Positions of objects in the sky are specified by a
right ascension,  declination and an epoch.  The
epoch is a date needed because RA and declination are
measured relative to the Earth's pole and orbit,  both
of which shift with time.
   In the past,  1950 (or "B1950.0") was a commonly used
epoch.  More recently,  almost all astronomical data has
been updated to the 2000 (or "J2000.0") system,  which is
the system this program uses by default.  The "J" means
Julian epoch;  the "B" refers to the older Besselian

J2000 position at epoch 
   Star positions given as "J2000 position at epoch" have
been corrected for proper motion,  but the coordinates
have been left in the J2000 reference frame.

Jalali (Persian) calendar 
   The Jalali (Persian) calendar is the official calendar of
Iran and of parts of surrounding areas such as Afghanistan and
Central Asia.  The year begins on the day of the vernal equinox;
if this occurs before midday,  Teheran local time,  that day is
1 farvardin ("New Years Day").  Otherwise,  the next day is
1 farvardin.
   The result is that most years are 365 days long,  with almost
a quarter having an extra day,  much as in the Gregorian calendar;
but the pattern is not as "orderly" as the Gregorian pattern.
Usually,  leap years are four years apart,  but sometimes,  they
are five years apart.  The months are:

farvardin (frvrdyn)     (31 days)
ordibehesht (ardybhSt)  (31 days)
khordad (Krdad)         (31 days)
tir (tyr)               (31 days)
mordad (mrdad)          (31 days)
shahrivar (Shryvr)      (31 days)
mehr (mhr)              (30 days)
Aban (Aban)             (30 days)
Azar (AZr)              (30 days)
day (dy)                (30 days)
bahman (bhmn)           (30 days)
esfand (asfnd)          (29 days;  30 in leap years)

   Some sources suggest that,  instead of being based on the vernal
equinox,  the calendar uses a pattern of 683 leap years in a cycle of
2820 years,  which closely matches the vernal equinox rule.

   The jansky is a fundamental unit for radio astronomers,
used in measuring the amount of radio energy detected from
an object.

   It's common for accretion disk objects to also show
jets.  Jets of outflowing matter form at right angles to
the plane of the accretion disk. Often they move at a good
percentage of the speed of light,  in which case Doppler
shift makes the jet approaching us bluer than normal,  and
that moving away from us redder than normal.
   The mechanism causing jets is still not well
understood.  It should be clear,  though,  that any matter
flowing out from an object with an accretion disk would
only be apparent at right angles to the disk.  Otherwise,
it would run into the disk and we would not find it easy
to see.

   The Johnson magnitude system is a widely-used standard for
devices measuring stellar magnitudes.  It defines a series of
photometric bands and the filters used to measure them.
For stars from the Hipparcos and Tycho catalogs,  Guide is
able to provide a "Johnson V" (visual) magnitude;  this comes
either from observations made on the ground,  or is computed from
the BT magnitude and VT magnitude.

Julian calendar 
   The two calendars most frequently used by astronomers
are the Julian and Gregorian calendars.
   The Julian calendar was created by Julius Caesar's
resident expert in such matters,  a Greek named Sosigenes.
He set up the months as we now know them and added an
extra day in February every fourth (leap) year.  This
provides where each year averages 365.25 days,  which is
pretty close to the actual length.
   However,  there is an error of about three days every
four centuries.  By 1582,  the calendar was about 10 days
out of adjustment to the real world.  Pope Gregory XIII
took two steps to deal with this.  First,  he decreed that
4 Oct 1582 would be followed by 15 Oct 1582.  Secondly, to
keep this problem from recurring,  he decreed that three
out of every four century years (those ending in 00) would
not be leap years.  Thus,  1700, 1800, and 1900 were not
leap years,  even though they are divisible by 4,  but
2000 will be a leap year.  This is the calendar used in
most of the world today.
   Regrettably,  it took from 1582 to 1918 for the Julian
calendar to completely die out,  so you do have to make
clear which one is being used for dates in that interval.
(Now,  except for some religious purposes,  the Julian
calendar is obsolete.)

