time dialog 
   The Time Dialog box provides all control needed to reset
the time,  time zone,  and calendars used by Guide.  The
date and time currently set appear at the top of the box.
Buttons above and below the time let you make changes of
tens of days, single days,  thousands of years, centuries,
and so on,  all the way down to changes of one second.
   You can also click on the day,  year,  hour,  minute,  or
second to change these values directly.
   The calendar lets you reset the day of the month (or move
to the preceding or following month) in one mouse click.
   Clicking on the "Time Zone" button brings up the time
zone dialog.  Of course,  this button also tells you to
what the zone is presently set.
   Clicking on the "JD" button will prompt you to enter
the desired Julian Day.  Again,  the button also serves
the purpose of telling you what JD is currently set.
   The next button toggles between the Julian and
Gregorian calendars.
   Clicking on "Current Time" tells Guide to read your
computer's built-in clock and reset the time accordingly.
   The array of twelve buttons at the bottom allows one to
reset the month in one mouse click.
   You can reach this dialog box by hitting Alt-T;  or by
clicking on the time shown in the legend;  or by using the
menu items "Setting" and "Time".

time zone dialog 

   The time zone dialog box allows one to reset the
default time zone.
   Time zones are listed explicitly,  using the common
three letter abbreviations,  for the four US time zones
and their daylight savings time (Summer Time) versions,
and Greenwich Time,  or Universal Time,  is also
listed. If you want to use a different local time,
clicking on the  "Enter time diff" option will let you
enter a difference,  in hours,  between Greenwich and
local time.  For example, EST would be entered as -5;
Tokyo time as 9;  and so on.

user object dialog 

   The user object dialog provides a way to maintain your
own list of objects.  It also allows you to load this list
onto a Sky Commander.
   To use this option,  find the object (or point in the sky)
of interest and center on it.  Bring up this dialog,  and
click on "Add Object".  You'll be prompted for the object
name;  enter this,  and the object will be added to the list.
   Once you have added objects,  controls will be available
to delete and "go to" (recenter the chart on) them.
   You can access this option with the F5 hotkey.

   Hitting [,  or clicking on the "Faster" button in the
animation dialog, lets you increase the rate of animation.

   Hitting ],  or clicking on the "Slower" button in the
animation dialog, lets you decrease the rate of animation.

a sin i 
   The spectrum of a binary star will show each line
split,  representing the different stars orbiting each
other.  The amount of split,  plus the time it takes the
stars to orbit one another,  tells you the (minimum)
distance between them.  This quantity is called a sin i.
'a' is the distance between the two stars,  measured in
astronomical units.  The 'sin i' is the sine of the
angle between the orbit of the stars and a line from the
stars to the earth;  it basically means that a sin i is
the minimum possible distance between the two stars,  and
we don't know the maximum.

   The USNO (US Naval Observatory) has created an enormous
catalogs of stars,  showing almost 500 million objects.  The
first version,  A1.0,  consumes a set of ten CD-ROMs;  the
updated version,  A2.0,  consumes eleven CD-ROMs,  and is the
current record-holder for most detailed dataset.  The two have
strong similarities,  and are referred to as Ax.0 in situations
where their differences are unimportant.

    Getting a set of the CD-ROMs for Ax.0 is not easy; the USNO
didn't make many,  and reserves them for especially deserving
applicants.  You can,  however,  download Ax.0 data for a small
region through


   (Note that this URL may change!)  You can view downloaded Ax.0
data in Guide.  If you have the CDs,  you can extract data from
them and view it in Guide through the Get Ax.0 data option
in the Extras menu.
   The USNO does make single-CD "Selected Ax.0",  or SAx.0,
versions available freely;  check


   for current availability information.  The SAx.0 data can also
be viewed in Guide,  in the same manner as Ax.0 data.  You can
also create a list of Ax.0 stars with the Create Star List
option,  and can click on them for "more info",  just like the
"usual" GSC and Hipparcos stars.

