The Bright Star catalog sometimes will remark on
binary and multiple stars.  Such comments may give the
time it takes for the stars to orbit one another,  their
relative masses and spectral types,  and so on.

   The DSS (Digital Sky Survey) is an enormous dataset consisting of
the scanned images from the Palomar Observatory Sky Survey and several
other sky surveys,  compressed to fit on 100 CD-ROMs.  (RealSky is a
version of the DSS which has been compressed still further,  allowing
it to be stored on fewer CDs at the cost of losing some image quality.)
   Like RealSky,  DSS is available from the Astronomical Society of the
Pacific,  but at considerable cost.  But you can download a DSS image for
any desired area from at least three Web servers.  See

   for a current list.  Once the images have been downloaded,  you can
display them in Guide,  using the Add DSS Image option.

Dual-mode Cepheid 
   Dual-mode Cepheid stars are like normal Cepheid stars,
except that their variations consist of two cycles,  one
on top of the other.  Usually the more rapid cycle takes
about 71% as long to complete as the slower cycle,  and the
slower cycle runs from 2 to 7 days in length.

   The DM,  or Durchmusterung,  catalog was compiled in
the late 1800's and contains magnitude estimates made by
eyeball and guesswork.  Later catalogs used photoelectric
gear,  getting more precise values,  but the DM catalog is
still in use for historical reasons.  A DM specification
for a star consists of two numbers.  The first defines a
declination to within a degree;  the second defines the
star's order among those in that one-degree zone in order
of right ascension.
   The DM is also divided into three sections,  the BD
or Bonner Dorchmusterung, compiled for declinations of
about -20 degrees and north;  the CD or Cordoba Dorch-
musterung,  compiled for declinations between about -20
and -50 degrees;  and the CPD,  or CP,  Cape Photographic
Durchmusterung,  for stars south of -50 degrees.

Reference: Dickel, H.R., Wendker,H. and Bierlitz, J.H.: 1969,
Astronomy and Astrophysics 1,270. The Cygnus X Region V. Catalogue and
Distances of Optically Visible HII Regions.

Dynamic Parallax 
   Dynamic parallax is a method used to measure the
distance to binary stars.  It's not too difficult to
measure the angular separation between the two stars,
and it's also easy to measure how long it takes them
to orbit one another.  If you can also get some idea
as to how massive they are (usually from the spectra
of the stars),  you can then do the math to figure out
how far apart they are physically... and from that,
determining their distance from Earth becomes a
straightforward problem.
   This method is important because we have very few ways
to determine the distance to a star.  The most common
method,  geometric parallax,  only works accurately out
to perhaps 100 light-years.  This method gives us a
little more distance and a way to check our results.

Dynamical Time 
   Dynamical Time,  or TD (from the French abbreviation),
is the time system used in most astronomical calculations.
The problem with using Universal Time (UT) is that it
matches the Earth's rotation,  which is not entirely
regular;  it speeds up and slows down erratically,  and
sometimes a "leap second" has to be inserted at the end of
a month in order to correct for this.  Dynamical Time,  on
the other hand,  is a uniform time system based on atomic
clocks;  it is a successor to "Ephemeris Time",  an earlier
system based on planetary motions that served the same
function (though not as precisely).
   The difference TD-UT,  also known as Delta-T,  is
currently about a minute;  it can't be well-predicted into
the future because of the irregular changes in the earth's
rotation (and hence in UT) mentioned earlier.  It is shown
in Guide when you ask for Quick Info.

   Most objects that circle the Sun have orbits only
slightly distorted from a circle,  and therefore are
always at roughly the same distance from the Sun all the
time.  The Earth is in this category:  in January,  we're
but a mere 3.3% closer to the Sun than we are in December.
The measurement of this departure from a circular orbit
is eccentricity.
   An eccentricity of 0 means the orbit is a circle.  An
eccentricity close to 1 means a very elliptical orbit,
where the object swings in close to the Sun,  then swings
far out (as most comets do).

