The Telrad is a simple telescope aiming device;  it
uses an LED,  lens,  and beam-splitter to cause three
concentric circles to appear in the sky at an apparently
infinite distance.  The circles are 4 degrees,  2
degrees,  and .5 degree across,  and are aligned with the
telescope axis.
   The measurements dialog provides an option to toggle
the display of a Telrad at the screen center.  In the DOS
version,  you can toggle display of the Telrad with the
Ctrl-F6 hotkey.

   The terminator of a planet is the line between sunrise
and sunset.  The term can be applied to any planet,  but you
will usually see it mentioned in reference to the Moon.  It
is useful there because objects are best seen when they are
near the terminator.  If they are well in the sunlit side of
the moon,  then they cast no shadows and are difficult to see
in the glare.  If they are on the dark side,  then of course,
there is no light to observe them.  When you "click for more
info" about the moon,  you'll be given some information such
as the colongitude and selenographic position of the sun
that can be useful in figuring out where the terminator is.

text alignment menu 
   This menu allows you to reset the alignment that will
be used for text added to the overlay.

   Ticks are the cross-hatches indicating intervals of
right ascension and declination that are common to
many star charts.  You can adjust their spacing and turn
them on and off within the measurements dialog.

   Consider two points on the Moon,  one the point closest
to the Earth,  the other the farthest from the Earth.  The
point closest to the Earth feels more gravitational force
than the rest of the moon does.  The farthest point feels
less force.  This difference,  which tries to pull the
moon into an egg shape,  is a tidal force.
   Left to themselves,  rocks put at these two points
would drift apart.  They don't do this because the moon
has its own gravity,  much more powerful than the tidal
force.  Similarly,  the moon exerts a tidal force on us.
The rocks in the earth are rigid and aren't affected much
by this,  but the oceans,  being fluid,  tend to "stretch"
so that the water piles up a little at the points closest
and farthest from the moon.  As the earth turns,  these
points move around the globe and the ocean,  at any given
point,  seems to rise and fall.
   If a moon gets very close to a planet,  the tides will
become more powerful than the satellite's own gravity,
and the stress will rip it apart.  This may have happened
to form Saturn's rings,  and definitely keeps the rings
from lumping back up into a satellite.

   Titan is the largest moon of Saturn.  It is slightly
larger than Mercury and about 50% larger than the Moon.
   Because it is so large and so cold,  Titan is able to
hold an atmosphere of methane and other compounds.  This
gives Titan a reddish-brown appearance,  and also prevented
the Voyager probes from actually seeing the surface.  It
has been widely speculated that Titan could have life on
it,  if the atmosphere retains enough heat. The atmosphere
bears some resemblance to that of the Earth's just before
the emergence of life.

   A TLE (Two-Line Element) is a special,  standard way of
expressing a set of orbital elements for an Earth-orbiting
satellite.  It contains the "usual" elements,  such as mean
anomaly,  longitude of the ascending node, and eccentricity,
plus some which are specific to satellites,  such as atmospheric
drag terms.  Files containing TLEs can be downloaded from many
Internet sites;  see for links to such
   Once you have downloaded a TLE file,  you can use the
TLE=(filename) option to view the satellites contained in it.
   Because artificial satellite orbits tend to change rapidly,  you
will need to update your TLE files frequently.  A given orbit will
be good for a few days to a few months,  depending on the satellite
and the level of precision you want.

   The TLE=(filename) option allows you to select a new TLE
(Two-Line Element) file of orbital data for artificial satellites.
By default,  Guide draws satellites from a small file called
BRIGHT.TLE.  But the orbits of artificial satellites change rapidly,
and you will eventually want new satellites.  Also,  BRIGHT.TLE
contains only a few of the brighter satellites;  you may wish to
use a file containing data for geosynchronous satellites,  or those
in low-earth orbit,  and so forth.
   The best sources for up-to-date elements are all on the Internet.
See for links to such sites.

Toggle user datasets 
   The Toggle user datasets option in the Extras menu
provides a way to control the display of user added datasets.
Select this option,  and Guide will list the currently
installed datasets.  When you click on one of these datasets,
you will be provided with a standard data display dialog.
   You can also reach this option with the Alt-F9 hotkey.

Toolbar Dialog 
   By default,  the toolbar contains a few buttons that cover some
major features in Guide;  but it is expected that users will want to
adjust this to include only those icons of interest to them.  This
can be done using the Toolbar Dialog,  accessed through the
Settings menu.
   This dialog contains lists all buttons that can be put into the
toolbar.  Those in use are marked with an asterisk.  You can double-
click an item to toggle it;  or,  you can select a group of buttons
and turn them all on or all off,  using the two buttons provided for
that purpose.
   Also,  there is a checkbox given to shut off and turn on the toolbar.
   Finally,  you can click "OK" to indicate that you want to keep the
changes you've made,  or "Cancel" to cancel those changes.

