Celestron skyexpedition 60, Instruction Manual, Page: 17 / 26

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 1700s 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
The point in the Northern hemisphere around
Pole
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 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 parallax of one
second of arc. It is equal to 3.26 light–years, 206,265
astronomical units, or 30,800,000,000,000 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 The angular distance of a celestial
ascension : (RA) 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
telescope 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, relations
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 coincided 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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