eclipsing binaries 
eclipsing binary 
   There are a few binary stars where,  in the course of
orbiting one another,  one star will block the light of
the other as seen from Earth.  These are called eclipsing
binaries,  or Algol-type stars.
   The most notable example is the star Beta Persei,  also
known as Algol (from Al Ghoul,  Arabic for the Demon
Star).  This is usually a magnitude 2.1 star,  made up of
a large,  dim star and a small, bright star.  Every
2.867315 days,  the large star eclipses the bright star.
For about two hours,  Algol's magnitude drops to 3.4.
   You'll notice that this process,  unlike most variable
stars,  doesn't involve any real change in power output
from the star.  It just looks that way to us on Earth,
because some light has been cut off from view.
   Another kind of eclipsing binary is exemplified by the
star Beta Lyrae.  Beta Lyrae consists of two stars so
close together that their gravity stretches them out into
egg shapes.  As a result,  the light varies depending on
how much of the eggs are visible.  Here,  the variation is
continuous,  rather than the relatively abrupt eclipses of

   The ecliptic is the plane of the Earth's orbit,  as
well as (roughly) the plane in which most objects in the
solar system travel.  It (roughly) follows the
constellations of the zodiac.
   By default,  Guide displays the ecliptic;  you can
toggle it on or off in the Measurements dialog,  or
(in the DOS version) with the Alt-F1 hotkey.

Ecliptic coordinates 
Ecliptic coordinate 
ecliptic latitude 
ecliptic longitude 
   Ecliptic coordinates are an alternative way to specify
positions in the sky.  The ecliptic itself is defined by
the plane of the earth's orbit;  thus,  points with a zero
ecliptic latitude are in that plane (which corresponds
pretty well to the plane of motion of most objects in the
solar system).  The zero point,  or "prime meridian", for
ecliptic longitude is one of the two points where the
ecliptic intersects the plane of the Earth's equator.  (This
point is also known as the "first point of Aries",  since
at one time,  it lay in that constellation.  It is also
the zero point for right ascension.)
   Ecliptic coordinates are rarely used,  but they are
sometimes convenient for describing positions of solar
system objects.  The orbital elements for solar system
objects,  for example,  are almost always expressed in this
   You can have Guide show the ecliptic 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 ecliptic coordinates,
click on the ecliptic position shown in the legend,  or hit
the Alt-, hotkey,  and Guide will prompt you to enter
the new position.

Edit comet data 
   If you wish to change the data for a comet or enter a new
one from its orbital elements,  you should click on the
Edit comet data option in the Extras menu.  This will
bring up a list of some recent comets,  along with "new
comet" and "new asteroid" options.  Click on any of these,
and Guide will show an orbital elements dialog box for
the currently stored elements (for an existing object) or
default data for a new object.

elliptical variable 
   An elliptical variable is a kind of variable and binary
star.  It varies because the stars are so close together
that their mutual gravity stretches them into egg shapes.
As they orbit each other,  we see the eggs from different
angles,  and therefore see different amounts of light.
   The amount of stretching can't be too great,  or the
stars would have ripped each other apart long ago.  This
means the changes in brightness can't be more than a few
tenths of a magnitude either.
   Sometimes the stars eclipse each other as well,  making
bigger changes in brightness.  This kind of star is called
a Beta Lyrae type variable.

   As planets circle the Sun,  their apparent distance
from the Sun in the sky varies.  This distance is called
the elongation from the Sun;  when it is small,  the
object is close to the Sun and difficult to see in the
glare.  When it is close to 180 degrees,  the object will
rise at sunset and set at sunrise,  so you can see it all
night,  far from the Sun's glare.

emission line 
   When you heat up a gas and look at the spectrum of
light it emits,  you'll find that it tends to emit light
at certain frequencies,  or wavelengths.  Different gases
will emit at different frequencies;  these are called
emission lines,  and by analyzing them,  you can determine
what gases you're looking at.

emission nebula 
   An emission nebula is a nebula which is close to a
star (or several stars) emitting short-wave (blue or
ultraviolet) radiation.  This radiation ionizes and/or
excites the atoms in the nebula.  These atoms then get rid of
their energy by emitting a photon,  much as excited gas atoms
in a neon light will emit radiation of a certain color.
   This is quite different from a reflection nebula,
where the light from the nebula is simply reflected light
from the central star.  You can tell the difference by
looking at the spectrum of the nebula and comparing it to
that of the stars providing the initial energy.  If it's a
reflection nebula, then the spectra will match;  if it's an
emission nebula, it will show emission lines of its own.

