A star's luminosity is a measure of how much energy
it puts out.  The Sun's luminosity is defined to be 1
unit;  other stars are measured by comparison.  Sometimes
a star that looks dim really has a high luminosity;  it's
simply so far away that it looks dim.  A few stars that
look bright have low luminosities,  but are close by.
   A consequence of this is that if you know a star's
luminosity,  and you know how bright it appears (its
apparent magnitude),  you can tell how far away it is.

luminosity class 
   A star's luminosity is a measure of how much energy it
generates.  It's possible for a star of high luminosity to
appear dim,  simply because it is far away.
   A star's luminosity class gives an idea of its actual
luminosity relative to other stars.  Luminosity classes
are expressed as Roman numerals from I to V,  with V being
the least luminous and I being the most luminous.  Thus,
stars such as Rigel and Deneb that pour out tremendous
amounts of energy are class I;  stars such as Tau Ceti,
which is even less luminous than the Sun,  are class V.
The luminosity class often appears right after the
spectral type;  for example,  Tau Ceti,  with spectral
type G8,  is listed as a G8V object,  meaning "a star
slightly cooler than the sun,  and emitting less energy."

   Luna,  the Earth's moon,  has about a quarter the
diameter of the earth.  Because it is smaller and made of
less dense materials than the Earth,  it has but one-sixth
the surface gravity that we have.  This is not enough to
hold an atmosphere,  and the Moon is totally airless.
   It circles the Earth once every 27.5 days,  and goes
through a complete cycle of phases once every 29.5 days;
this is why a month is about 30 days long.
   Because it is the closest to us of all celestial
bodies,  it exerts the most extreme tidal forces on us.
The surface is heavily cratered;  Earth probably received
about as many craters during its history,  but we have
weather and water to smooth out the evidence.  There is,
at most,  minimal geological activity,  such as volcanoes.
   Because of the tidal forces the Earth exerts on the
Moon,  the Moon shows only one side to us.  The other side
was totally unknown until the Russian probe Lunik III took
television images of it in 1959.  During the 1960s,  the
U.S. Lunar Orbiter series gathered detailed imagery of
most of the surface,  followed by the Surveyor series that
soft-landed and did soil testing.  Finally,  from 1969 to
1972,  seven manned Apollo missions were sent to the moon;
six successfully landed and explored the surface.

lunar data table 
   You can generate a lunar data table through the Tables
Menu in Guide.  This table will show a row of data for
each day.  First,  it will show the rise and set times for
the moon on that day.  (There will be days when one entry
or the other is blank,  even at tropical latitudes.)
   Next,  the total libration in degrees,  for 0:00 UT on
that date,  is shown.  Next,  the limb angle of the part of
the moon tilted toward you is shown.
   Finally,  the colongitude of the sun is shown.

intercalary month 
   Some calendars,  like the Islamic calendar,  are "lunar-only";  they
correspond well to the actual motion of the moon,  but tell you nothing
about the seasonal variations due to solar motion.  Most calendars,
like the Gregorian and Julian calendars,  follow the solar cycle well,
but the months have little relation to lunar motion.

   A few calendars,  including the Chinese and Hebrew calendars,  are
true lunisolar calendars:  the first day of the month is usually close
to a New Moon,  and a given month occurs at a given time of the solar year.
To accomplish this feat,  it's necessary to have both extra days added to
some months in some years;  and to insert an extra intercalary month in
some years (so a year can have either 12 or 13 months).

Sky Commander 
   There are quite a few automated telescope systems now
on the market.  Some simply provide information on the
telescope position ("digital setting circles");  others,
such as the Meade LX-200,  allow a computer to control
the telescope.  Automated telescopes are supported by
Guide through the F11 and F12 hotkeys and the scope
control dialog.
   The LX-200 and the Sky Commander encoder system are
both supported by Guide.  However,  the Sky Commander
cannot actually move the scope,  so the "Slew Scope"
option is grayed out when this option is selected.
   Also supported is a method useful for home-built
systems,  where data is transferred through the DOS IACA
(Inter-Application Communication Area).  In this system,
Guide sends and receives positions to the telescope
control software,  through a portion of memory available
to both Guide and the controller software.

Lynd's Bright 
   Lynd's Bright Nebulae (LBN) catalog contains most of
the known emission nebulae.

