Previous page  Page 21   Next page    Table of Contents 

clusters,  the Zwicky clusters,  or both. All three are possible, and a
check mark is placed in the menu beside the current selection.

   The Asteroids submenu also shows some extra options controlling
the labelling of asteroids:  asteroids labelled by number;
asteroids labelled by number if available, provisional designation
otherwise;  asteroids labelled by name, provisional designation
otherwise. By default,  asteroids are labelled with their number.
(Unnumbered asteroids are therefore also unlabelled asteroids,  and are
simply shown as cross-marks.)
                                           
   For the Planets there is a panel with two switches: "Full precision"  
and "Label by Name".  The first toggles between normal precision (the  
default) and full precision for positions.  Planet positions are
calculated using either the VSOP ("Variations Seculaires des Orbites
Planetaires") theory or the PS1996 theory,  both compiled at the Bureau
des Longitudes in Paris.  In each theory planetary positions are
calculated as the sum of a long series of trigonometric terms.  When
full precision is used,  all terms in the optimal theory are used, and
planetary positions are precise to roughly .01 arcsecond. Computing
them can be slow on older computers.  (If you have a more modern
computer,  you will probably not even notice the slowdown.)

   Note that for certain time spans and objects,  a still better method
may be used:  JPL ephemerides.  These are the current "standard" for
planetary ephemerides;  VSOP and PS1996 are based on the JPL ephemerides.
(The method used for a given planet or satellite will be shown in the
"more info" section for that object.)

   "Normal" precision  provides a precision of about an arcsecond over
the period 0 AD to 4000 AD. It is based on VSOP, but omits many of the
smaller terms.  It is the truncated version used in Jean Meeus'
_Astronomical Algorithms_ (Willmann-Bell, 1991).  For normal use
arcsecond precision  is more than sufficient,  but there are cases where
the additional level of accuracy in "full precision" is truly necessary.
It also allows Guide to claim higher accuracy than other software.  Those
wanting specific details of Guide's accuracy should look here:

http://www.projectpluto.com/accuracy.htm

7c:  Planet display             

   When Guide shows a planet or natural satellite,  and the field of
view is so large that the object would appear as only a few dozen
pixels,  that object is shown as a small cross-mark labelled with a
symbol.  You can change its color from the Data Shown dialog,  and
you can click the "Label planets by name" option in that dialog to
cause Guide to use names in place of symbols.

   The real capabilities in how Guide shows planets only become apparent
when you zoom in far enough for the object to show a disk.  For many
objects,  a "bitmapped surface" is provided.  Zoom in on,  for example,

 Previous page  Page 21   Next page    Table of Contents