heliocentric 
   Heliocentric means "using the Sun as the center".  It
usually refers to the concept that the Earth and other
planets orbit the Sun,  as opposed to the earlier idea
that everything revolves around the earth (the geocentric
theory of Ptolemy and others).
   In this program,  a "heliocentric" datum means
"measured from the Sun",  while a "geocentric" datum is
measured from the center of the Earth.  A topocentric
datum is measured from the observer's position on the
surface of the Earth.

Heliocentric position 
   When you ask for "more info" on a planet,  you will be
told its heliocentric position,  the place where it
would be found relative to the Sun.  (Helios is the Greek
word for the Sun.)

HH 
Herbig-Haro objects are small emission nebulae, which are
shock-exited by the interaction of gaseous material, expelled
from protostars, with the surrounding material.
Reference: Herbig, G.H.: 1975, Lick Observatory Bulletin, No. 658;
Draft catalog of Herbig-Haro objects.

HICK 
Hickson 
   The Hickson catalog lists 100 compact groups of
galaxies,  such as Stephan's Quintet and similar groups.
It was compiled by searching the entire set of Palomar
Observatory Sky Survey plates for small groups of
galaxies.  Many of these groups are extremely dim,  but
they should be of interest to CCD observers and those
with extremely powerful telescopes.
   A galaxy in the Hickson catalog is described by a
number and a letter.  HICK 34C,  for example,  is the
third galaxy found in cluster #34.  Some clusters contain
up to seven galaxies.  You can find an object by its
Hickson number by using the Go to Hickson option in the
Go to Galaxy menu in the Go To menu.

High-Precision Subset 
HPS 
   Of the stars in the PPM,  31,841 belong to the HPS,
or High-Precision Subset.  The positions and proper
motions of stars in this subset are considered to be to
better than normal accuracy.  Positions are usually good
to about .12 arcsecond;  proper motions are usually
accurate to about .24 arcseconds/century.

Hilda 
   The gravity of Jupiter has a strong effect on the
orbits of asteroids,  making some periods of revolution
around the Sun more likely,  others less so.  In
particular,  if an asteroid takes 2/3 as long as Jupiter
to orbit the Sun,  Jupiter's gravity will tend to "hold"
it in that orbit.  This is an example of resonance.
   The first asteroid found with this property was Hilda.
Subsequent examples are members of the Hilda orbital
family.

Hipparcos 
HIP 
   The Hipparcos (HIP) data was collected as part of the
mission of the Hipparcos satellite.  It contains the most
precise measurements of positions,  parallaxes,  proper
motions, and magnitudes ever made,  far better than in any
other catalog.  Unfortunately,  it only contains data for
118 218 stars;  so for most dimmer stars,  other data sources
must be used.  It contains most stars down to magnitude 8,
with a sampling of dimmer stars.  The dimmer stars were mostly
chosen for unusual or interesting properties:  variables,
multiple stars,  nearby stars,  and so forth.

Hipparcos satellite 
   The Hipparcos satellite was a European Space Agency
experiment to collect vastly improved information about the
fundamental properties of stars.  Because it was above the
atmosphere,  it was able to collect extremely precise
parallax,  proper motion,  and magnitude data for
a total of over 1 million stars.  The Hipparcos and
Tycho datasets resulting from this effort are used by
Guide as the basic data for all stars brighter than
about magnitude 10.5,  and many stars fainter than this.