   Photography (both of the celestial and "normal" sort)
relies on chemicals that will change on being exposed to
light.  The mix of materials applied to a film or glass
plate is called an emulsion.
   Most celestial objects do not emit a lot of light,  so
the emulsions used for photographing must provide good
sensitivity over long exposure times.  Quite a bit of
human ingenuity has gone into making emulsions that will
record fainter objects in less time.

Enter caption 

   You can click on this menu item to change the caption
that can be shown in the legend at lower left.  Notice
that this just lets you change the text.  Turning the
display of the caption on or off is done with the
Caption on/off menu option;  clearing it is done with
the Clear Caption menu option.

Enter Latitude 

   The Enter Latitude option lets you reset your
geographic latitude. (You can get this from USGS maps and
from most others.)  You enter a latitude as a compass
sign (N or S) followed by degrees,  minutes,  and
seconds.  For example, the latitude of Bowdoinham,  Maine
could be entered as

N 44 1 30

   Sometimes longitudes are expressed as degrees and decimal
minutes,  or decimal degrees.  You can use these instead:

N 44 1.5
N 44.025

   The default latitude and longitude for this program
are for Bowdoinham,  Maine.  You need not be nit-pickingly
accurate in finding your latitude and longitude;  as long
as you're within a few kilometers,  you'll be okay for
most purposes.
   You can reach this option at any time with the CTRL-Y
hotkey,  or through the Location dialog in the Settings menu.

Enter RA/DEC 
   You can use this item to go to any desired RA and
declination.  When you click on it,  you will be asked
for a RA;  you can enter this in hours,  minutes and
decimal seconds,  as hours and decimal minutes,  or as
decimal hours.  The program will recognize either 4.4h
or 4h24m or 4h24.0m or 4h24m0.0s as the same RA.
   Next,  you are asked for a declination.  Once again,
the program is reasonably good about understanding
different formats:  it will recognize 63.3 or N63.3 or
+63d18' or 63d18m0.0" as the same declination.
   Once you have entered a position in a given format,
Guide will use that format to display all latitude
and longitude values.
   This item can be reached at any time via the ALT-E
hotkey.  Also,  if the legend is shown,  you can click
on the RA/dec shown (if that's turned on) to reach this
option;  or you can use the 'Enter RA/Dec' option in the
Go To menu.

Enter Time 
   In general,  most users of Guide find that the Time Dialog
is the simplest way to reset the date and time Guide uses for its
display and calculations.  Some people,  though,  prefer to type
in the time directly and avoid use of the mouse.  The Enter Time
menu option in the Extras menu is intended for that purpose.
   Click on Enter Time,  and you'll be prompted to enter the new
date and time.  Here are some examples of valid dates or times:

13/6/1987      to reset the date to 13 Jun 1987
13/6           to reset the date to 13 Jun of the current year
13             to reset the date to the 13th of the current month
16:45:02       to set the time to 16:45:02 of the current day
16:45          to set the time to 16:45 of the current day
16             to set the time to 16:00 of the current day
13/6 16:45:02  to set the time to 16:45:02 on 13 Jun, current year

   You can also use any of the "date" formats followed by any of
the "time" formats.  Also,  the following are valid:

+27.3         to advance the current date by 27.3 days
-10.4h        to back up the current date/time by 10.4 hours
+2356m        to advance the current date/time by 2356 minutes
-63.1s        to back up the current date/time by 63.1 seconds
-100y         to back up 100 years from the current date/time
J2450540.321  to set the current date/time to JD 2450540.321

   You can also reach this option with the Ctrl-F9 hotkey.

