constellation borders 

   In 1930,  the International Astronomical Union (IAU)
officially defined 88 constellations and gave them
borders.  These borders are similar to the rectangular
borders in some Western U.S. states that run exactly
north/south or east/west.  The constellation borders run
along lines of declination and right ascension.  (At
least,  they did in 1875.  Since then, precession has
added a very small rotation to this.)
   The display of these borders is controlled in the
Overlay menu.

constellation labels 
   This program can label each constellation on the screen
with its three-letter abbreviation.  These labels are
stored as overlays,  and their display can be adjusted in
the Overlay menu.  You also can click on the labels
themselves to get a full constellation name,  and can
also then click for "more info" on the constellation.

constellation lines 
   Most people find constellation lines a helpful aid
to learning their way around the sky.  For each constell-
ation,  a set of lines is defined that either matches
what the constellation is supposed to represent (such as
the Big and Little Dippers or the Southern Cross) or
simply helps in remembering which stars are where.
   Keep in mind that there are no standards defining how
these lines are set up,  and the set in this program  may
not exactly match a set you've seen elsewhere.
   The display of these lines is controlled in the
Overlay menu.

contact binary 
   In a contact binary star,  the two stars orbiting one
another are close enough together for matter to flow
between them.  This will at least make the star an
elliptical variable,  and probably an eclipsing binary as
well.  Because of the ellipticity,  it can be hard to tell
when eclipses begin or end.  Periods are less than one
day.  The variation is usually less than .8 magnitude.

   The COPERNICUS satellite made observations of the
spectra of a large number of stars.  In particular,  it
measured spectra in ultraviolet,  a feat much easier to
do from orbit than from ground level.  (The ozone layer
tends to absorb ultraviolet light;  COPERNICUS,  of
course,  was well above the ozone.)

   Most telescopes are designed to produce a well-focused
image at the center of the field of view.  Usually,  stars
become blurrier as you move away from the center of the
field.  This isn't usually much of a problem,  since you
are usually only interested in an object at the center of
the field.  But if you want to take a survey photo of a
large area of the sky,  you need a corrector to improve
the image over the entire field of view.  This can be one
("singlet") small lens,  placed near the focus of the
telescope.  However,  if you use a two-lens ("doublet") or
more lenses,  you can get better correction over a larger

common proper motion 
   Suppose you see two stars close together and think they
might be binary.  On the other hand,  maybe one is very
close and the other far away,  and they just look close
together.  How can you tell if they are really binary,  or
just faking it?
   One good clue is to measure their proper motion.  If
they have the same,  or common,  proper motion (CPM),
then you know that at the very least they are going in the
same direction and are probably a real binary star.

Critical List 
   The Minor Planet Center maintains a Critical List of
asteroids that are in particular need of having positions
measured via astrometry. Objects on this list often have not
been measured for a few years,  and the precision of their orbits
is beginning to degrade;  given a few positional measurements,
though,  the orbital elements can be recomputed and greatly
improved.  Further information is available at

   The catalog you just clicked on is one of several seldom-used
catalogs that are mentioned in the Atlas of Galactic Nebulae.
If you are interrested in the exact reference,  please contact
Project Pluto.

   The Ctrl-B hotkey causes Guide to ask you to enter
a Bayer or Flamsteed designation,  using the usual
abbreviations;  or a common name.  For example:

alp umi
40 eri
Bet Per

   (The use or non-use of capitals is ignored.)  Guide
will then redraw the chart,  centered on that object.
   This is provided as an alternative to the Go to
Bayer/Flamsteed menu option;  both accomplish the same
task of finding a numbered or lettered star,  but one
method emphasizes the keyboard,  while the other uses
the mouse.

   In the DOS version,  you can use the Ctrl-E hotkey to reset
the default epoch from J2000 to a different value.  Hit Ctrl-E,
or click on the epoch shown in the legend area,  and Guide will
prompt you to enter the new default epoch.
   Unless otherwise mentioned,  all positions shown in Guide (or
entered at the keyboard) will be in this default epoch.  (There
are exceptions,  but they are all clearly labelled.)

   "Normally",  the cursor keys can be used to move the
mouse.  This can be especially useful if your system
lacks a mouse;   you can then still click on objects and
such using the keyboard ("insert" is used for the left
mouse button,  "delete" for the right mouse button).
   In the DOS versions of Guide,  you can use this option
to toggle from this behavior to one where the cursor keys
instead pan the field of view,  in the direction of the
cursor keys.

