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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 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.
O -
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.
P -
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 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. 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.
S -
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.
T -
Terminator: The boundary line between the light and dark
portion of the moon or a planet.
U -
Universe: The totality of astronomical things, events, rela-
tions and energies capable of being described objectively.
V -
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.
W -
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.
Z -
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.