 30,8000,000,000,000 km. (Apart from the Sun, no star lies within one parsec of us.) Point Source An object which cannot be resolved into an image because it to too far away or too small is considered a point source. A planet is far away but it can be resolved as a disk. Most stars cannot be resolved as disks, they are too far away. R - Reflector A telescope in which the light is collected by means of a mirror. Resolution The minimum detectable angle an optical system can detect. ...

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