American Association of Variable Star Observers 
   The AAVSO (American Association of Variable Star
Observers) is an organization gathering and distributing
data regarding variable stars.  Observing variables is a
fairly common amateur activity,  and the data gathered can
be extremely useful;  there are a large number of
variables whose behavior is still not well understood. It
is an area of astronomy where you can produce useful data
without a large telescope and an immense grant;  in fact,
observations made with anything from binoculars on up can
be used.
   Those interested in further information should contact:

American Association of Variable Star Observers
25 Birch St.
Cambridge MA 02138

Internet:  aavso@aavso.org
Tel. (617) 354 0484       FAX (617) 354 0665

   The Abell catalog of clusters of galaxies is one of
two such catalogs available in this program.  (The other
is the Zwicky catalog.)  It contains 5,250 objects
covering the entire sky,  including the southern half.
   You can find an object by its Abell number by using the
Go to Abell Cluster option in the Go to Galaxy menu in the
Go To menu.
   This shouldn't be confused with the Abell catalog of
planetary nebulae;  that catalog is an entirely separate

   The earth's motion introduces a very slight distortion
in the apparent positions of celestial objects.  Readers
of some science fiction stories will recognize the effect;
in a spaceship moving at a large percentage of the speed
of light,  the stars would seem to "creep ahead" and
cluster around the forward direction.
   The Earth moves at a much smaller speed,  of course,
but stars and planets do cluster very slightly toward the
direction of our motion around the Sun.  This effect is
called aberration;  it never amounts to more than about
20 arcseconds,  enough to be noticeable with precise
instruments.  Along with nutation,  the effect is
included in converting a mean position at current epoch
into an apparent position at current epoch.

Above is the computed position for J1991.25 
   The Hipparcos satellite gathered data for several
years,  with 2 April 1991 being close to the center of
the observations.  So the catalog therefore gives
positions for the date J1991.25,  also known as 13:30
UT on 2 Apr 1991. This is the standard epoch of
observation for the catalog.  The catalog data is most
precise for this date,  and becomes gradually less
precise for dates in the past and future.
COORDINATE SYSTEM.  Essentially,  the Hipparcos and
Tycho catalogs list the J2000 position for stars as
they appeared on 2 Apr 1991,  at 13:30 UT.

absolute magnitude 
   This term has two different meanings:  one for stars,
one for asteroids.
   The absolute magnitude of a star is the magnitude it
would have if it were ten parsecs away from us. This
measurement lets us see bright objects and dim objects as
corrected for their different distances. While it is often
a figure of great interest,  it can be difficult to
   The absolute magnitude of an asteroid is the magnitude it
would have if it were one AU from the Sun,  as seen by an
observer on the sun.  It is usually given the symbol 'H'.
It is used along with the slope parameter to compute the
asteroid magnitude for any position and distance,  and also
allows Guide to compute an assumed asteroid diameter.

Acceleration in RA 
Acceleration in dec 
   The Hipparcos satellite examined a large number of binary
stars in an effort to analyze their orbits.  In many cases,  this
was unsuccessful;  the satellite could tell that the stars were
not moving in straight lines (and were therefore presumably
pulling on one another),  but the data wasn't precise enough
to compute an orbit.
   In such cases,  the only really useful data that was derived
was an acceleration in RA and an acceleration in dec,  usually
given in units of milliarcseconds per year per year.  For example,
an acceleration of -13 mas/yr/yr means that,  each year,  the
proper motion of that star decreases by -13 milliarcseconds/year
along that axis.

accretion disk 
   Many stars and quasars have accretion disks.  These
form when the object has lots of dust and gas orbiting it.
As the dust particles and gas molecules collide,  they
lose energy to one another,  so they wind up in lower
orbits.  Eventually,  after a fair number of collisions,
the matter hits the star itself.  This entire process
turns the kinetic energy of the gas and dust into heat
energy;  in other words,  the disk is hot,  and disks can
sometimes be spotted by the infrared energy they emit.
   The disk shape results from the fact that the matter
falling in will almost never head straight for the object
at the center.  One direction of orbiting will be favored
by most of the matter,  and anything going the wrong way
will get hit by gas and dust until it does go the right
   Often the energy poured on the central object will
raise jets,  which will flow out from the object at right
angles to the disk.

