Orion 9536 Manual page 24

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 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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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 angu-
lar direction of the object from each position at the ends
of the baseline has been measured. The traditional meth-
od 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 eye-
piece 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 consid-
ered 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.