Escape velocity 
   If an object leaves Earth's atmosphere at about 11
km/sec (7 miles/second,  or about 25000 miles/hour),
it will escape the earth's gravity;  below this speed,
it will eventually come back to earth.  This speed is
called the escape velocity of the earth.
   When you click for "more info" on a planet,  the
escape velocity is listed for that planet.  It can be
useful to know that an object orbiting a planet near
its surface moves at about 71% of escape velocity;
for example,  an object in low-earth orbit travels at
about 8 km/second.

   The ESO/Uppsala catalog is based on plates taken at
the European Southern Observatory,  and shows most of the
galaxies in the southern half of the sky.  ESO objects
are labelled with two numbers and,  sometimes,  a letter.
   You can find an object by its ESO number by using the
Go to ESO/Uppsala option in the Go to Galaxy menu in the
Go To menu.

ESO/Uppsala identifier 
   For galaxies in the southern half of the sky,  most
of the data given comes from the ESO (European Southern
Observatory) extension of the Uppsala General Catalog.
Each galaxy in the ESO/Uppsala extension has an
identifier,  listed when you "click for more info" on such
a galaxy.

   Europa is the smallest of the four large satellites of
Jupiter found by Galileo.  It is covered in a shield of
ice from pole to pole,  broken occasionally by cracks.  It
is thought possible by some that life might form in the
ocean under the ice,  given some internal source of heat.

Exp. Length 
   This item tells you for how long the plate that this
Guide Star Catalog star was found on was exposed.

Explanation of solar eclipse data 
   The solar eclipse data shown in these tables is directly copied from
Fred Espenak's "Catalog of Solar Eclipses".  The full catalog is
available at:

   When you create a list of solar eclipses using the option in the
Tables Menu,  the following data is given for each eclipse:

   The date and time,  in UT,  of greatest eclipse.  The Gregorian
calendar is used for dates after 1584 October 15.  The Julian calendar
is used before that date.

   The type of each eclipse is given by a capital letter:  A=Annular;
T=Total;  P=Partial;  H=Hybrid (the eclipse is annular at some points
along the path,  total at others.)

   In several cases,  an extra lowercase letter is given.  Those letters
mean the following:

      "m" = Middle eclipse of Saros series.
      "n" = No northern limit.
      "s" = No southern limit.
      "+" = No northern limit and no center line.
      "-" = No southern limit and no center line.
      "b" = Saros series begins (first eclipse in series).
      "e" = Saros series ends (last eclipse in series).

   Next,  the Saros number is given.  Solar eclipses tend to occur in
series spaced about 18 years apart;  several such series are running
at any given time.  A given Saros series will run about 13 centuries;
each series has a Saros number.

   Next,  the "Gamma" value is given.  "Gamma" is the distance of the
shadow cone axis from the center of Earth (units of equatorial radii)
at the instant of greatest eclipse.  A Gamma of zero means that the
shadow axis at mid-eclipse runs right through the center of the
earth;  one of +.997 means that it runs near the edge of the earth,
near the north pole;  and a Gamma of -.997 means that it runs near
the edge of the earth close to the south pole. (The value is .997 to
reflect the fact that the earth is slightly flattened,  by about

   If the value falls outside the range -.997 to +.997,  then it is
very likely to be a partial eclipse;  within this range,  it is likely
to be a total or annular eclipse.

   Next,  the "magnitude" of the eclipse is given.  This does not have
the standard astronomical meaning of "brightness";  instead, it refers
to the portion of the sun covered by the moon.  When the magnitude is
greater than or equal to 1,  the sun is totally eclipsed. When the
magnitude is nearly zero,  the sun is only very partially eclipsed.

   To be exact,  the magnitude is defined as follows.  It is the length
of the line segment running through the center of the sun and moon,
measured from the solar limb nearest the center of the moon to the
lunar limb nearest the center of the sun;  divided by the apparent
diameter of the sun.  (In a partial eclipse,  this reduces to just
"the fraction of the apparent diameter of the sun covered by the moon.")

   Next,  the latitude and longitude where maximum eclipse
would be seen is given,  along with the altitude of the sun at this
point.  (In the case of a partial eclipse,  that altitude will always
be zero.  In other words,  if the eclipse is a partial one,  then the
greatest eclipse will be seen someplace where the event occurs on the
horizon,  and in a polar area.)

   Next,  the width of the eclipse path is given for all non-partial
eclipses,  in kilometers,  at the time of greatest eclipse.

   Finally,  the duration of the eclipse is given for all non-partial
eclipses,  at the location of greatest eclipse.  (The eclipse will be
shorter elsewhere,  for total eclipses;  or longer elsewhere, for
annular eclipses.)

