Hi Dave,
There are many co-ordinate systems used in astronomy, but as an amateur,
about the only one you will need to be familar with is that based on Right
Ascension (RA) and Declination (Dec) referenced with respect to what is
known as the J2000.0 Epoch.
Nearly all magazines, published catalogs, star charts and planetarium software
packages express celestial positions with respct this co-ordinate system.
An analogous co-ordinate system used to find your location on earth is
longitude and latitude. Go and grab a globe or an atlas and familiarize
yourself with it.
RA and Dec are like lines of longitude and latitude marked out on the
celestial sphere. In fact, if you were to stand at the North Pole, you would
be at latitude +90 and a Dec of +90 would be in the sky at the zenith
(i,e, directly above you). Same at the South Pole except it would be a latitude
and Dec of -90.
Whereas lines of longitude are fixed at any location on Earth, lines
of RA are (more or less) 'fixed' to the heavens and since the Earth rotates
around its own axis and also orbits the Sun, if you could see lines of RA they
would appear to move slowly across the sky.
An excellent printed reference is Hartung's Astronomical Objects of
Southern Telescopes, which I believe is out of print but you may find in
a library. See page 11 of "Coordinate systems". Undoubtely a Google
search will help you find some other references.
Just as one can locate a place on Earth by knowing its latitude and longitude,
one can locate an object by knowing its RA and Dec. In other words, for
most practical purposes, the RA and Dec position of a celestial object is fixed
which means its co-ordinates are fixed despite the fact the object appears
to move across the sky.
In reality, due to wobble's in the Earth axis, its rotation around the Sun,
the finite speed of light, the refraction of light through the atmosphere and
the fact that many objects such as stars are actually moving through our galaxy,
the RA and Dec co-ordinates of any object are not in fact fixed but are
constantly changing, albeit slowly. For that reason, whenever a RA and Dec
position is quoted it should also be cited with respect at one moment in
time those co-ordinates were good for. At the present time, most objects
are quoted with respect as to where they were at noon in Greenwich on Jan
1, 2000. From then until 2008, most objects have only shifted a tiny fraction
of a degree from these co-ordinates.
Objects within our own solar system are influenced by gravitational
interaction between the Sun, planets and other bodies. Most, as you know are
in orbit. Since these objects are physically closer to us compared to distant
stars and galaxies, their relative motions appear much faster for an
observer standing on the Earth. Therefore the RA and Dec co-ordinates of
objects such as planets are changing at a much faster rate, to the extent that
from a practical standpoint, one needs to look up a new RA and Dec co-ordinate
for them for each night you observe.
Hope this is a helpful introduction. We happen to manufacture a computer that
attaches to a telescope and helps you locate and identify objects. To initialize
the device, one needs to simply align it on any two objects (for example bright stars
or planets) you know. After that, it "knows" where it is pointing and it in fact
removes the complexity of having the operator know the RA and Dec of an object
and how to find it. It takes into account the rotation of the Earth and its orbit around
the Sun and even the fact that the Earth's axis wobbles and other subtle effects.
Just as satnav systems alleviate the need for many of us to be expert navigators,
devices such as the one we manufacture do the same for navigating the night sky.
Thus this is a topic that we are familar with.
Best Regards
Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au