   The Ctrl-F8 hotkey recenters the CCD frame on the
current cursor position.  The frame will also be turned
on,  if it was not on already.

Legend on/off 
   This item lets you turn the legend on or off.  When on,
the legend takes up room in the lower left corner of the
chart.  Some users may not be very happy about losing part
of their field of view to the legend,  thus the need for
this menu item.
   When you toggle the legend ON,  the Legend dialog will
be shown.  This menu lets you select which items are shown
in the legend (for example,  captions,  time,  RA/dec,
latitude/longitude,  and so on.)
   You can toggle the legend at any point in Guide with
the CTRL-L hotkey.

   In DOS,  the Ctrl-U hotkey can be used to set the
display of labels showing the magnitudes of stars in the
chart.  For example,  if you wanted to make a chart
labelling the magnitudes of stars down to mag 12,  you
would hit Ctrl-U and enter 12.  To get rid of the labels,
you would hit Ctrl-U again.
   In the Windows version,  this option is found in the Star
Display dialog.

Enter Longitude 

   The Enter Longitude option lets you reset your
geographic longitude. (You can get this from USGS maps
and from most others.)  You enter a longitude as a
compass sign (E or W) followed by degrees,  minutes,
and seconds.  For example, the longitude of Los Angeles
could be entered as

W 118 15 30

   Sometimes longitudes are expressed as degrees and decimal
minutes,  or decimal degrees.  You can use these instead:

W 118 15.5
W 118.25833

   The default latitude and longitude for this program
are for Bowdoinham,  Maine.  You need not be nit-pickingly
accurate in finding your latitude and longitude;  as long
as you're within a few kilometers,  you'll be okay for
most purposes.
   You can reach this option at any time with the CTRL-X
hotkey,  or through the Location dialog in the Settings menu.

Enter Altitude 

   The Enter Altitude option lets you reset your
geographic altitude,  in meters.  For most purposes,  this
is not an important factor,  but if you're looking for very
precise timings of some events (to within a fraction of a
second), it can be useful.
   You can reach this option at any time with the CTRL-Z
hotkey,  or through the Location dialog in the Settings menu.

dark nebula 
   A dark nebula is visible only in that it blocks the
light of stars in the background.  There are quite a few
such objects,  particularly in the plane of our galaxy.
Indeed,  without them,  the sky would be a much brighter

data display dialog 
   The data display dialog provides controls over the way
in which most on-screen data in Guide is shown.  It appears
when you use the Toggle User Datasets option in the Extras menu,
or on the Toggle Overlays option in the Overlays menu.  Also,
when you right-click on many objects on the chart,  the resulting
dialog box often includes a "Display" button that will lead to
this dialog.

   The dialog will generally look something like this:
[ ]--- Quasars ----------------[X]
|  ( ) On           [X] Labels   |
|  ( ) Off                       |
|  (o) Auto                      |
|  ( ) Fixed                     |
|                                |
|  Magnitude limit:  __14.0_     |
|                                |
|  Show at: __1_ - _180_         |
|                                |
| [ Options...]    [ Color ]     |
|                                |
|  [  OK  ]       [ Cancel  ]    |
   You can turn a dataset On or Off, so that either everything is
shown or nothing is shown. You can set 'Fixed' and have everything
shown down to a magnitude limit, and that limit stays constant as
you zoom in or out. 'Auto' does the same thing, except that Guide
will adjust the magnitude limit as the field of view changes. And you
can turn labels on or off and adjust the color used to display that
class of object.

   In some cases, certain options will be grayed out. For example, turn
a dataset on or off, and the magnitude limit doesn't matter anymore and
is grayed out. Some user-added datasets (such as the "Millennium Star
Atlas pages", "RealSky North plates", and so on) don't have any
magnitudes, and the 'Auto' and 'Fixed' options are grayed;  magnitude
limits don't make much sense for,  say,  constellation borders
either. Some datasets don't have labels, and that option is therefore
grayed out.

   The "Show at A to B degrees" option is available for overlays and
user-added datasets,  and defaults to 0-180: that is, Guide will show
this dataset no matter what field of view you have. But if you wish,
you can use this option to tell Guide that a given dataset should only
appear at certain fields of view.