Julian Day 

   The usual method of dealing with dates (expressing them
in days,  months,  years,  hours,  minutes,  and seconds)
can be a little tricky to deal with mathematically.  Try
to figure out the difference,  in days,  between 4 Jul
1776 and 1 Sep 1939,  and the problem will be evident.
   To avoid this problem,  astronomers often express a
time in terms of Julian Day.  (The "Julius" involved is
not Julius Caesar,  and this system is unrelated to the
Julian calendar.)  JD 0.0 corresponds to 1 Jan -4712;
any other JD is the number of days since then.  Thus,  1
Jan 2000 is the same day as JD 2451545.  Decimal fractions
correspond to fractions of a day.  This program uses the
JD system internally,  and it is used almost universally
when astronomical calculations are made.  It is possible
that the time of an astronomical event will be given to
you as a JD,  in which case you can enter it inside the
Time dialog.

Julian epoch 
Besselian epoch 
   The "J" (or sometimes "B") before an epoch introduces
a minor twist,  negligible by all save people intent on
fanatic-grade accuracy.  The "J" indicates a Julian epoch,
as distinguished from the older "B" or Besselian epoch.
Julian epochs measure the year as 365.25 days exactly,
the length of a year in the old Julian calendar;
Besselian epochs have 365.2421988 days,  matching a "real"
(tropical) year.  This makes for a small difference in the
epoch date,  and therefore in actual positions.  If there
is no letter given,  assume 1950 and earlier epochs to be
Besselian,  and later to be Julian.  Alternatively,
ignore the difference unless you need an accuracy of
better than about .1 arcsecond.

   Jupiter,  the fifth planet from the Sun, is
appropriately named for the king of the gods.  This planet
has eleven times the diameter of our own.  If you took
the surface of the Earth and stretched it out on Jupiter,
it would look like the area India covers on the Earth.
   Jupiter is five times as far from the Sun as we are,
causing it to take twelve years to go around the Sun
and providing it with 1/25 as much sunlight for a given
   The planet's atmosphere is about 80% hydrogen, 20%
helium,  plus slight traces of methane and ammonia.  The
nature of its interior is still mostly unknown.  It has
fairly violent weather;  one hurricane,  the Great Red
Spot,  was seen by Galileo in 1610 and is still there now.
   Jupiter has four major moons,  each roughly the size
of our own,  found by Galileo: Io,  Europa,  Ganymede,
and Callisto.  If you zoom in on Jupiter in this program,
the four moons will appear.  There are at least a dozen
other moons,  all of much lesser size.  You can see the
big four with binoculars,  and easily with a small scope
(which is all Galileo had back then.)
   Most of what we know about Jupiter and its satellites
was found by four probes,  Pioneer 10 and 11 and Voyagers
1 and 2.  Each returned many pictures and measurements of
these objects.  In 1995,  the Galileo probe should carry
out further observations.

Jupiter satellite events 
   When you request "more info" on Jupiter,  among other
data, a list of the events of Jupiter's moons is shown.  As
Jupiter's four largest moons orbit Jupiter,  they cast
shadows are themselves eclipsed by Jupiter,  and pass in
front of and behind Jupiter. All of these events can be
observed with small telescopes.
   The list shows events for the next 7 days.  For each event,
the number of the moon is given (I=Io,  II=Europa,  III=Ganymede,
IV=Callisto),  followed by the type of the event:

   Sha = Shadow cast by the satellite on Jupiter
   Ecl = Moon is eclipsed by Jupiter
   Tra = Moon crosses the disk of Jupiter
   Occ = Moon is blocked from view by Jupiter

   Also,  each line tells you if this is the beginning or end of the
event,  and its time and date.

