It should also be noted that the first edition of Hartungs publishes
co-ordinate positions in terms of the B1950.0 co-ordinate system
whereas the revised second edition by Malin and Frew uses J2000.0
co-ordinates.
This is an important practical point if relying on the co-ordinates in the
book.
As most readers will be aware, due to a complex series of wobbles
in the Earth's axis of rotation, the apparent position of stars and
other celestial objects will appear to shift over the years in a RA/Dec sense.
The Earth's poles precess like a wobble in a spinning top.
It comes as a surprise to many enthusiasts how significant the rate
of precession is.
It shifts at about 50.3 arc seconds a year - about the diameter of Jupiter.
So going between the B1950.0 Epoch to J2000.0 is about 2515 arc seconds,
which is equivalent to 41.9 arc minutes. Depending upon how extended the
target object is, that can make quite a difference in reliably
locating and identifying it through 'blind pointing', if one relies
on the B1950.0 RA/Dec co-ordinates alone.
Interested readers might like to know what is the significance of the 'B' and 'J' prefixes.
The 'B' stands for Besselian, named after Wilhelm Bessel, and the
"Besselian epochs" such as B1950.0 were based on the tropical year
(i.e. a solar year) in 1900, the reference being when the Sun's
longitude was 280 degrees toward the beginning of the year.
The 'J' stands for Julian and such epochs are based on a year
being exactly 365.25 days long.
An additional subtlety is the fact that the J2000.0 system uses a revised
reference frame, known as FK5, which in 1974 replaced the previous
system known as FK4. For example, FK5 takes into account the orbital
motion of stars around the Galactic center - a very tiny difference -
but nevertheless arose as knowledge of the Galaxy grew.
These days astronomers have adopted a revised reference frame known
as the International Celestial Reference Frame (ICRF), which is the
realization of the IAU International Celestial Reference System (ICRS).
This has superseded the J2000.0 co-ordinate system.
However, in practical terms, the difference between J2000.0 co-ordinates and
ICRF co-ordinates are minuscule, by design, and for amateur observers
they can be thought of as equivalent.
There will never be a J2050.0 epoch. The IAU resolved in the year 2000
that all co-ordinates from now on will be with respect the ICRF.
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