   The "Options..." button is usually grayed out,  but not always.
It leads to a dialog box with options specific to the current data type.
For example,  for artificial satellites,  it would lead to an option
to reset the TLE filename.  For comets,  it would lead to options to
reset the line of variation and to add MPC comets/asteroids.
For planets,  it would lead to a rather complex planet display dialog.

data inferred 
   When you ask for "more info" about a star for which
Hipparcos or Tycho data is available,  Guide will first
show that data,  and will then show some quantities derived
from them.  For example,  both catalogs have parallax and
magnitude data;  Guide can use the parallax to determine
a distance to the star,  and can then compute the luminosity
and absolute magnitude.  This data appears in a separate
"data inferred" section.
   The idea is that this information doesn't actually appear
directly in the catalog;  it is just computed based on the
catalog data.

Data Shown 
   This menu lets you determine what kinds of objects are
shown on the chart.  From this menu,  you can turn
Messier and NGC objects, nebulae,  planets,  variables
and suspected variables,  and other classes of celestial
objects on or off,  or control how they are shown.
   This menu can be reached at any time via the CTRL-D

date of perihelion 
   A comet or other object moves around the sun in an
ellipse,  parabola or hyperbola.  The point where it is
closest to the Sun is its perihelion.  Guide will list
this date for comets.  If the comet is periodic,  it
will return to another perihelion;  Guide will list
the date closest to the current one.

right ascension 
RA and declination are the measurements astronomers use
to specify locations in the sky.  They are very similar to
latitude and longitude on the Earth.  The declination
of a point in the sky,  like the latitude on the Earth,  is
a number between -90 and 90 degrees.  The RA (or right
ascension) of a point on the sky is very similar to
longitude,  except that instead of being expressed in
degrees,  it is expressed in hours, minutes and seconds,
and can range from 0h0m0s to 24h0m0s.  Given an RA and a
declination,  any point in the sky can be specified
precisely.  For example,  Sirius,  the brightest star in
the sky,  is at RA 6h45m08.9s,  dec -16.716 degrees.
   One difference between RA/dec and latitude/longitude is
that the celestial equivalents of the poles,  equator and
prime meridian drift over time,  so in giving an RA/dec,
you usually specify its epoch,  the time for which it is
valid.  In the past,  it's been common to use epochs for
the years 1900 and 1950;  by now,  most people have
switched to 2000,  also shown as J2000.  The default epoch
for the legends,  grids and other markings in this program
is J2000.
   Latitude and longitude on Earth are measured from
the Equator and the Prime Meridian.  RA and declination
are measured from the vernal equinox and the celestial

   The term degree here is meant as a unit for measuring
the angle between objects in the sky.  The angle between
the horizon and the point straight overhead (the zenith)
is 90 degrees.  The angle between the "pointer stars" in
the Big Dipper is about five degrees.  The angle across
your thumb at arm's length is about one degree,  which can
make a pretty useful measuring tool.  The Moon and Sun are
about half a degree across.
   For measuring small angles,  the units arcminutes and
arcseconds are used.  There are sixty arcminutes to a
degree,  and sixty arcseconds to an arcminute.

Delta Cep 
Beta Cep 
   There are several types of Cepheid variable.  The Delta
Cep type resembles the star Delta Cepheus.  These are
young stars with very regular periods of a few days and
have a certain set relationship between the amount they
vary,  their period of variation,  and their actual
intrinsic brightness.  The Beta Cep type of variable have
periods of .1 to .6 days,  and vary by .1 to .3
magnitudes,  less than the variations in Delta Cepheids.
Most simply expand and contract radially,  but a few have
oddball "quivering" motions that give them more complex
behavior (and more random-looking light curves.)

Delta Sct 
   The Delta Sct type of variable star is a low-mass,
pulsating star.  The pulsations aren't symmetric;  the
star quivers like a ball of Jello.  This means the changes
in light aren't very regular either.  They vary from .003
to .9 magnitudes from minimum to maximum;  usual changes
are a few hundredths of a magnitude. They also expand and
contract,  just like normal Cepheid variables.  There
are usually two or more major pulsations at slightly
different frequencies.  Sometimes one set of pulsations
will be just out of synch with another set,  and the
changes in brightness will go away for a while.

   Delta-T is the difference between Dynamical Time (TD),
the uniform time system based on atomic clocks,  and Universal
Time (UT),  the slightly irregular system based on the earth's
rotation.  It is widely used in astronomical formulae.
   Guide uses observed values of Delta-T for dates from 1620
to 1994,  and a formula to approximate values before 1620.  (Our
knowledge of the earth's rotation,  and hence of UT,  is limited
for medieval and ancient dates.)  For dates in the future,  it
uses a separate formula that makes some assumptions about how the
rotational speed will vary.  It shows the value used for Delta-T
in the Quick Info section.

