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
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 dif- fuse 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 hemi- sphere 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 sud- denly becomes explosively bright at the end of its life cycle.
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Open Cluster: One of the groupings of stars that are con- centrated 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.
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Parallax: Parallax is the difference in the apparent posi- tion 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 length 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 paral- lax of one second of arc. It is equal to 3.26
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.
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Reflector: A telescope in which the light is collected by means of a mirror.
Resolution: The minimum detectable angle an optical system can detect. Because of diffraction, there is a limit to the minimum angle, resolution. The larger the aperture, the better the resolution.
Right Ascension: (RA): The angular distance of a celes- tial object measured in hours, minutes, and seconds along the Celestial Equator eastward from the Vernal Equinox.
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Sidereal Rate: This is the angular speed at which the Earth is rotating. Telescope tracking motors drive the tele- scope at this rate. The rate is 15 arc seconds per second or 15 degrees per hour.
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Terminator: The boundary line between the light and dark portion of the moon or a planet.
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Universe: The totality of astronomical things, events, rela- tions and energies capable of being described objectively.
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Variable Star: A star whose brightness varies over time due to either inherent properties of the star or something eclipsing or obscuring the brightness of the star.
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Waning Moon: The period of the moon’s cycle between full and new, when its illuminated portion is decreasing.
Waxing Moon: The period of the moon’s cycle between new and full, when its illuminated portion is increasing.
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Zenith: The point on the Celestial Sphere directly above the observer.
Zodiac: The zodiac is the portion of the Celestial Sphere that lies within 8 degrees on either side of the Ecliptic. The apparent paths of the Sun, the Moon, and the planets, with the exception of some portions of the path of Pluto, lie within this band. Twelve divisions, or signs, each 30 degrees in width, comprise the zodiac. These signs coin- cided with the zodiacal constellations about 2,000 years ago. Because of the Precession of the Earth’s axis, the Vernal Equinox has moved westward by about 30 degrees since that time; the signs have moved with it and thus no longer coincide with the constellations.
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