Jupiter System I 
Jupiter System II 
Jupiter System III 
   Measuring longitude values on Jupiter is made difficult
by the fact that the planet rotates more rapidly near the
equator than it does at the poles.  So three systems are
used.  Jupiter System I is used for features within about
10 degrees of Jupiter's equator,  where a full rotation
takes about 9 hours, 50.5 minutes.  Jupiter System II is
used for features north and south of this zone (such as the
Great Red Spot), where a rotation takes about 9 hours,
55.677 minutes.
   There is another system,  Jupiter System III,  which
is based on the rotation of Jupiter's interior;  it is used
for radio observations,  and is not particularly useful for
visual observers.  This rotation time of 9 hours, 55.495
minutes probably reflects the rate at which the solid core
of Jupiter rotates,  far below the cloud layers.

   Pieter Dirckszoon Keyzer was a Dutch explorer of the
1590s.  During some trips around Africa to the Far East,
he had an opportunity to be one of the first Europeans to
observe the southern sky,  and came up with twelve
constellations that have survived to the present:  Apus
the Bird of Paradise, Chamaeleon the Chameleon,  Dorado
the Goldfish (a large fish found in tropical oceans),
Grus the Crane,  Hydrus the Sea Monster,  Indus the
(American) Indian, Musca the Fly,  Pavo the Peacock,
Phoenix the Phoenix,  Triangulum Australe the Southern
Triangle, Tucana the Toucan,  and Volans the Flying Fish.
Keyzer may have borrowed these groups from the local
peoples.  He managed to get them into Bayer's 1603 atlas,
thereby ensuring their continued use.

   A kiloparsec (kpc) is,  logically enough, 1000 parsecs,
or roughly 3,260 light-years.  It is a convenient unit for
measuring distances to globular clusters. (For comparison,
the center of our galaxy is about ten kpc from Earth.  Our
galaxy is about 30 kpc in diameter,  and the nearest spiral
galaxy to our own,  M-31 in Andromeda,  is about 900 kpc

   One km (kilometer) is equal to 1000 meters,  or
.6214 miles.  There are 149597870 kilometers in one AU.

Kuiper belt 
   In 1951,  the astronomer Gerard Kuiper suggested that
there might be billions of small,  icy objects just beyond
the orbits of Neptune and Pluto.  These objects would be
spread out over so large a volume that they wouldn't tend
to collect to form actual planets.  They would also give a
possible explanation as to why there are so many
short-period comets.
   This idea had to remain in the realm of theory for four
decades;  these objects were too dim to be detected until
1992.  Since then,  several dozen of these Kuiper belt
(or "transneptunian") objects have been found.  They
tend to brightnesses of about magnitude 23.

La Caille 
   In the 1750s,  the astronomer La Caille filled in some
of the gaps then existing between constellations with some
of his own.  Most are scientific apparatus of the era
(Antlia the Air Pump, Telescopium the Telescope,  Fornax
the Chemical Furnace).  He also created Sculptor and
assorted tools,  such as Caelum the Chisel,  Pictor the
Easel,  and Norma the Level,  supposed to be used by
Sculptor in his work.  There are a total of 14 La Caille

Language menu 
   The Language Menu,  within the Settings Menu,  allows you to
switch Guide's user interface to English,  French,  German,  Spanish,
Italian,  Japanese,  Dutch,  or Russian.  All menus and dialog boxes,
and much of the text used elsewhere in Guide,  will be shown in the
new language.  (The "help" data currently exists only in English,
German,  Italian,  and Dutch,  so it obviously can't be shown properly
in the other languages yet.)
   As of this writing,  some versions are incomplete translations;
some text will still appear in English.  More complete versions will be
available at


   as they are created.  Also,  more languages may be posted if translators
are found.
   It should also be noted that the Japanese version cannot work in the
DOS software,  due to font limitations.  Also,  the Windows software can
have problems showing Japanese or Russian if you don't have suitable fonts