hotkey 
? 
?1 
Click for first page of hotkeys 
!Cursor keys pan (or move mouse;  use CTRL-F4 to toggle in DOS)
Space bar to stop drawing           ! Switch to French
Enter to force a redraw             ~ Switch to German
Del  "right mouse button"           " Switch to Italian
Ins "left mouse button"             ' Switch to Spanish
5 (on numeric keypad) centers cursor (DOS)
% toggle colored stars              ALT-F1 toggle ecliptic (DOS)
$ toggle star outlining             ALT-F2 toggle horizon (DOS)
# star display dialog               ALT-F3 toggle gal. equator (DOS)
& enter alt/az                      ALT-F4 quit
( set cross-marks on trails         Alt-F5 Language menu (DOS)
) go to new level                   Alt-F6 Create RealSky image
* zoom in                           Alt-F8 "Full" cursor (DOS)
+ brighten stars                    Alt-F9 Toggle user datasets
, Glossary                          Alt-F11 Clear all RealSky images
- dim stars                         CTRL-F1 or F11 aim LX-200
/ zoom out                          CTRL-F2 or F12 recenter on LX-200 loc
1-9 go to level 1-9                 CTRL-F3  Toggle Chart Mode
; find an asteroid                  CTRL-F4 use mouse to pan (DOS)
= find a planet                     CTRL-F6 toggle Telrad (DOS)
> select constellation              CTRL-F8  Recenter CCD frame
? this list                         CTRL-F9  Enter current time as text
@ toggle overlays menu              Ctrl-F11 Reset line of variation
                                    Ctrl-F12 Add objects from an MPC file
ALT-, Enter an ecliptic coordinate (DOS)
ALT-. Enter a galactic coordinate (DOS)
Alt-/ Set GRS position (DOS)        CTRL-A toggle animation
Alt-[ Coordinates menu              CTRL-B enter Bayer/Flam desig
ALT-0 set time to 0:00 UT           CTRL-D data shown menu
ALT-1...9 go to level 10-19         CTRL-E enter new epoch (DOS)
ALT-A animation menu                CTRL-F CCD frame on/off
ALT-B back up one chart             CTRL-H set home planet
ALT-C printer setup menu (DOS)      CTRL-K add a new comet
ALT-D display menu (DOS)            CTRL-L toggle legend
ALT-E enter RA/dec                  CTRL-N find NSV star
ALT-F File menu                     CTRL-T measurements menu
ALT-G "go to" menu                  CTRL-U set mag text limit (DOS)
ALT-H Help menu                     CTRL-V or \ find variable
ALT-I inversion dialog              CTRL-X enter longitude
ALT-K go to comet                   CTRL-Y enter latitude
ALT-L "flashlight" mode (DOS)       CTRL-Z enter altitude
ALT-M select video mode (DOS)       F1 get help
ALT-N show full sky view overhead   F2 print (DOS)
ALT-O overlay menu                  F3 set current time
ALT-P Create a PostScript chart     F4 scope control
ALT-Q "quick info"                  F5 user object dialog
ALT-R red mode toggle               F6 reset level size
ALT-S settings menu                 F7 toggle to prev chart (DOS)
ALT-T time dialog                   F8 create list of stars (DOS)
ALT-U find 'opposition' point       F9 color settings menu
ALT-V "freeze" view (DOS)           F10 "stretch" printouts (DOS)
ALT-W horizon dialog                [ double anim rate
Alt-X Extras menu                   ] halve anim rate
ALT-Z north/zenith up               { go to mark menu
I find IC object                    | delete a mark
M find Messier object               } save a mark
N find NGC object
SHIFT-F1 rotate chart (DOS)
SHIFT-F2 change scope delay time (DOS)
SHIFT-F3 add to print queue (DOS)
SHIFT-F4 flush print queue (DOS)
Shift-F9 Tables menu (DOS)
Shift-F11 Extract A1.0 (DOS)
Tab full-screen mode (DOS)

Click on a point with right mouse button to get info on an object
Click on a point with left mouse button to recenter on that point
Click on a point with middle mouse button to recenter on that point
   and simultaneously zoom out one level
Click and drag right button to find distance and position angle
Click and drag left button to zoom and recenter

Hpmag 
   Magnitudes given by the Hipparcos dataset were measured using
a device sensitive to a wide range of wavelengths (mostly visual).
These magnitudes are unique to the detector on the satellite;  they
correspond roughly,  but not exactly,  to visual magnitudes,  and
are called "Hipparcos magnitudes",  symbolized as Hpmag.

HR 
Bright Star 
   The Yale Catalog of Bright Stars, or HR catalog,  is
a list of 9,096 of the brightest stars in the sky.  This
isn't a lot of stars -- in fact,  it's much smaller than
the 258,857 stars in the SAO catalog or the 18 million
objects in the GSC catalog -- but it does contain a lot
of interesting information on each star,  such as its
Bayer and Flamsteed designations, proper motion,
parallax,  magnitude,  and miscellaneous comments.
   You can find a star's HR number by clicking on it with
the right mouse button.  (Of course,  it may not have
one.)  You can go to any HR star by using the Go to Yale
(BSC) option in the Go to Star menu,  in the Go To
menu.

Hubble class 
   Some galaxies shown by this program have been
classified as elliptical (E),  spiral (S),  barred spiral
(Sb),  or irregular (I),  or other categories,  known
as the galaxy's Hubble class.

