10
The celestial equator is a full 360º circle bisecting the celestial sphere into the northern celestial hemisphere
and the southern celestial hemisphere. Like the Earth's equator, it is the prime parallel of latitude and is
designated 0º.
The celestial parallels of latitude are called "coordinates of declination (Dec.)", and like the Earth's latitudes
they are named for their angular distances from the equator. These distances are measured in degrees,
minutes and seconds of arc. There are 60 minutes of arc in each degree, and 60 seconds of arc in each arc
minute. Declinations north of the celestial equator are "+" and declinations south are "-". The north pole is
+90 and the south pole is -90 .
The celestial meridians of longitude are called
"coordinates of right ascension (R.A.)", and like the
Earth's longitude meridians they extend from pole to
pole. There are 24 major RA. coordinates, evenly
spaced around the 360º equator, one every 15º. Like
the Earth's longitudes, R.A. coordinates are a measure
of time as well as angular distance. We speak of the
Earth's major longitude meridians as being separated
by one hour of time because the Earth rotates once
every 24 hours (one hour = 15°). The same principle
applies to celestial longitudes since the celestial sphere
appears to rotate once every 24 hours. Right
ascension hours are also divided into minutes of arc
and seconds of arc, with each hour having 60 minutes
of arc and each arc minute being divided into 60 arc
seconds.
Astronomers prefer the time designation for R.A. coordinates even though the coordinates denote locations
on the celestial sphere, because this makes it easier to tell how long it will be before a particular star will
cross a particular north-south line in the sky. So, R.A. coordinates are marked off in units of time eastward
from an arbitrary point on the celestial equator in the constellation Pisces. The prime R.A. coordinate
which passes through this point is designated "O hours O minutes O seconds". We call this reference point
the vernal equinox where it crosses the celestial equator. All other coordinates are names for the number of
hours, minutes and seconds that they lag behind this coordinate after it passes overhead moving westward.
Given the celestial coordinate system, it now becomes possible to find celestial objects by translating their
celestial coordinates using telescope pointing positions. For this you use setting circles for R.A. and Dec. to
find celestial coordinates for stellar objects which are given in star charts and reference books.
Polar Alignment
Polar alignment is the process by which the telescope’s axis of rotation is aligned (made parallel) with the
Earth’s axis of rotation (see figure 9). Once aligned, a telescope with a motor drive will track the stars as
they move across the sky. The result is that objects being observed through the telescope appear stationary
(i.e., they will not drift out of the field of view). If your telescope does not use a motor drive, all celestial
objects in the sky (day or night) will slowly drift out of the field. This motion is caused by the Earth’s
rotation. Even if you are not using a motor drive, polar alignment is still desirable since it will reduce the
number of corrections needed to follow an object and limit all corrections to one axis (R.A.). There are
several methods of polar alignment, all of which work on a similar principle, but are performed somewhat
differently.
For each hemisphere, there is a point in the sky around which all the other stars appear to rotate. These
points are called the celestial poles and are named for the hemisphere in which they reside. For example, in
the northern hemisphere all stars appear to move around the north celestial pole (see figure 8). When the
telescope’s polar axis is pointed at the celestial pole, it is parallel to the Earth’s rotational axis.
Figure 7
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