Reference: Dorschner, J. and Grtler, J.: 1964, Astronomische
Nachrichten 287, 257. Verzeichnis von Reflektionsnebeln.

diffuse nebula 
   Diffuse nebulae are large masses of gas in interstellar
space.  In some cases,  the gas is heated up by stars
inside to the point where it glows and forms a bright
diffuse nebula;  in other cases,  the gas simply blocks
the light that would normally come from background stars,
forming a dark diffuse nebula.  Diffuse nebulae are most
common near the core of our galaxy,  in the direction of
the constellation Sagittarius.

Dim Stars 
   You can hit the - key at any point in Guide to make
the stars a little dimmer,  and to lower the limiting
magnitude for all objects. You can use it repeatedly
until all stars have been dimmed into oblivion,  though
you'll probably want to stop before that point.  Each
use of this option subtracts .5 from the current limiting
   You can also add a button for this option to the toolbar,
by using the Toolbar Dialog.

Direct to screen 
   Usually,  when Guide draws a new chart,  it erases the screen
and you watch as the chart is drawn.  This is "Direct to Screen"
mode.  In Windows,  you can turn this mode off in the Display
Menu.  Do so,  and when charts are drawn,  the old one will
remain while a new chart is being drawn;  the new chart will
appear to instantly replace the old one.
   This option exists for two reasons.  First,  some people
prefer it to seeing the charts drawn.  Second,  on some systems,
it can be a little faster;  this is especially true on slower
video cards.

Discovered at 
   This line shows where the asteroid was discovered.

Discoverer's ID 
   The various observers of double stars have been given
designations,  ranging up to three letters.  A double can be
specified with the discoverer's ID and a number,  such as
HJ 2574,  the 2574th double catalogued by John Herschel.

Discoverer's name 
   This line shows the name of the discoverer of the

Discovery date 
   This line shows the date on which the asteroid was
found and recognized to be an asteroid.  Sometimes it
turns out that the asteroid was photographed at an earlier
date,  but went unrecognized;  such cases are called
prediscoveries.  (For example,  Pluto,  discovered in
1930,  was later found on old plates dating back to 1914.)

display menu 
   The Display menu offers the following options for
determining what markings are shown on your chart:

   Star Display dialog (set the sizes for stars and
      how they are labelled)
   Data Shown (Windows only;  in DOS,  this is part of the Main Menu)
   Legend on/off
   CCD Frame on/off
   Inversion dialog (flip the chart north/south and east/
      west;  rotate the chart;  put the zenith at the top,
      or ecliptic or galactic north)
   Measurements dialog (ticks, grids,  etc.) All markings
      used to indicate distances and coordinates
   Screen fonts (Windows only)
   Printer fonts (Windows only)
   Direct to screen (Windows only)
   Background Dialog

distance at closest approach 
time of closest approach 
   If you click for "more info" about an asteroid,  and
the object is within 1 AU,  Guide will assume that the
object is about to make a close approach to your home
planet (or has done so recently),  and will calculate the
time when the object was closest,  and how far away it was
from you at the time.  This data is shown as the distance
at closest approach and the time of closest approach.

Doppler shift 
Doppler effect 
   The Doppler shift is most familiar from the sounds
vehicles make.  An approaching train,  for example,  makes
a higher-pitched sound than one moving away from you.  The
sound waves are "pushed together" in the first case,  and
"stretched apart" in the second.
   Light waves do much the same thing.  An object
approaching us rapidly appears shifted toward blue;  one
receding appears redder than usual.  This effect,  though
small,  can be measured and can be used to tell you how
fast an object is moving toward or away from you.  (It
can't tell you anything about sideways motion.)  It makes
the spectrum of an object appear shifted relative to
what one might "normally" expect.
   Most galaxies have fairly large red shifts,  meaning
that they seem to move very rapidly away from us.  Hubble
figured out that there is a direct relationship between
the shift (and therefore the velocity) and the galaxy's
distance from us.

double star 
   It is common to find stars in pairs or larger groups.
Sometimes,  the pairing is by coincidence:  the objects
simply appear near to one another as seen from Earth,
even though one star may be much farther away than the
other.  Often,  however,  the stars are really physically
bound to one another by gravity,  as the Earth is to the
moon;  such objects are called binary stars.
   Double stars are listed in a variety of catalogs;  if
you click on Go to Double Star in the Go to Star menu,
you will see about 130 catalogs listed,  such as the ADS
and Struve catalogs.