   Latitude and longitude are numbers used to specify
points on the Earth's surface.  For example,  Project
Pluto's corporate headquarters is located at latitude N
43.01 degrees,  longitude W 69.70 degrees; those two
numbers specify its position to within a mile or so.
(With enough digits,  the location of individual atoms
could be specified.)  You can find your latitude and
longitude on a USGS map,  or,  with some training and a
good watch,  you can calculate it from observing stars.
   This program uses latitude and longitude when figuring
out such information as rise and set times or the altitude
and azimuth of an object.  Also,  if you want to find
solar eclipses,  you must provide the lat/lon of the place
where the eclipse is visible.  (A solar eclipse visible in
one place won't necessarily be visible elsewhere.)  To
enter a new lat/lon,  use the Location dialog in the
Settings menu.

Level Size 

   The Level Size option lets you reset the field of view
shown at a particular level.  For example,  by default,  the
field of view at zoom level 9 is 30 arcminutes,  or .5
degree.  Suppose you would like to reduce this to 24
arcminutes,  or .4 degree.  You would go to level 9,  and click
on this item.  You are asked to "Enter new screen size:"  Enter
24',  or .4 (degrees),  or 1440".  The screen will be redrawn
at the new size.  You can also access this feature with the
F6 hotkey at any point in the program.

   In general,  we see only one face of the moon.  In
reality,  the moon appears to rock slightly from side to
side and from top to bottom.  This tilting is called
libration,  and it allows us to see about 59% of the
moon's surface,  instead of the 50% we would otherwise
   There are three causes of libration.  Most significant
is the fact that the moon spins at a fairly constant rate,
but orbits us at a somewhat variable rate because its
orbit is not a perfect circle.  This is called the optical
libration,  because it is caused by our viewpoint and not
by any real changes in the moon's spin.
   Also,  as the earth turns,  the angle at which an
observer sees the moon changes slightly.  This effect can
be as great as about one degree,  and isn't quite as
important as the optical effect.  This is called the
diurnal (daily) libration.
   Finally,  there is a real,  but very minor,  rocking
motion that amounts to a few arcminutes.  This effect is
called the physical libration.

light curve 
   If you plot the brightness of a variable star over
time on a graph,  you have produced a light curve.  The
shape and dimensions of a light curve can tell you what
kind of variable star you're looking at and what sort of
processes are going on in it.

   A light-year is the distance light travels in a year.
Note that this is a unit of _distance_, not time.  (The
'year' part confuses some people.)  It is equal to 5.88
trillion miles,  or 9.46 trillion kilometers.  It's a
standard unit for measuring distances to objects outside
our solar system.

limb angle 
   The moon's libration is often expressed as a total
amount and a limb angle.  The limb angle tells you
which part of the moon is tilted toward you.  It is
measured from lunar north,  at 0 degrees,  counter-
                                  Mare Humboldtianum