IC 
Index Catalog 
   The IC,  or Index Catalog,  is an extension of the NGC
list of galaxies,  nebulae and clusters.  It contains
positions and descriptions for over 5,000 objects.

illuminated 
   As planets and moons move in their paths,  the amount
of area that appears illuminated varies as well.  For
example,  at Full Moon,  almost 100% of the Moon is
illuminated;  at New Moon,  almost 0%.  A solar system
object's apparent brightness varies according to how
fully illuminated it is,  how close it is to you,  and
how close it is to the Sun.
   Among planets,  Mercury and Venus range from 0% (no
illumination) to 100% (full illumination).  They can do
this because they orbit more closely to the Sun than we
do,  and can pass between us and the Sun (just as the
Moon can).  The other planets are always close to 100%
illuminated.

inclination of orbit 
   Most objects that circle the Sun have orbits in roughly
the same plane as the Earth's orbit,  called the ecliptic.
That's why the planets,  and most asteroids,  are always
found in a fairly narrow band around the sky,  passing
(mostly) through the zodiac constellations.
   The angle of an object's orbit with the Earth's orbit
is the inclination of orbit,  measured in degrees.  For
most asteroids,  this is under ten degrees.

index marks 

( 
   The index marks menu option toggles between the addition
of index marks to planetary trails,  and their omission.
   When you click this item on,  you'll be asked for the
frequency at which index marks should be added to trails.
If you are creating (or have created) a trail with the
animation step size set to two days,  for example,  and
you would like an index mark every ten days,  you would
enter 5 here.
   You can reach this option at any point in Guide with
the ( hotkey.

infrared 
   The human eye can see a range of visible light from
red to blue.  Light just outside the red limit is called
infrared,  or "heat",  radiation.  Its wavelength ranges
roughly from 1 millimeter down to 700 nanometers.
   As you might expect from "heat" radiation,  infrared
radiation is associated with warm objects.  For example,
some night-vision equipment detects warm-blooded animals
by the infrared radiation they emit.  The IRAS satellite
examined a wide range of objects,  such as galaxies,
comets,  and asteroids,  in the infrared.

interference 
interferometry 
speckle interferometry 
interferometer 
interferometric 
   When you have two (or more) sources of light at the
same wavelength,  the waves can either add together or
cancel out.  This process is called interference,  and
the gadgetry that makes use of this fact is called an
interferometer.
   The first astronomical use of this was in the 1920s,
when it was used to find the angular diameter of the star
Betelgeuse in Orion.  Betelgeuse is large enough so that,
with the right equipment and the correct setting, the
hills in interference patterns caused by light from one
side of the star would just fill up the valleys in those
caused by from light from the other side.  By measuring
where that happens,  you can determine the angular diameter
of the star.
   Unfortunately,  current interferometers can only measure
the sizes of a few stars with very large apparent diameters.

Inversion dialog 

ALT-I 
   The Inversion dialog lets you invert or flip the screen
to match what you see in your telescope.  Because your
average scope does involve mirrors and lenses,  it's
likely that by the time an image passes through and is
reflected by all of them,  it will be oriented in one of
the following ways:








 upright       inverted    flipped N/S    flipped E/W

   All four options are available in this menu,  with the
current choice marked with a check mark.  The menu also has
an option for adding any desired rotation,  and four radio
buttons to select between celestial north,  the zenith,  or
ecliptic or galactic north at the top of the chart.
   Which way your scope actually flips the image depends
on what kind of scope you have and what eyepieces you are
using.
   You can reach this menu at any time with the ALT-I
hotkey.   Also,  if the legend is shown and the compass
box is turned on,  you can click on the compass box to
reach this menu.
@c 50,135,15   head
@m 50,150      start at neck
@l 50,180      go to crotch
@l 35,210      right foot
@m 50,180      crotch
@l 65,210      left foot
@m 20,125      right hand
@l 35,155      rt should
@l 65,155      lft should
@l 70,185      left hand
@c 150,195,15   head: inverted guy
@m 150,180      start at neck
@l 150,150      go to crotch
@l 165,120      right foot
@m 150,150      crotch
@l 135,120      left foot
@m 180,205      right hand
@l 165,175      rt should
@l 135,175      lft should
@l 130,145      left hand
@c 250,195,15   head: flipped N/S guy
@m 250,180      start at neck
@l 250,150      go to crotch
@l 235,120      right foot
@m 250,150      crotch
@l 265,120      left foot
@m 220,205      right hand
@l 235,175      rt should
@l 265,175      lft should
@l 270,145      left hand
@c 350,135,15   head: flipped E-W guy
@m 350,150      start at neck
@l 350,180      go to crotch
@l 365,210      right foot
@m 350,180      crotch
@l 335,210      left foot
@m 380,125      right hand
@l 365,155      rt should
@l 335,155      lft should
@l 330,185      left hand