ACT catalog 
   The ACT catalog was produced by the USNO,  and combines data
from the Tycho and Astrographic (AC) catalogs to get very precise
proper motions.  One problem with the Tycho catalog is that the
data was collected over a short time (the satellite was only used
for a few years),  which leads to very rough proper motion data.
The USNO fixed this by using Tycho positions combined with AC
positions;  the AC positions all date from around 1900,  so they
provide a long baseline for computing proper motions.  Guide uses
Hipparcos data when possible,  but that only covers about 118,000
stars.  For the remaining stars,  Guide uses Tycho positions
combined with ACT proper motions wherever it can, for the greatest
possible accuracy.

Add a Trail 
   Use the Add a Trail option to add a planetary (or asteroid
or comet or artificial satellite) trail to the display.
   To use this,  you should first set the current time,
in the Time dialog,  to that for the start of the trail.
Right-click on the object for which you want a trail.  (This
option,  and the somewhat similar Make Ephemeris option,  won't
work if you haven't clicked on a solar system object.)
   Finally,  when you click on "Add a Trail",  you'll
be asked for the desired number of steps and the step size.
If you've set the step size to 2 days/step,  for example,
and would like to cover a 90-day period,  you would enter 45
at this point.  Guide will calculate the position of the
object for 90 days at 2-day intervals,  and show the
resulting trail.
   A button is also provided to set the color for the trail,
and you can select the frequency at which index marks are added
to the trail.

Add DSS image 
   If you have downloaded a DSS (Digital Sky Survey) image from the
Internet,  you can use the Add DSS image to view it in Guide,  just
as if it were an image from the RealSky CDs.
   To do this,  click on the Add DSS image option.  In Windows,  this
will bring up a file dialog;  in DOS,  you'll be asked to type in the
file name.  Do so,  and Guide will analyze the FITS file header and
will add it to the list of images to be drawn.
   When you click on "Clear RealSky Images",  it will be dropped from
the list.  Of course,  you can then just click on Add DSS Image to
restore it to the list.

Add MPC comets/asteroids 
   Gareth Williams,  at the Minor Planet Center,  has very kindly
provided orbital elements for comets and asteroids in the native
format of Guide at the MPC's Web site.  You can get data for
currently visible comets, Critical List minor planets, and distant
minor planets (for example, Kuiper belt and Centaur-type objects.
   You can download these files from


   in either ASCII text or HTML format (Guide can use both,  so
it doesn't matter which you use.)  Once you have downloaded the
data you'd like to use,  click on Add MPC comets/asteroids in the
Extras menu.  Guide will ask for the name of the file you've
downloaded;  provide it,  and Guide will add new objects and update
old objects using that data.
   You can also access this option through the Ctrl-F12 hotkey.
   Also,  if you subscribe to the Minor Planet Electronic Circulars
(MPECs),  you can specify a file containing one or more Circulars.
Guide will comb through it for orbital data,  and add the orbits to
its database.

   The Aitken Double Star,  or ADS,  catalog,  lists most prominent
double,  or binary,  stars.  The catalog includes information about
the position of the stars relative to each other,  their magnitudes,
and their angular separation.  The WDS catalog often contains this

   The AGK (Astronomisches Gesellschaft Katalog) is an older catalog
covering stars in the northern half of the sky.  AGK numbers are given
for reference purposes.

   An object's albedo is a measurement of how much light
it reflects.  An object with a high albedo reflects most
of the light hitting it;  Venus,  for example,  has
clouds that reflect 70% of the sunlight that hits it,
which is one reason it's so bright.  An albedo of 0 means
no light is reflected (totally black);  an albedo of 1
means all light is reflected (perfectly polished).
   Most asteroids have low albedos,  around 10% or so.
437 Rhodia has the highest measured asteroid albedo;  it
56% of the light hitting it.  From up close,  it would
probably look like chalk.  Most asteroids are much more
dull than this.

Alpha CVn 
   The Alpha CVn class of variable stars show small
changes in brightness caused by the star's rotation.
One side of the star is slightly different from the other,
and as the star spins,  the brightness changes by from .01
to .1 magnitude.  These are mostly hotter stars than the
Sun (spectral types B8 through A7),  with strong magnetic
fields.  It is thought the fields may help make different
parts of the star's surface of different compositions and
brightness,  in a manner similar to how the Sun's magnetic
field makes sunspots.  The spectrum of this kind of star
shows a lot of silicon,  strontium,  chromium and rare
earth elements.