   Topocentric means "measured from the Earth's surface."
For example,  when you get information on an object in
Guide (such as its position,  distance,  etc.),  this
data is usually measured from the Earth's surface,  from
the position described by the latitude,  longitude and
height above sea level you entered into the Settings menu.
   Usually,  the difference between a topocentric and a
geocentric (measured from the Earth's center) position is
not very great;  objects tend to be far away,  and your
position on Earth doesn't matter very much.  But a nearby
object,  such as the Moon,  can vary in position by two
degrees depending on where you are on the Earth,  and an
observatory in Australia recently missed an asteroid that
passed close to the Earth because they used geocentric,
not topocentric,  positions.

transit time 
   When you click on an object,  this program calculates
when that object will appear highest in the sky.  This
instant is called the transit time.

transition variable 
   Some variable stars that are in the process of evolving
from one type to another are classified in this program as
transition variable stars.  They are either just
starting or ending to vary,  changing types,  or simply
haven't been studied enough to tell what type they really

   Jupiter's gravity tends to push asteroids into certain
preferred orbits and places.   Two such places form
equilateral triangles with the Sun and Jupiter,  such that
one node travels in front of Jupiter,  the other behind
it.  An asteroid in either node will take the same time to
orbit the Sun as does Jupiter (about 11.86 years),  and
will stay in roughly that location.
   Some asteroids have fallen into these nodes.  The first
to be discovered was 588 Achilles.  By convention,  objects
found in these nodes have been named after characters in
the Iliad;  hence,  the term Trojan asteroids.
   In reality,  these asteroids have been named after both
Trojan and Greek characters.  Those in one node are named
after Greeks;  those in the other,  after Trojans.

           Leading Trojan asteroids (named for Greeks)

                                         (Jupiter and
                                         asteroids go
                                         clockwise around
   Jupiter         Sun                   the Sun)

           Following Trojan asteroids (named for Trojans)
@c 132,360,12    Sun
@c 12,360,5     Jupiter
@c 72,255,1
@c 74,454,1
@c 76,251,1
@c 78,456,1
@c 70,253,1
@c 68,458,1
@c 66,259,1
@c 64,462,1
@c 72,260,1
@c 72,460,1
@k 14            Dim gray
@c 132,360,120   Jupiter's orbit

   The Tycho data was collected as part of the overall
mission of the Hipparcos satellite;  it represents a catalog
of position,  parallax,  proper motion, and magnitude data
collected for over 1 000 000 stars.  In most cases,  its
precision is much greater than all earlier catalogs.  About
the only case in which the Tycho data would be ignored would
be if Hipparcos data is available instead.
   The Tycho data is essentially a survey of all stars that
were bright enough to be measured by the detector,  and is
essentially complete to about magnitude 10.5,  with
somewhat incomplete coverage to magnitude 11 or 11.5.

Tycho Input Catalog 
   When the Hubble Guide Star Catalog was put together,
it contained stars from magnitudes 9 to 15.  Since the
brighter stars weren't needed to point the telescope,
they were left out.
   Later on,  the bright stars were added in from the
Tycho Input Catalog,  a separate source.

   The Hubble Guide Star Catalog,  or GSC,  classifies each
object as a star,  non-star,  galaxy,  blend of two
objects,  or as an artifact (such as a scratch mark on a
photographic plate).  Since the same object sometimes
appears on several different,  slightly overlapping
plates, it is not uncommon for an object that looks like
a star on one plate to appear as a non-star on another.
Guide will list these types for you when you ask for
"more info" on a GSC star.

Type I supernova 
   A type I supernova is one of the two types of
supernovae.  This particular type shows little or no sign
of hydrogen in its spectrum.  After it peaks,  it drops
by about .1 magnitude a day for 20 to 30 days;  then the
rate of decrease slows down to about .014 magnitude/day.
   Type I supernovae are divided into three types: Ia, Ib,
and Ic.  Ia supernovae occur when a white dwarf is in
close orbit with another star,  and is pulling matter from
its companion and growing in mass.  Eventually,  it gets
to 1.4 times the mass of the Sun.  This is the most
massive a white dwarf can be;  beyond this point,  it must
collapse into a neutron star,  releasing its energy in
a supernova explosion while it does so.  Because the mass
limit (called Chandrasekhar's limit) is so exact,  these
supernovae are always of about the same brightness,  so
they make a good means of determining the distance to
distant galaxies.
   Type Ib and Ic supernovae are probably type II
supernovae where the parent star has lost its outer layer
of hydrogen (for Ib) or hydrogen and helium (for a Ic).