Exposure start 
   The Hubble Guide Star Catalog,  or GSC,  was created
by scanning in photographic plates of the night sky.  The
time when the plate was taken is among the information
given when you ask for "more info" on a GSC star;  this
time is the exposure start.

Extras menu 
   The Extras menu contains a set of "advanced" options
that you will probably not use at first.  It contains the
following menu items:
   RealSky image
   Add DSS image
   Clear RealSky images
   Shut off images
   Get Ax.0 data
   Edit Comet data
   Add MPC comets/asteroids
   Enter Time
   Fixed levels
   Line of variation
   Toggle user datasets
   Make PostScript file
   Make .BMP File (Windows only)
   Find conjunction
   Show eclipse (or occultation,  or transit)

   In DOS,  you can always reach this menu with the Alt-X hotkey.

Getting help for a menu item 
   You can get some help information for any menu item
in Guide by moving the mouse cursor over the item in
question and hitting F1.  Guide will then show some
help data specific to that menu item.  If you hit F1
while in the chart area,  Guide will provide some
introductory "how to use Help" information.

   This option is intended solely for a few Panasonic
printers with slightly faulty Epson emulation.  A few of
these printers "stretch" their output by 1/8,  resulting
in printouts that are distorted.  For those printers,
use this option and enter 1.125.  That will introduce an
opposite "stretching",  resulting in correct output.
   This option can be reached at any time with the F10

Slew scope 

   This option should send commands to the telescope that
will slew it to the current screen center.  It will
produce a series of comments ("port opened", "set up",
and so on) as it does this.  It appears in the main
menu when you have selected a port in the scope control
dialog,  and have chosen a scope system that supports
   This option can be reached at any time via the F11
key,  or (useful for those with keyboards that have no
F11 key) the CTRL-F1 key.
   The counterpart of this is the F12 key,  which moves
Guide to where the telescope is.

Slew Guide 
   This option should send commands to the telescope that
will find out where the telescope is pointed,  and
recenter Guide at that point.  It will produce a series
of comments ("port opened", "set up", and so on) as it
does this.
   Once you have selected a telescope port in the scope
control dialog box,  this option will appear in the main menu.
   It can be reached at any time via the F12 key,  or
(useful for those with keyboards that have no F12 key) the
CTRL-F2 key.
   The counterpart of this is the F11 key,  which moves
the telescope to Guide's current chart center.

  You can reset Guide's time to the present moment
(as measured by the computer's built in clock) with
the F3 key,  or by clicking on "Current Time" in the
time dialog.

   Hitting the F7 key results in toggling the previous
chart.  This lets you flip back and forth between two
views.  It differs from the ALT-B key,  which backs
up a view at a time,  up to five views.


Create star list 
   This option lets you generate a list of positions for GSC and
Tycho stars in the area currently on the screen. The list is
displayed in Guide's help system;  you can then print the list or
save it to a file as you would any help topic.
   When you select this option,  you are prompted to enter a
limiting magnitude,  which lets you filter out stars too dim to be
of interest.  An example of the resulting output:
  GSC #       RA (J2000)    dec           mag spec  PPM    HIP    HD     SAO
5549   935 13h45m52.351s  -11 13' 16.04"  7.66 F8 227806  67169 119852 158151
5549  1127 13h45m35.709s  -11 28' 59.79"  9.79 K0 227796        119803
5549   371 13h44m45.604s  -10 48' 07.38"  9.99 A5 227772        119669 158136
5549   192 13h44m26.834s  -10 23' 52.48" 10.59  0  044B
5549   827 13h44m28.435s  -10 34' 53.87" 11.79  0  044B
5549  1155 13h44m15.506s  -11 26' 10.00"  8.09  3  044B
   Each line of the output shows data for one star.  The GSC identifier
is the first item in each line;  next are the RA and declination for
the star.  The format for the position is the format you currently have
selected in the Settings Menu.  Next,  the magnitude of the star is
given.  If the star came from the Tycho or Hipparcos catalogs,  you
then get the spectral type and some cross-references to four catalogs.
   If the data comes from the GSC,  then there is no spectral type
data available.  Instead,  an object type is shown (usually 0 for a star
or 3 for a non-star) and the identifying number of the plate on which
that GSC star was imaged.

Field rotation rate 
   When you ask for Quick Info,  one of the many items shown will be
the field rotation rate for the center of the screen,  at your
currently set position on the earth and date and time.  If you attempt
to do CCD or photographic imaging from an alt/az mount, you can run
into the problem that the image rotates with time.  You can get around
this by adding an extra "field derotator" motor.  (Though this is
mostly theory;  such motors appear to be quite rare in reality.) In
some parts of the sky,  you can take advantage of the fact that the
rotation rate is zero when the object is directly east or west
(azimuth is 90 or 270 degrees).
   But in general,  you'll want to know this figure when imaging
from an alt/az mount.