Lynd's Dark 
   Lynd's Dark Nebulae (LDN) catalog contains most of the
known dark nebulae:  those visible only because they block
the light of stars behind them.

m sin3i 
   If you look at the spectrum of a binary star,  you'll
see the spectral lines are split into pairs of lines,  one
for each star.  You'll also see these lines cross in a
regular period as the stars orbit one another,  telling you
exactly how long they take to complete an orbit.
   The amount of gap between the lines tells you how fast
one approaches the earth while the other recedes.  Given
this speed and the time of an orbit,  you can find the
minimum total mass of the stars,  called m sin3i.  The
'm' stands for the total mass,  measured in units of the
Sun's mass;  the 'sin3 i' means that the stars may be more
massive than this,  but the orbit is tilted away from the
Earth. That's why this is only a minimum total mass;  the
real mass isn't determined.

Magellanic Cloud 
   Our galaxy has several smaller galaxies orbiting it.
Two are particularly large and bright;  these are the
Small and Large Magellanic Clouds.  They are only
easily seen from the Southern Hemisphere,  and Ferdinand
Magellan was the first European to see them.  Both
contain a variety of interesting nebulae and clusters.

magnetic variation 
   Some X-ray variable stars have compact members with
intense magnetic fields.  As matter falls on the magnetic
poles of the compact object,  the resulting light can have
varying circular or linear polarization.  These objects
are also known as "polars",  in analogy to pulsars.

   The brightness of stars and other celestial objects is
described in terms of magnitude.  The magnitude scale was
created by the Greek astronomer Hipparchos sometime near
180 BC.  He labeled the brightest stars he could see as
magnitude 1,  the next brightest as magnitude 2,  down to
the dimmest at magnitude 6.  The definitions were pretty
arbitrary,  since he didn't have photoelectric cells to
make actual measurements.
   When people started using telescopes,  the scale was
extended to dimmer stars,  meaning magnitudes greater than
6.  Also,  decimals were added to allow one to distinguish
stars of roughly the same brightness;  for example,
Polaris,  the North Star,  is of magnitude 2.02.  Very
bright objects can even have zero or negative magnitudes;
for example,  Sirius,  the brightest star in the sky,  is
of magnitude -1.58;  the planet Venus reaches mag -4.4;
the full moon is about magnitude -11;  the Sun,  mag -26.
This program can show almost all stars down to magnitude
15,  about 4000 times too faint to see with an unaided

Magnitude of tenth-brightest member 
   Careful statistical study of clusters of galaxies led
to the finding that they tend to contain similar
distributions of brighter and dimmer galaxies.  This means
that the tenth-brightest members of all clusters should
have roughly the same actual light output,  and any
differences we see are due to the object's distance.
This in turn means that if you have the apparent
magnitude of the tenth-brightest galaxy in a cluster,
you can calculate with fair accuracy the distance to the
   Finding this magnitude is relatively easy.  However,
it can then give you the cluster's distance,  a number
that would otherwise be very difficult to measure.

main sequence 
   If you plot the spectral type of a star against its
intrinsic luminosity,  and repeat that process for a lot
of stars,  a pattern will emerge.  You'll find a lot of
stars on a diagonal,  from very luminous O and B (blue-
hot stars with hight surface temperatures) through more
modest F and G type stars (like the Sun) down to very
dim,  cool,  red,  class K and M stars.  This diagonal
is called the main sequence,  and it's where most stars
spend most of their lives.
   Stars do leave and reenter the main sequence as they
evolve.  Depending on where they are on the graph of
spectrum vs. luminosity,  they may be red giants or
supergiants (cool stars with high luminosity),  white
dwarfs (extremely compressed stars no larger than the
Earth,  with high temperatures and low luminosity),  or
other types.

Make .BMP File 
   By using the Make .BMP File option in the Extras menu
of Windows Guide, you can generate charts in .BMP format for
use with most Windows-based graphics software.
   To make such a file,  first set up your chart on the
screen in the manner in which you want the .BMP file to
appear.  Then click on the Make .BMP file option.
   In the Windows 95 software,  Guide will ask for the name
of the new .BMP file,  and create one of the same size as the
current Guide window.  In the Windows 3.1 software,  much the
same thing happens,  but Guide first asks for the number of colors
in the .BMP file (you can have 2,  16,  or 256) and for the size
of the chart in pixels.

Make Ephemeris 
   Use this option to create an ephemeris for a planet,
asteroid,  comet,  or artificial satellite.
   To use this,  you should first set the current time,
in the Time dialog,  to that for the start of the ephemeris.
Right-click on the object for which you want an ephemeris.
(This option,  like the Add a Trail option,  won't work if
you haven't clicked on a solar system object.)
   Finally,  when you click on this option,  you'll be able to set
the desired number of steps and the step size.  If you set the step
size to 2 days/step,  for example,  and ask for 45 steps,  the
result will cover a 90-day period.
   Click "OK",  and Guide will calculate the position of the
object for 90 days at 2-day intervals (or whatever step size and
number of steps you require),  create the ephemeris,  and display
it in the help system.  You can then print the ephemeris or save
it to an ASCII file as you would any other help topic.