Type II supernova 
   A type II supernova is one of the two types of
supernovae.  Unlike a type I supernova,  a type II will
show mostly hydrogen and helium in its spectrum.  This
type also shows wider variations in how they fade away,
but usually,  after 40 to 100 days,  they are fading by
.1 magnitude per day.
   Type II supernovae occur in very massive stars,  at
least ten times the mass of the Sun.  Such stars start
out by fusing hydrogen into helium.   Later,  they start
forcing helium nuclei together into carbon,  oxygen,  and
still heavier nuclei,  until they produce iron.  The iron
cannot be fused to provide further energy to maintain the
core temperature;  eventually,  the core collapses under
the weight of the upper layers,  rebounds,  and throws off
the upper layers of the star.

UCAS galaxy catalog 
   The UCAS galaxy catalog is a special catalog of galaxies
between RA 10h58m and 12h48m,  declination S 21 45' to
N 10 48' (J2000 coordinates).
   The objects were examined from a series of UK Schmidt
IIIa-J plates (originals, not copies) taken for an
asteroid survey conducted by Bobby Bus.  26 of the best
plates were chosen to scan for galaxies. Brian Skiff did
this by examining them on a light-table with a 6x loupe
containing a compass and millimeter reticle.  He got
morphological types for all the galaxies appearing in some
catalogue (via the Dixon et al overlays for the POSS-I) and
others that were reasonably large/bright, such that they
would have appeared in Zwicky's catalogue had it extended
south of the equator, plus whatever else caught his
attention. He measured for size and position angle all
the NGC/IC galaxies plus some other of the largest
non-NGC/IC galaxies.

UCAS galaxy size 
   The sizes in the UCAS galaxy catalog include somewhat
more of the outlying areas of a galaxy than is usual.
Catalogs such as the RC3 measure the size of a galaxy out
to the point where its surface brightness drops to 25
magnitudes per square arcsecond (i.e.,  if you took a
chunk of the sky from here that was one arcsecond on a side,
and somehow isolated it,  it would show up as a 25th
magnitude object.)
   The UCAS measurements,  on the other hand,  didn't stop
until the surface brightness dropped to 26 or 27 magnitudes
per square arcsecond.  So galaxy sizes from UCAS tend to
be a bit larger than those listed in other catalogs.
   In general,  you should use RC3 sizes in preference to
UCAS sizes.

Uppsala General Catalog 
   The UGC,  or Uppsala General Catalog,  contains
galaxies too faint to appear in the NGC catalog.  It
was compiled by astronomers in Uppsala,  Sweden,  and has
detailed information on the position,  size,  and other
characteristics of almost 13,000 galaxies north of about
declination S 2 degrees.
  You can find an object by its UGC number by using the
Go to UGC option in the Go to Galaxy menu in the Go
To menu.

   The human eye can see a range of visible light from
red to blue.  Light just outside the blue limit is called
ultraviolet,  or UV,  radiation.  Its wavelength ranges
roughly from 5 to 400 nanometers.  It's difficult to
observe celestial objects in the ultraviolet,  because the
ozone layer absorbs almost all of it.  In general,  this
is just as well,  since the sun emits enough UV to kill
us all.  Most UV observing has been done from satellites,
such as Copernicus,  IUE,  and EUVE (Extreme Ultraviolet
   UV is mostly emitted by extremely hot objects;  things
such as Markarian galaxies and cataclysmic variables,
for example.

Ultraviolet FeII 
   The spectrum of a star shows lines corresponding to
elements heated to high temperatures.  One such line,  in
the ultraviolet part of the spectrum (between visible
light and X-rays),  is due to iron.  The symbol for iron
is Fe,  and many stars show this ultraviolet FeII line.
(A given element usually produces a lot of different
lines;  they are indicated by Roman numerals which indicate the
ionic state of the element. The type of spectrum changes with
the number of electrons in the last, partly filled, shell.
NaI (neutral sodium) has a hydrogen-like spectrum; the spectrum
of NaII (sodium once ionized) is very much like that of Ne.
FeII means iron once ionized; OIII means oxygen twice ionized)