File Menu 
   The File Menu provides controls to:

   Save a Mark (store your position and settings to a file)
   Go to Mark (recover position and settings from a file)
   Delete a Mark
   Printer Setup

   The GSC was scanned from a collection of photographic
plates.  Each plate was taken with a particular
combination of filter,  emulsion,  and telescope.  There
are ten different combinations,  labelled SERC-J1,
SERC-J2,  SERC-V,  PAL-V1,  PAL-V4,  PAL-V5,  PAL-J,
PAL-E,  GPO-V,  and BBO-B.  When you ask for information
about a GSC star,  the plates it appeared on are listed,
and so is the filter used for that plate.

Find Conjunction 
   This option,  in the Extras menu,  provides a way to find the
time and location of the conjunction of two objects.  To do this,  you
must right-click on the first object in question, click OK,  and then
right-click on the second object and click OK.
   Once you've done this,  the "Find Conjunction" option will no longer
be grayed out.  Click on it,  and Guide will find the nearest conjunction
and will display it for you.
   If one object is the Moon,  then you can click this option repeatedly
to find successive conjunctions.  Unfortunately,  you can't do that for
other cases,  such as conjunctions of Jupiter with Saturn (yet).
   If you click on two non-moving objects,  Guide will figure out that
it cannot show a conjunction,  and this option will remain grayed out.

find distance and position angle 
   You can "drag" a line to determine the distance and
position angle between two points.
   First,  click with the right mouse button on the
first point.  Holding that button down,  move the mouse
to the second point;  a line will drawn between the two.
When you let go of the mouse button,  the distance and
position angle between the two points will be shown.

Fixed levels 
   By default,  Guide draws charts with fixed levels,  in
which the size of a chart progresses in 20 steps,  from 180
degrees to 90 degrees,  and so on down to one
arcsecond.  You can't readily get,  for example,  a 3-degree
field of view,  because Level 7 has a two-degree field of view
and Level 6 has a five-degree field of view.
   But if you click on the Fixed Levels option in the Extras
Menu,  this restriction is relaxed.  Drag open a box with the
mouse,  and it opens smoothly,  instead of in steps.  You can
then turn Fixed Levels back on when you decide you want
specific fields of view.
   You can also access this option through the Ctrl-G hotkey.

FK Com 
   The FK Com type of variable star is a rapidly rotating
giant of spectral type G or K (like the Sun or a little
cooler,  but much larger).  Their surfaces are nonuniform
in brightness,  and as they spin,  we see different
amounts of light.  They may be a later version in the
evolution of contact binary stars.

Fundamental Katalog 5 
   The FK5,  or Fifth Fundamental Catalog,  contains
1,535 stars distributed in a roughly even manner across
the entire sky.  Its purpose is to provide a reference
standard for determining the positions of other stars.

   Almost all stars visible to the unaided eye (that is
to say, those brighter than magnitude 6) have a
Flamsteed number.  These stars are referred to by the
Flamsteed number and constellation,  such as 61 Cygni or
40 Eridani.  When you see a star with a number on it in
this program,  that's its Flamsteed number.  You can turn
these numbers off inside the Star Display dialog.
   You can find a particular Flamsteed star through the
Go to Bayer/Flamsteed menu under the Go To menu.

   Occasionally,  there will be a brief,  high temperature
outburst in the solar atmosphere (chronosphere).  These
appear as bright areas on the sun,  called flares.  They
are usually associated with sunspots,  and can cause
magnetic and radio disturbances on the earth.
   A few stars,  such as UV Ceti,  have flares of their
own of such violence that the star will shoot up in
brightness by several magnitudes,  usually within a
few seconds.  This type of variable star is called a
flare star.  (The Sun's flares aren't quite in this

equatorial radius 
polar radius 
   A rotating planet will tend to bulge out slightly at its equator,
resulting in an "oblate spheroid". For example,  the earth has an
equatorial radius of 6378.14 km, but a polar radius of 6356.75
km.  This difference of 1 part in 298.257, or .003353,  is the
flattening of the earth.
   Jupiter and Saturn each rotate more than twice as rapidly as
we do and have much lower densities; the combination gives them
much greater flattening,  of 0.06487 and 0.09796.

Fluctuating X-ray 
   Fluctuating X-ray stars are a form of X-ray variable
where the levels of X-ray and visible radiation fluctuates
over a few hundredths of a second.  Examples are Cygnus
X-1, also known as V1357 Cyg,  and V821 Ara.