   Sinus Roris
                                          Mare Marginis

      90                                     270

                                      Mare Australe

   For example,  if the limb angle is about 220 degrees,
Mare Australe is tilted toward the earth and is in a
better than usual position for viewing.  The total amount
of tilt can range up to about 8 degrees.  As a result,
about 59% of the moon can be seen at one time or another.
@c 220,240,120   Lunar disk
@m 327,220  Mare Crisium
@l 328,209
@l 325,199
@l 314,190
@l 306,199
@l 308,209
@l 316,220
@l 327,220
@m 334,224  Mare Undarum
@l 335,230
@l 329,228
@l 331,224
@l 334,224
@m 338,220   Mare Marginalis
@l 337,220
@l 333,203
@m 334,203
@m 108,199   Oceanus Procellarum (limb)
@l 112,240
@l 124,260
@l 151,271
@l 148,281
@l 141,281
@l 154,299
@l 161,299
@l 167,294
@l 170,298
@l 168,301
@l 171,308
@l 183,305
@l 183,299
@l 202,299
@l 211,294
@l 206,260
@l 189,246
@l 204,246   Sinus Medii
@l 232,240
@l 242,224
@l 238,209
@l 225,203
@l 218,209
@l 220,220
@l 226,224
@l 204,234
@l 204,224
@l 214,218
@l 204,209
@l 234,184
@l 255,209
@l 259,240   mare nectaris
@l 266,240
@l 266,260
@l 279,260
@l 276,281
@l 283,284
@l 292,281
@l 295,266
@l 291,260
@l 284,260
@l 287,236
@l 297,240
@l 304,230
@l 306,232
@l 300,244
@l 299,260
@l 313,266
@l 303,277
@l 304,286
@l 317,281   Mare Foecunditatus
@l 326,240
@l 310,222
@l 296,222
@l 295,220
@l 305,220
@l 292,199   paulus somnii (s edge)
@l 284,199
@l 286,207
@l 273,201
@l 276,199
@l 276,199
@l 270,177
@l 290,177
@l 279,163   lacus somniorum
@l 287,159
@l 269,149
@l 266,140
@l 266,140
@l 243,135
@l 235,138   Mare Frigoris
@l 182,137
@l 157,139   Sinus Roris (S edge)
@m 158,170   Sinus Iridum
@l 168,163
@l 174,153
@l 185,154
@l 194,149
@l 220,151
@l 223,160
@l 238,160
@l 233,167
@l 256,172
@l 257,168
@l 250,160
@l 271,162
@l 258,149
@l 230,149
@l 211,140
@l 196,143
@l 185,141
@l 173,150
@l 159,151
@l 150,167
@l 153,170
@l 158,170

Line of variation 
   If you are trying to recover or identify a long-lost asteroid or
comet (or one whose orbit is simply not well-determined),  it's very
likely that the object won't be detected very close to its predicted
position.  Small errors in the old orbit pile up over time,  to the
point where (in extreme cases) the object could be just about anywhere.
   In less extreme cases,  the object may be a few arcminutes or a
few degrees away from the prediction,  but it probably still lies
along a certain line called the line of variation,  or LOV.
Knowing where that line is can be useful in limiting the region to
be searched, and Guide provides the ability to display that line
for all asteroids and comets.
   This is a somewhat specialized function,  but if you are trying
to recover an object,  it can be very valuable.
   To turn it on,  click on the Line of Variation option in the
Extras Menu.  Guide will ask for the length of the line in days;
a starting value of one day is usually a good idea.  (This
corresponds to a guess that the object may be one day "ahead of
prediction" or "behind prediction".) Guide will display a one-day
LOV for all asteroids and comets on the screen.
   To turn it off,  return to this menu option and enter an LOV
length of zero days.
   You can also access this option through the Ctrl-F11 hotkey,
or by right-clicking on an asteroid or comet;  then selecting
"Display",  then "Options".

Local Events only 
   The Local Events only option appears in the Extras menu,  when
in eclipse mode.  It provides a way to limit Guide to finding only
events that will be visible from a particular position.
   Suppose you have a chart of a solar eclipse path on the screen,
generated using the Show Eclipse function.  If you click on the
Local Events Only option,  a check-mark will be added next to it.
Zoom in on the part of the world in question,  and hit Next.  Instead
of just going to the next eclipse,  Guide will keep searching until
it finds an event visible from that position.
   One drawback of this can be that it may be a long time before the
next eclipse comes your way;  Guide may therefore take a long time to
find it.  This is especially true when the Partial Events option is
turned off;  in this case,  Guide will be hunting for a total or
annular event,  and these can be quite rare indeed!

Local Group 
   Our own galaxy,  the nearby Andromeda galaxy,  and
a few other much smaller galaxies form a small cluster
called the Local Group.

long. ascending node 
longitude of the ascending node 
   Among the numbers making up the orbital elements of an
object is the longitude of the ascending node.  This
number is part of the definition of how the orbit is
oriented in space relative to the Earth's orbit.  It is
usually represented by an uppercase Omega.