Make PostScript file 
   By using the Make PostScript file option in the Extras
menu, you can generate charts in PostScript format for use with
a PostScript printer,  GhostScript,  or any of several graphics
   To make such a file,  first set up your chart on the screen in
the manner in which you want the PostScript file to appear.  You
may decide to reset margins in the Margins menu to do this,  and
to do other things normally associated with setting up a printout.
   Then click on the Make PostScript file option and select a
filename.  Guide will create a PostScript chart with that name.
   You can also access this option through the Alt-P hotkey.

Margins menu 
   By default,  Guide prints with half inch (1.25 cm) margins
on all sides.  You can reset any or all of these sizes in the
Margins menu by clicking on the appropriate side(s), and
typing in the new size.
   If you have set your language (in the Language Menu) to
English,  the sizes will be in inches.  In any other language,
the sizes will be in centimeters.
   The Margins menu has four check-boxes so you can decide which
edges will get side labels when printing.

   Mars is the fourth planet from the Sun.  It looks
blood-red,  doubtless accounting for why it is named after
the god of war.
   Mars has half the diameter of the Earth,  making it a
little too small to muster the gravity to hold a thick
atmosphere.  It maintains about 1/100 the pressure of the
Earth's,  enough to produce dust storms and to weather
the rocks.  Because it is 50% farther from the Sun than
we are,  it's a little colder,  getting up to about 50 F
(10 Celsius) on a warm day.  The thin air lets most of the
heat escape at night,  bringing temperatures down to -100
Fahrenheit (-75 Celsius).
   Mars comes close to the Earth once every 780 days (2
years,  50 days).  Once every fifteen or seventeen years,
when this close approach is near August of the year,  the
approach is unusually close,  about 35 million miles (56
million kilometers),  and our view of Mars is as good
as it ever gets.  The last such approach was in 1988.
   During the 19th century,  several astronomers were
convinced that Mars was covered in a network of canals,
presumably carrying water from the ice caps at the poles
to Martians in warmer latitudes.  In 1965,  the first
probe to Mars,  Mariner 4,  effectively ended that idea,
showing a barren,  cratered landscape unlikely to support
life.  Later,  the Viking probes sent back images and
data from the surface,  confirming the harshness of the
conditions on the surface.

mean anomaly 
   Among the numbers making up the orbital elements of an
object is the mean anomaly.  This number (an angle from 0
to 360 degrees) defines where the object is in its orbit
on the date given by the epoch of elements.  A small angle
means the object is just past its perihelion,  or closest
point to the Sun.  An angle of 180 degrees would put the
object at aphelion,  its farthest distance from the Sun.
An angle close to 360 degrees would mean the object is
approaching perihelion.

Mean position at current epoch 
   Positions given as "mean position at current epoch"
have been corrected for proper motion,  and have then
been corrected for precession to bring them to the
equator and equinox of the current date and time.  This
second correction means the coordinates have been
transformed from the "fixed" J2000 system to one that
matches the current orientation of the earth.  However,
the effects of nutation and aberration are _not_
included here;  they show up in the "apparent position
at current epoch".

   Mercury is the planet closest to the Sun,  named for
the god of thieves and darkness.  It's a small planet,
not much larger than our Moon,  and because of this and
because it never gets far from the Sun's glare,  it takes
some patience to find it.
   Mercury takes 88 days to orbit the Sun.  It takes 2/3
of this time,  or 59 days,  to rotate,  leading to very
long,  hot days and cold nights.
   Because it is so close to the Sun, daytime temperatures
run up to about 700 degrees Fahrenheit (375 Celsius).  The
planet has been examined by one satellite,  Mariner 10,
back in 1974 and 1975.  It showed a surface not too unlike
the Moon,  heavy with craters.
   Recent radar evidence indicates that there may be some
water ice near Mercury's poles.

Merged Catalog of Galaxies 
   The Merged Catalog of Galaxies contains detailed data
for faint galaxies in the northern part of the sky.

   The meridian is the imaginary line running from a point
due south,  through the zenith,  to a point due north.
Objects are highest in the sky,  and therefore best placed
to be observed,  when they pass through the meridian.  At
any particular moment,  the objects on the meridian will
have a right ascension that is equal to the current
local sidereal time.