   FOKAT-S is a star catalog for the southern sky,
assembled from photographs taken around 1984 in a Soviet-
Bolivian cooperative effort.  The entire region was
photographed four times,  which helped to reduce errors
significantly.  Many of the positions and proper motions
listed in the PPM come from this survey.

FU Orionis 
   FU Orionis type variable stars will gradually brighten
by about six magnitudes over several months,  then either
stay almost constant of slowly drop down by one or two
mags.  They are probably a stage in the development of T
Tauri type stars.  All presently known FU Ori variables
are coupled with reflection nebulae.

full screen 
   Hitting the Tab key toggles between full screen mode
and "menu visible" mode in the DOS version.

Galactic latitude 
Galactic longitude 
galactic coordinate 
galactic equator 
   Galactic coordinates are an alternative way to specify
positions in the sky.  The galactic equator runs roughly in
the plane of our galaxy;  thus,  points with a zero
galactic latitude are in the galaxy,  and as the latitude
departs from zero,  one gets away from the Milky Way.
The zero point,  or "prime meridian", for galactic
longitude is in the direction to the center of the galaxy,
in Sagittarius.  Galactic lat/lon is usually given in
decimal degrees.
   Many objects tend to concentrate in the plane of the
galaxy (certain types of nebulae,  for example).  The
convenient thing about galactic coordinates is that you
can quickly tell how close to the plane of the galaxy a
given object is.  This is not a truly immense convenience,
which is why galactic coordinates don't get a lot of use.
   By default,  Guide displays the galactic equator;
you can turn it off in the Measurements dialog,  or
with the Alt-F3 hotkey.
   You can also have Guide show the galactic coordinates
of the cursor in the legend;  this is turned on or off
through the legend dialog.
   If you want to select a position in galactic coordinates,
click on the galactic position shown in the legend,  or
hit Alt-. (Alt-period),  and Guide will prompt you to
enter the new position.

   Galaxies are collections of from thousands to
trillions of stars.  Our own galaxy,  the Milky Way, is
a spiral of about a hundred billion stars stretching a
hundred thousand light-years across.
   Galaxies come in several forms.  Our own is a spiral;
it has a dense core from which two arms point out and
spiral around the center like a pinwheel.  Others are
elliptical galaxies,  which vary from being almost round
balls of stars to being stretched out into almost cigar
shapes.  Others are irregular galaxies;  as the name
implies,  these have no particular shape.
   Galaxies are shown in this program as ovals followed
by a Messier,  NGC,  IC,  or PGC number.
   You can find a galaxy by using the Go to Galaxy option
in the Go To menu.  You can choose from any of several
catalogs or use a "common name" such as "Maffei I" or
"Whirlpool".  (You can also find it by Messier, NGC,  or
IC number.)

galaxy inclination 
   The galaxy inclination measures the angle at which a
spiral galaxy is tilted from us.  A galaxy with a 90
degree galaxy inclination is seen edge on.  One with a 0
degree galaxy inclination is seen face on.

Galaxy RV 
   A Galaxy RV (radial velocity) is a measure of its
velocity toward (or,  more frequently) away from us.  It
is usually determined by measuring its "redshift",  the
reddening of light caused by the Doppler effect.  When you
examine the light from a galaxy,  you'll notice that its
spectrum shows a shift to redder (longer) wavelengths.
By measuring the amount of shift,  one can determine the
radial velocity of the object.
   The velocity can be determined by looking at emissions
from neutral hydrogen or from the shift in optical light.
The observations will give a velocity relative to the
earth. They are then corrected to give a velocity relative
to the sun (a heliocentric velocity),  and can then be
corrected to give velocity relative to the GSR (the
galactic standard of rest),  or to the 3K background,
the microwave emissions from the Big Bang.

Gamma Cas 
   The Gamma Cas type of variable star rotates so rapidly
that centrifugal force can push matter at the equator into
a ring or disk.  When this happens, the star may fade by
up to 1.5 magnitudes.  These are usually Be stars.

gamma ray 
   Gamma rays are emitted by the most energetic and
usually cataclysmic of cosmic events.  Gamma radiation is
really just light with a very short wavelength;  it is
produced by some astronomical objects,  such as quasars.
It's not easily studied from the ground,  but the Compton
gamma-ray observatory has,  from orbit,  studied many
objects at gamma-ray wavelengths.
   In particular,  "gamma-ray bursters" have grabbed a lot
of interest.  These objects release huge amounts of gamma
radiation for a brief time (usually a few seconds),  and
then vanish from view.  There are many theories as to
their cause,  including such exotic possibilities as
colliding neutron stars.