Io 
   Io is the innermost large satellite of Jupiter.  It is
about the size of the Earth's Moon,  and is one of the
four large Jovian satellites found by Galileo.
   Most of what we know about Io comes from the Voyager
flybys.  These show a geologically active planet, complete
with volcanoes and a thin atmosphere.
   Io is about as far from Jupiter as the Earth is from
its moon.  From Io,  Jupiter's diameter would appear to
be that of about forty full moons.

ionized 
ionizes 
ionization 
   "Normally",  an atom will have a net electrical charge of
zero;  it will have a nucleus with a certain number of
protons and neutrons,  surrounded by electrons, and the
number of electrons and protons will be equal to each other.
   Under some circumstances,  though,  one or more electrons
will be stripped from the atom;  this is known as ionization.
It takes energy to accomplish this separation;  this can
be provided by having the atom absorb a highly energetic
photon (of ultraviolet or higher energy),  or sometimes by
having it collide with another atom in a very hot gas.
   Eventually,  the atom and its lost electron are apt
to recombine;  when they do,  they will emit a photon.

IRAS 
   Many galaxies will have listed,  under their "alternate
designations",  an IRAS designation,  from the list of
objects observed by the Infrared Astronomy Satellite.  In
the early 1980s,  this satellite performed the first
detailed examination of the sky in the infrared.

irregular variable 
   Variable stars classified in this program as irregular
variable stars are objects that haven't been studied well
enough to determine how they vary and,  in some cases,
what their spectral type might be.

Islamic calendar 
   The Islamic calendar is a lunar-only calendar.  It makes only a minor
effort to track the solar cycle,  so a given month can occur in any season.
There are twelve months:

Muharram (30 days) (holy month)
Safar (29 days)
Rabi'a I (30 days)
Rabi'a II (29 days)
Jumada I (30 days)
Jumada II (29 days)
Rajab (30 days)  (holy month)
Sha'ban (29 days)
Ramadan (30 days)  (fasting month)
Shawwal Dhu (29 days)
Dhu al-Q'adah (30 days)  (holy month)
Dhu al-Hijjah (29 days;  30 in leap years)  (holy month)

   As you can see,  months alternate between 29 and 30 days,  for an
average month length of 29.5 days.  The actual average "lunar month"
is 29.530588 days;  to accommodate this,  an extra day is inserted
into the last month in 11 years out of every 30.  The resulting
calendar month is about 2.9 seconds shorter than the real lunar month.

   In theory,  months begin with the first sighting of a crescent moon.
This gives rise to several problems,  such as the fact that you cannot
create a calendar in advance (though some work has been done to improve
predictions of crescent visibility).  The above system,  used in Guide,
is also used in some Muslim countries.

   The Islamic calendar begins with 16 July 622 (Julian calendar),  which
corresponds to 1 Muharram 1.  Be aware that there is some dissent on the
subject of this starting point,  with some sources stating that the date
should be 15 July 622.

isophotes 
   By default,  Guide draws the Milky Way and some prominent
nebulae with their actual shapes.  This is controlled with the
isophotes option in the Display menu.  Turn this option off,
and the Milky Way and nebula outlines will vanish.
   The isophotes shown by Guide were created by Eric-Sven
Vesting as part of the Nebula Databank,  and were derived from
RealSky images.  This makes them extremely detailed and precise.
Unfortunately,  RealSky doesn't extend below -16 declination,
and the nebulae isophotes therefore stop at that point.

IUE 
   The IUE,  or International Ultraviolet Explorer,  was
launched in early 1978,  and placed in geosynchronous
orbit over the Atlantic Ocean.  Its sole purpose is to
examine the spectra of objects in the ultraviolet,
in the 50 to 3000 Angstrom range (just below the range
visible to humans).  This has to be done from space,
since light in that range is absorbed by the Earth's
ozone layer.
   The spacecraft has a 45 centimeter (18 inch) telescope,
with two spectrographs and television cameras to collect
the data.  It has collected a great deal of data about
most of the brighter stars,  as well as assorted novae,
comets,  and supernovae.  It was intended to last
three years,  but as of this writing (January 1993),  is
still collecting data.