Alpha Cyg 
   The Alpha Cyg type of variable star changes in
brightness because the star physically pulsates,  in an
uneven manner like a ball of Jello.  This makes for very
irregular changes in brightness.  The changes have to stay
small or the star would be really unstable and might split
or blow up,  and in fact,  this kind of star usually
varies by less than a tenth of a magnitude.

   This option sets the time to 0 hours UT (midnight) for the
current day.
   This option can be reached at any time with the ALT-0 key.

   This option allows you to back up to previous views,
up to five views.  It differs from the effect of the F7
key,  which toggles between two charts.  This option
can be reached at any time with the ALT-B key.

   The Alt-F4 hotkey allows you to exit Guide from any point
in the menus,  instead of requiring you to back up to the main
menu.  When you hit Alt-F4,  Guide will check to make sure you
really want to quit.  Also,  this lets you exit the program while
in full screen mode.

   In the DOS software,  hitting Alt-F8 toggles between the usual,
small cursor and a full-screen "cross" cursor.  This was added because
a few people find this large cursor easier to use,  especially at

   This option toggles to "flashlight" mode:  black on a
red screen.  This can provide enough light to be useful
in finding dropped objects,  etc.  when out in the field.
   This option can be reached at any time with the ALT-L
key in the DOS version.

Set Video mode 
   In the DOS versions of Guide,  the default video mode is
640x480,  16 color;  this is "standard" VGA.  In the Settings
menu,  there is an option to reset the video mode to a higher
   Be aware that not all cards support all resolutions!  If you
find that GUIDE won't select the new video mode,  try using
WGUIDE instead;  or if WGUIDE fails,  try GUIDE.  (It's not very
common for _both_ GUIDE and WGUIDE to fail,  although it does
happen sometimes.)
   You can reach this option with the Alt-M hotkey.

   Hitting the Alt-N hotkey causes Guide to recenter at the zenith
with a 180-degree field of view,  with the time reset from the PC
clock. The resulting chart looks like the foldouts in the center of
many astronomy magazines,  showing a "horizon-to-horizon" view.
For certain tasks while observing (such as following artificial
satellites or telescope control),  such a feature can be quite

   This option gives one wider control over what is shown
on the chart,  and lets you do things that might usually
be considered "ugly",  or too slow,  or otherwise unusual,
such as displaying tenth-magnitude stars at level 2.
   When this option is turned on,  the data shown on the
screen is "frozen".  You can zoom in and out to any level,
and the data used will be the same.  The program will
cease to make assumptions about what makes a "reasonable"
   When the option is turned off,  the program will resume
making its usual decisions in these matters.
   This option can be toggled at any time with the ALT-V

   This item lets you switch between zenith up and
celestial north up.  The first puts the zenith at the top
of the screen;  the second puts the celestial north pole
at the top.  You usually want the first to match the
screen to the real sky,  and the second to match star
atlases and charts.  When you switch from one to another,
the usual effect will be that what you see on the screen
rotates around,  sometimes upside down.
   You can reach this option at any time with the
Alt-Z hotkey;  or you can go into the Inversion dialog
and select "zenith at top" or "north at top".

Alternate names 
   Dozens of catalogs of galaxies have been compiled for
different purposes,  and they tend to have a huge degree
of overlap.  It is common for objects to be listed in
seven or more catalogs.
   Both the PGC and RC3 catalogs of galaxies contain
extensive information as to alternate names an object
may have been given in other catalogs.

   You can use the altitude and azimuth of an object to
find it in the sky.  These figures tell you where an object
is,  as seen from a particular place at a particular time,
in the sky.  Altitude tells you how high the object is
above the horizon.  An altitude of 45 degrees puts the
object halfway between the horizon and zenith (straight
overhead);  of 90 degrees,  straight overhead;  of zero
degrees,  on the horizon;  and so on for values in
between.  An altitude of less than zero puts the object
below the horizon,  invisible to all lacking X-ray vision.
   Azimuth tells you in which direction to look.  An
azimuth of zero degrees puts the object to the north;  of
90 degrees,  to the east;  of -90 or 270 degrees, to the
west;  of 180 or -180 degrees,  to the south.
   Therefore,  if you click on an object and are told
that it is at altitude 10 degrees,  azimuth 100 degrees,
you should look for it a little south of due east,  and
about a ninth of the way up from the horizon to the