   The Messier catalog is a list of 110 objects such as
nebulae, galaxies, and clusters of stars.  It was
put together by a French astronomer named Charles Messier
about 200 years ago.  His intent was to list objects that
might be mistaken for comets.
   The list is a good one for amateur astronomers looking
for "interesting" objects.  Each object has a number from
1 to 110,  and is referred to as, say, M-13.  Many of the
objects also have names,  such as the Andromeda Galaxy
(M-31) or Ring Nebula (M-57).  Messier objects appear on
the screen as symbols followed by an "M-" and the Messier
number,  all in yellow.
   You can find a Messier object by using the Go to
Messier option in the Go To menu.  You can set the
display of Messier objects in the Data Shown menu.

meteor shower 
   The earth is continuously bombarded by debris from outer
space,  in pieces ranging from pinheads up to huge rocks.
Almost all such objects vaporize on hitting the atmosphere,
creating the trail of a meteor or "shooting star".
   On any clear night,  you can expect to see a few meteors
per hour.  On certain nights,  however,  the rate will pick
up to a few dozen per hour,  as the earth crosses a region
with a greater density of meteors.  Such events are called
meteor showers.  Since the earth's orbit repeats once a
year,  it is not surprising that these showers tend to be
(roughly speaking) annual events.

Click for a list of meteor showers

   A meter is 1/1000 of a kilometer,  or about 39.37
inches, or a little over a yard.  There are about 1,609
meters in a mile.

   The Michigan Henry Draper,  or MHD,  catalog covers
most of the southern sky.  It contains both spectral type
and luminosity class information for all stars,  and
often has some comments on the star.

   Microwave radiation is radiation between about 30 cm
and a few millimeters in wavelength.  It's often emitted
by vast,  cool clouds of interstellar gas,  such as
neutral hydrogen clouds.  Also,  the 3K background
radiation is mostly microwave radiation.

Minor Planet Circular 
   The Minor Planet Circulars distribute news concerning
new minor planets and their names and designations.  They
are available by subscription, in paper and electronic form.
For more information,  see:

Minor Planet Electronic Circular 
   The Minor Planet Center provides several services to distribute
up-to-date information about comets and asteroids.  One such
service,  available by subscription,  is the MPEC (Minor Planet
Electronic Circular).  The MPECs are sent out by e-mail,  and contain
data about recently-discovered or recovered objects,  lists of objects
in special need of observation (the Critical Lists),  and so forth.
   If you store your MPECs in a file,  you can ask Guide to search
them for orbital elements to be added to Guide's database.
   Further information on how to order an MPEC subscription is
available at

Mira type 
Long period variable 
   The star Omicron Ceti (also known as Mira) climbs up
in brightness to magnitude 2,  then drops down over
months to magnitude 10,  then climbs back up again,  all
over a period of about 332 days.  Sometimes it's brighter
or dimmer,  and sometimes it runs a little behind or
ahead of schedule.
   There are many of these long period variables in the
sky;  Mira is the most famous of them.  They are all old,
red,  spectral type M or dimmer stars,  and vary over
periods ranging from a few months to a year or so.  Other
notable LPVs are Chi Cygni (varies from magnitude 5.1 to
13.3 over about 407 days) and 13 Lyrae (also known as R
Lyrae,  with a period of about 50 days).

   The Bright Star catalog sometimes will remark on
assorted aspects of a star under the Miscellaneous

   The MK,  or Markarian,  catalog of galaxies is a list
of galaxies that show a continuous spectrum in the
ultraviolet.  Quite a few objects will emit some UV,  but
in specific lines.  To emit UV at _all_ UV wavelengths,
you need a fairly hot object contained in the galaxy.  The
nature of the objects in Markarian galaxies that produce
this radiation is a subject of some debate.
   You can find an object by its Markarian number by using
the Go to Markarian option in the Go to Galaxy menu in the
Go To menu.

Morphological Catalog of Galaxies 
   The Morphological Catalog of Galaxies (MCG) is a list
of galaxies with their shapes,  magnitudes,  and other
characteristics.  Objects in it are designated by MCG
followed by three numbers,  such as MCG+02-28-015.  You
can find an object by its MGC number using the Go to MCG
option in the Go to Galaxy menu in the Go To menu.

Minor Planet Center 
   The Minor Planet Center (MPC) is a clearinghouse for data
concerning asteroids and comets.  It gathers the astrometric
data required to find the orbital elements for these objects,
for example.  For more information,  see: