Brightness level extremely approximate 
   For most nebulae in the Nebula Databank,  the brightness
level could be gathered from the Lynd's Bright Nebula catalog,
IC,  NGC,  Sharpless,  or van den Bergh catalog data,  or
from comparison to nearby nebulae. But in some cases in the
Southern sky,  no brightness level data was available at all;  in
such cases,  an extremely approximate value was assigned based on
membership in other catalogs (that is, a nebula appearing in many
catalogs was assumed to be brighter than a nebula appearing in
only one catalog.)

BT magnitude 
VT magnitude 
   The BT magnitude and VT magnitude values are blue and
visual magnitudes as measured by Tycho.  They correspond
pretty closely to "standard" Johnson visual magnitudes
and B magnitudes.  The BT system has a peak at 435
nanometers; the VT,  at 505 nanometers.

BY Draconis 
   BY Draconis-type variables are dwarf stars of
spectral type K or M,  with quasiperiodic light changes
ranging from a few hundredths to .5 magnitude.  The period
will run from a few hours up to 120 days.
   These objects vary because some parts of their surface
are brighter than others (like sunspots,  but on a larger
scale).  As they rotate,  we see more or fewer spots and
therefore more or less light.  Some of these stars also
show flares,  similar to UV Ceti stars,  in which case
they belong to both types of variable star.

   Callisto is the outermost of the four large satellites
of Jupiter found by Galileo.  It has a heavily cratered
surface,  indicating minimal recent geological activity.
(Recent volcanoes and such would eliminate the craters,
as they do on earth.)

Caption on/off 
   Clicking on this menu item toggles the display of the
caption in the legend area of the chart.  (To change the
caption,  you should use the Enter Caption  menu item.)

carbon star 
   Most stars get their energy through nuclear fusion:
they use a process in which four hydrogen nuclei are fused
into one helium nucleus.  This is,  for example,  how the Sun
produces its energy.
   A carbon star is a star,  usually of spectral type R,
N, or S,  that has run low on hydrogen.  It has switched
over to fusing helium nuclei into carbon,  and also to a
cycle in which carbon nuclei fuse with four hydrogen nuclei,
then emit a helium nucleus (same end effect as the usual
fusion method).  This method usually results in a cooler
surface temperature,  and therefore,  a very red star.

cataclysmic variable 
U Gem 
   The U Gem type of variable star is also often called
a dwarf nova,  or a cataclysmic variable.  These are
binary stars,  one a white dwarf,  the other a subgiant
star of spectral type K or M (cooler than the Sun,  and
larger in diameter).  They are close enough that tides
from the white dwarf rip gas from the bigger star;  this
matter spirals in on the white dwarf, forming an accretion
disk.  Usually the stars orbit one another every 1 to 12
   Most of the time,  these stars have small,  sometimes
rapid variations in light.  From time to time,  though,
the system will rapidly get brighter by a few magnitudes
and,  over a period of days to months,  go back to normal.
How often this happens varies from star to star,  but any
given U Gem star has something resembling a period;  that
is,  the star will blaze up at semiregular intervals.  The
greater the interval,  the greater the increase in
   These stars are also often eclipsing binaries.  They
are divided into SS Cyg,  SU UMa, and Z Cam types.

Cause(s) of variability 
   Stars vary in brightness for a number of reasons,  and
sometimes the same star will vary for more than one
reason.  This program will list those reasons (if known)
for any variable star you pick on,  in the "Click for more
info" section.

CCD frame 
   The CCD frame provides a way to "frame out" a
rectangular region of the screen,  to show the area of
coverage of a photo or CCD image.  The CCD frame dialog
provides the controls needed to manipulate the frame.

CCD Frame on/off 
   This menu item toggles the display of the "CCD Frame",
a rectangle useful in setting up images with CCD or film
   When you toggle this option ON,  the CCD Frame Dialog
box will appear,  giving you control over where the frame
is shown,  its size and orientation,  and whether it stays
in the center of the chart.
   You can toggle this option using the Ctrl-F hotkey.

Reference: Cederblad, Sven: 1946, Meddelande fran Lunds Astronomiska
Observatorium Series II, No. 119.  Studies of Bright Diffuse Galactic
Nebulae - With special regard to their spital distribution. Published
by the Observatory Lund, Sweden.

celestial equator 
   Imagine that the Earth was a transparent sphere with a
big light in the center.  Imagine further that someone ran
around the Equator and put a line of paint there.  The
shadow cast by that paint out into infinity would cut the
sky in half;  half the sky to the north,  half to the
south.  This dividing plane is called the celestial
equator.  It is the plane from which declination is
measured;  a point exactly on this plane has a declination
of 0 degrees.

   The Centaur class of asteroids consists of the (very few)
asteroids with orbits between Saturn and Neptune.  The first
such object discovered was 2060 Chiron in 1977;  this object
is now known to also behave a little like a comet,  with some
signs of a coma.  It orbits between Saturn and Uranus,  with a
period of 50 years.

central meridian 
   The central meridian of a planet tells you what part
of that planet is currently facing you.  For example,  the
feature Syrtis Major on Mars is located near longitude
300 degrees;  it is therefore best observed when the
central meridian is near this value.
   When the Earth is observed from other planets,  its
central meridian is also shown.  If you (for example) set
the "home planet" to be the Moon,  using the home planet
dialog,  and then click for "more info" on the Earth,
it might show a central meridian of 150 degrees,
indicating that the Pacific Ocean is facing the moon at
that time.

Cepheid variable 
   One of the earliest known variable stars was Delta
Cephei.  This star regularly grows brighter and dimmer by
about one magnitude over a period of about five days.
In this case,  the star is physically growing brighter and
dimmer (and its surface gets hotter and colder and its
spectrum varies as well.)  Stars that exhibit this sort
of behavior are called Cepheid variables.
   Cepheid type variables usually have periods ranging from
a few days to a few weeks.  The changes in some,  such as
Delta Cephei,  can be easily followed from night to night
by estimating its magnitude.  This is usually done by
comparing the variable's brightness to nearby comparison
   One unusual property of Cepheids is that the length of
a Cepheid's period can give a close idea as to its total
output of energy,  or luminosity.  Comparing that output
to the brightness we actually see gives an idea as to how
far away the star is.  For more normal stars,  determining
luminosity and distance is a much more difficult and
uncertain task.  The fact that Cepheid variables have this
property was used earlier in this century to determine the
distance to nearby galaxies.

   Many galaxies will have listed,  under their "alternate
designations",  a CGCG,  or Catalog of Galaxies and
Clusters of Galaxies,  designation.
   You can find an object by its CGCG number by using the
Go to CGCG option in the Go to Galaxy menu under the Go To

chart area 
   You can accomplish two tasks by clicking in the chart
area.  If you click in this area with the LEFT mouse
button,  the chart clears and recenters at the point you
clicked on.  You can pan around the sky in this manner.
   You can also click on an object in this area with the
RIGHT mouse button.  In this case,  the program will find
out information on that object and show a summary in the
center of the screen,  listing such information as the
object's name(s),  rise/set times,  magnitude,  and so
forth.  Underneath will be two buttons,  "OK" and "more
info";  clicking on the second button will cause Guide
to show more detailed information about the object.

Chart mode 
   Normally,  Guide shows white stars on a black background.
The Chart Mode option toggles to black stars on a white
background.  This option is especially useful in doing color
printouts in Windows;  you set this mode and reset the colors
as you would like them to appear on the printout.  (When you
return to white stars on black,  your other colors will be
restored as well;  a different set of colors is stored for
each mode.)
   You can also get this option with the Ctrl-F3 hotkey,  or
through the background dialog.

Chinese calendar 
   The Chinese calendar is both lunisolar (meaning the months
line up with lunar phases and the years line up with seasons,  both
at the same time),  and observational (meaning that it's based on the
real motions of the moon and sun,  rather than on a mathematical
approximation.)  The result is a calendar of truly immense

   The fact that it is lunisolar means that (as with the Hebrew calendar)
some years have twelve months and 353,  354,  or 355 days;  while
other "leap" years have thirteen months,  and 383,  384,  or 385 days.
And also like the Hebrew calendar,  months have 29 or 30 days.

   However,  the Hebrew calendar inserts leap years in a regular cycle,
with seven years out of 19 having the extra intercalary month;  and
this added month is always at the middle of the year.  The Chinese
calendar adds such months at almost exactly the same frequency,
but in an odd order and at any point in the year.  For example,  one
can have months "one",  "two",  "three",  "three (intercalary)",
"four", and so on up to "twelve".

   The Chinese calendar runs either on a sixty-year cycle,  or by
numbering years since an emperor ascended the throne,  or by numbering
from the sixty-year cycle that started in 2637 BC.  Guide follows
this last convention;  for example,  it will tell you that 28 January 1998
Gregorian is "Chinese New Years Day" for year 4635.  The years of
the sixty-year cycle are named,  and Guide will also show these names.
(Year 4635,  which is the fifteenth year of the current cycle,  is
named wu-yin,  for example.)

   Those interested in full details,  including how the calendar is
computed and C/C++ source code,  should look at:

cpolar text 
not vis. text 
   Depending on where you are on the earth,  some objects
will be either always visible (or circumpolar) or
eternally not visible.  Since such objects neither rise
or set,  times of rising and setting aren't listed for
them when you click on them;  you are merely informed
that they are circumpolar or never visible.  (If you
choose a different latitude in the Settings menu,  you
can find a place on the Earth where the object will be
visible by moving closer to the Earth's equator.)
   These objects do move in the sky;  they trace circles
around the poles,  hence the term circumpolar.

Civil twilight 
Nautical twilight 
Astronomical twilight 
   Twilight is defined in three ways.  Civil twilight
occurs when the sun is six degrees below the horizon.  The
next "darker" form of twilight is nautical twilight,  with
the sun 12 degrees below the horizon.  Astronomical
twilight occurs when the sun is 18 degrees below the
horizon.  In polar latitudes,  nights may pass where one
or more of these may not occur.
   Twilight times are given (if they occur for your
current position,  as defined by your latitude and
longitude in the Settings menu) when you "click for
more info" concerning the sun,  or ask for Quick Info.
   At the end of civil twilight,  one is legally required
(in some US states) to turn on automobile headlights.
During nautical twilight,  one can take sightings of stars
and planets (it is dark enough for these objects to be
visible,  but bright enough for the horizon to be seen).
At the end of astronomical twilight,  the sky has reached
its maximum darkness.

Clear caption 
   If you have entered some caption text,  clicking on
this option will clear it all out so you can start over.
It is grayed out if there is no caption text currently

Clear RealSky images 
   If you have generated a series of RealSky images,  you
will eventually want to clear them from your screen and
hard drive.  The Clear RealSky images option in the Extras
menu provides a way to do this.  Click on this option,  and
you'll be asked to confirm that you really do wish to
erase the data.  If you do confirm it,  Guide will delete
the RealSky images in the Guide directory and will redraw
the screen,  free of images.
   You can also access this option with the Alt-F11 hotkey.

Click for a list of constellations 
!Andromeda          Crater         Norma       Vela
Antlia             Crux           Octans      Virgo
Apus               Cygnus         Ophiuchus   Volans
Aquarius           Delphinus      Orion       Vulpecula
Aquila             Dorado         Pavo
Ara                Draco          Pegasus
Aries              Equuleus       Perseus
Auriga             Eridanus       Phoenix
Bootes             Fornax         Pictor
Caelum             Gemini         Pisces
Camelopardalis     Grus           Piscis Austrinus
Cancer             Hercules       Puppis
Canes Venatici     Horologium     Pyxis
Canis Major        Hydra          Reticulum
Canis Minor        Hydrus         Sagitta
Capricornus        Indus          Sagittarius
Carina             Lacerta        Scorpius
Cassiopeia         Leo            Sculptor
Centaurus          Leo Minor      Scutum
Cepheus            Lepus          Serpens
Cetus              Libra          Sextans
Chamaeleon         Lupus          Taurus
Circinus           Lynx           Telescopium
Columba            Lyra           Triangulum
Coma Berenices     Mensa          Triangulum Australe
Corona Australis   Microscopium   Tucana
Corona Borealis    Monoceros      Ursa Major
Corvus             Musca          Ursa Minor

Click for a list of meteor showers 
Alpha Aurigids       Delta Leonids       Ophiuchids
Alpha Bootids        Draconids           Orionids
Alpha Scorpiids      Eta Aquarids        Pegasids
Andromedids          Geminids            Perseids
April Fireballs      June Draconids      Phi Bootids
Arietids             June Lyrids         Piscids
Camelopardalids      Kappa Cygnids       Quadrantids
Capricornids         Kappa Serpentids    Scorpiids
Coma Berenicids      Leo Minorids        Sigma Hydrids
Corona-Australids    Leonids             Sigma Leonids
Delta Aquarids       Lyrids              Tau Herculids
Delta Arietids       March Geminids      Taurids
Delta Cancrids       Monocerids          Ursids
Delta Draconids      Mu Virginids        Virginids

Showers in April and May
Showers in January,  February,  and March
Showers in June,  July,  August,  and September
Showers in October,  November,  and December

Click for Greek alphabet 
Greek letter 
   Greek letters are included here because they are used
for Bayer designations for stars.
@g    (means "use greek letters")
 Alpha         Eta           Nu            Tau
 Beta          Theta         Xi            Upsilon
 Gamma         Iota          Omicron       Phi
 Delta         Kappa         Pi            Chi
 Epsilon       Lambda        Rho           Psi
 Zeta          Mu            Sigma         Omega
@g    (return to European)
   In general,  the Alpha star in a constellation will
be brighter than the Beta,  which in turn will be brighter
than the Gamma... but there are numerous exceptions to
this rule.

   It's not uncommon for stars to form in groups,  known
as clusters.  There are two types of clusters,  open
clusters and globular clusters.  The Pleiades are an
example of an open cluster;  M-13 (Messier 13) is an
example of a globular cluster.

clusters of galaxies 
cluster of galaxies 
   Guide supports the display of clusters of galaxies
from the Abell and Zwicky catalogs.  Data for these
objects include their sizes,  positions,  and magnitude
and redshift (in some cases).

Reference: Cohen, M.: 1980, Astronomical Journal 85, 29. Red and
nebulous objects in dark clouds. A survey.

   The lunar colongitude is among the data listed when
you click for "more info" about the Moon,  and when you
create a lunar data table.  It provides a simple way to
describe the location of the lunar terminator,  and is
therefore useful in figuring out which lunar features are
best positioned for observing.
   The colongitude describes the lunar longitude on the lunar
equator where the sun appears to be rising.  Near New Moon,
the sun is rising on the moon's western edge (near Mare Crisium),
at lunar longitude -90.  (It's common to always show a positive
colongitude,  however,  so it would be reported as 270.)
   Near first quarter,  the sun is rising on the center of the
near side,  at lunar longitude 0,  and the colongitude is
therefore also zero.  Near full moon,  the sun is rising near
the eastern edge of the moon,  at lunar longitude +90;  and at
last quarter,  it is rising as seen from the center of the
far side of the moon,  at lunar longitude 180.

   The Bright Star catalog sometimes will remark on
colors observed in stars.  Usually,  it's difficult to
see a star's color without some help (such as binoculars),
but some of the brightest stars have obvious blue or red
tints.  The colors are usually fairly subtle.  It doesn't
help matters that human vision at night is mostly in
shades of gray;  the parts of the eye that detect color
differences require more light than is usually available.
   The usual way of measuring color quantitatively is to
look at a star through colored filters and measure its
brightness.  The colors and densities of these filters are
standardized.  The difference in brightness is a good
measure of the color.  The most commonly used filters are
the B(lue) and V(isual) filters,  leading to a "B-V" value.

   "Comet" is a word derived from the Greek for "hairy
star",  and comets are usually distinguished by a tail
formed by gas and dust boiled from their surfaces when
they approach the Sun.  A few come close enough to be
spectacular,  even with the unaided eye;  many more are
either too faint or appear at the wrong time and place
to be seen without binoculars or a telescope.
   Most comets come in from interstellar space,  take a
pass by the Sun,  and head out into outer darkness,  never
to be seen again.  A few have their orbits adjusted by
other planets,  so that instead of heading back out,  they
wind up in short-period orbits.  Halley's Comet,  for
example,  comes back every 76 years;  Swift-Tuttle,  every
130 or so;  Encke's,  every 3.3 years or so.
   You can find comets through the Go To menu;  it has
an option that brings up a list of currently visible
comets.  Click on one,  and Guide will recenter on it.

common name 
   Many very bright stars were named thousands of years
ago,  such as Sirius and Betelgeuse and Polaris.  The
common names used in this program are almost always
Arabic, with a few Greek and miscellaneous names added
in.  This program recognizes about 200 or so of the most
commonly used names.  You can find them in the Go to
Common Star Name menu under the Go To menu.

   Ancient civilizations divided the stars into groups of
constellations.  They usually gave them names that,  to
some minor degree,  resembled the shape of the group of
stars.  Thus,  Orion looks a little like a human holding
an animal skin and wearing a belt,  while Scorpius looks a
little like a scorpion's claws and stinger.
   Naturally,  different cultures came up with different
sets of constellations.  In the interests of standardizing
things,  there are now 88 officially recognized constell-
ations.  Each has precisely defined borders so that one
can determine with which constellation a star belongs,
and each has a name and three letter abbreviation.  For
example,  Orion is abbreviated Ori;  Cassiopeia, Cas;
Taurus, Tau.  These constellations were officially defined
in 1930.
   In this program,  you can show constellation lines,
constellation borders,  and constellation labels.  You
can also find a desired constellation by clicking on the
Go to Constellation menu item in the Go To menu.

Click for a list of constellations