   Uranus,  the seventh planet from the Sun,  was found by
William Herschel in 1781,  using a small homemade
telescope.  At times,  the planet gets just bright enough
to see with an unaided eye under good conditions.
   Physically,  Uranus is much like Jupiter,  Saturn and
Neptune:  a large planet composed of mostly hydrogen,
methane and other gases.  It is 19 times farther from the
Sun than we are,  and therefore tends to stay at about
-350 degrees Fahrenheit (-200 Centigrade).  It is unusual
in that its poles are almost in the plane of its orbit.
If this were true for the Earth,  the Sun would be
straight overhead in June as seen from the North Pole;  it
would slowly spiral down to the horizon,  making a circle
around the sky once a day,  finally setting in September;
the Sun would go farther and farther below the horizon,
finally starting back up in December;  it would rise again
in March,  and spiral its way back up until June again.
You can see that this process of having a 'day' that lasts
all year might lead to some extreme weather conditions.
It takes Uranus 84 years to circle the Sun,  so first one
hemisphere gets most of the sunlight for 42 years while
the other half freezes;  then the hemispheres reverse for
the next 42 years.
   Uranus has several satellites,  all named for Shake-
spearean characters,  none much larger than 600 mi (1000
km).  The planet and its satellites were examined in some
detail by Voyager 2 in 1986.

   By default,  Guide uses an immense built-in data of orbital elements
for computing highly accurate positions of asteroids.  Because this data
is precomputed,  it can account for most planetary perturbations and
be very fast,  and show accurate positions over a long time span.
   However,  you can use the Use MPCORB option in the Extras menu
to cause Guide to instead use data provided by the Minor Planet Center.
Their data file,  MPCORB.DAT,  is available via anonymous ftp at:
  The MPC provides the database in two forms:  as "MPCORBCR.DAT" and
"MPCORB.DAT".  Both contain exactly the same data,  but the former is in
PC format (with a carriage return/line feed at the end of every line of
data),  the latter in Unix format (a line feed but no CR at the end of
each line). Guide will recognize either file.  MPC also makes both files
available in .ZIP format,  reducing the download size considerably.  The
files are updated daily.

   There are two important disadvantages to use of MPCORB.  First,  the
data exists at only one epoch,  always within 100 days of the present.
If you try to compute positions far away from that epoch,  the quality
of the results will gradually deteriorate.  The Guide CD provides
orbital elements at 50-day intervals over a span of several decades, so
the epoch of the data is never off by more than 25 days.  Thus, for
objects that were well-determined in the original Guide data, MPCORB
positions will be _less_ accurate.  It's best to avoid MPCORB, for
example,  in computing asteroid occultations,  which always involve
well-determined objects.  (In defense of MPC,  it should be pointed
out that if Guide had a built-in orbital integrator, this problem
would go away and MPCORB would be more accurate in all cases.)

   Another disadvantage to MPCORB will be immediately apparent:  it's
slow.  For the built-in data,  Guide has some precomputed information
it can use to immediately determine that 99% of the asteroids will not
appear in a given chunk of the sky.  That precomputed data doesn't
exist for MPCORB.

user added dataset 
   The user added dataset capability makes it fairly simple to
add your own datasets to Guide.  Many examples now exist;  if
you click on the Toggle User Datasets option in the Extras Menu,
you will get a full list and can turn them on or off and adjust
their display.
   The examples include datasets such as catalogs of quasars,
active galactic nuclei,  radio catalogs,  and so on.  These can be
of considerable use in their own right,  even if you do not have
datasets of your own that you want to add to Guide.
   For full details on adding your own datasets,  see the manual.
   If you've turned a dataset on and now you want to find a
particular object in that dataset,  you should use the Go to
.TDF object option in the Go To menu.

Using Help 

   Guide has an extensive help system to explain both the
operation of the program and some of the astronomical
information it provides you.  The information is in a
linked hypertext form;  if you click on a boxed item shown
in light blue,  the display will switch to show information
about that topic.  For example,  clicking on "Guide" above
will lead to a list of phone numbers,  addresses,  and so
   Some of the more useful help features are:

   Using the Glossary section
   Printing help information
   Writing help information to an ASCII file
   Getting a list of hotkeys
   Getting help for a menu item

Using the Glossary section 
   You can enter the Glossary section of Guide by hitting
the comma (,) key at any point.  You can also enter it by
clicking on the "glossary" item at the bottom of any help
   The glossary is divided into several pages;  when in the
Glossary section,  controls for selecting a page appear at
the bottom of the screen.

   The USNO (United States Naval Observatory) pursues a variety
of research tasks,  many involving astrometry.  In this regard,
it is the creator of several important star catalogs,  such as
the SA1.0 and A1.0.