   An ephemeris is a list of positions for an object,
usually a planet,  asteroid,  or comet.  (In Guide,
ephemeris creation is limited to those objects).  An
ephemeris can also list distances of an object from the
Sun and/or Earth,  as well as magnitudes.  (Guide will
usually include that information.)  For example,  this is
an ephemeris for the comet 1993v (McNaught-Russell).
It shows data at two-day intervals from 28 Feb 94 to 24
Mar 94,  for midnight UT,  as seen from Project Pluto
headquarters in Bowdoinham,  Maine.
Date       RA            declination    r      delta  mag
----       --            -----------    -      -----  ---
28 Feb 94  03h17m36.45s  S34 29' 30.3"  1.034  0.853  7.5
 2 Mar 94  03h21m39.21s  S32 56' 37.9"  1.016  0.824  7.3
 4 Mar 94  03h25m52.16s  S31 16' 22.4"  0.998  0.794  7.0
 6 Mar 94  03h30m15.18s  S29 27' 49.2"  0.981  0.765  6.7
 8 Mar 94  03h34m48.14s  S27 29' 57.6"  0.964  0.735  6.4
10 Mar 94  03h39m30.91s  S25 21' 40.5"  0.949  0.706  6.2
12 Mar 94  03h44m23.36s  S23 01' 44.2"  0.935  0.678  5.9
14 Mar 94  03h49m25.37s  S20 28' 49.8"  0.922  0.650  5.6
16 Mar 94  03h54m36.86s  S17 41' 34.6"  0.911  0.622  5.3
18 Mar 94  03h59m57.79s  S14 38' 35.7"  0.900  0.596  5.1
20 Mar 94  04h05m28.24s  S11 18' 35.7"  0.891  0.571  4.8
22 Mar 94  04h11m08.37s  S 7 40' 30.0"  0.884  0.548  4.6
24 Mar 94  04h16m58.51s  S 3 43' 38.6"  0.877  0.527  4.4

Ephemeris Items 
   The Ephemeris Items option in the Make Ephemeris dialog gives you
full control over what data is shown in an ephemeris.  You can,  for
example, create an ephemeris containing only data such as an objects'
alt/az, distance,  and magnitude.  A check-box is given for each
possible item in an ephemeris.
   There are a few options in this dialog that could use some explanation.
The "elongation" refers to elongation from the Sun,  of course.  The
"phase angle" is often given in MPC ephemerides;  it refers to the angle
Sun-target-Earth (target at the vertex).  When this is near zero,  the
object is near to opposition;  a phase angle of 180 degrees,  on the
other hand,  suggests that the object is transiting the Sun.
   The "In Shadow" field only works for artificial satellites;  it gives
an "I" if the satellite is Illuminated,  or an "S" if it is in Shadow.
   The "Lat/Lon" option also only works for satellites,  and gives the
point on the earth over which the satellite is currently situated (if
you stood at this point,  the satellite would be at the zenith.)
   The final three options provide ways to filter out unwanted observations
(and exist only in the Windows software).  Click on the "Sunlit
(satellites)" option,  and data will only be listed for a satellite if it
is actually sunlit.  Click on the "Above Horizon" option, and data for any
object will only be listed if it is above your horizon. Click on the "Sun
below altitude" box,  and you can ensure that only nighttime data is given;
an edit box allows you to tell Guide how far below the horizon the sun must
be.  The default is six degrees, corresponding to nautical twilight.
   The default is for all three of these options to be OFF,  so you see
every line in the ephemeris,  even if the satellite isn't at all visible.
Click on all three,  and data for a satellite will only be shown if all
three of the key criteria are satisfied:  the satellite must be sunlit,
it must be above the horizon,  and it has to be dark outside.

Ephemeris Time 
   Usually,  the times of astronomical events are given in
Universal Time.  UT is coordinated with the Earth's
rotation.  But the Earth's speed of rotation varies;  part
of this is because the tides raised by the Sun and Moon
are slowly dragging down the Earth's spin.  The result of
all this is that one year in UT may not be as long as
a different year.  Sometimes "leap seconds" must be added
at the end of a year.
   It's much easier to make astronomical calculations with
a system in which a year is always a year,  and a "day" is
always 24x60x60 seconds.  This system is Ephemeris Time.
ET provides a uniform system lacking the semi-random
fluctuations found in UT.  For the last century or so,
the difference between ET and UT (called "delta-T") has
been under a minute.  Delta-T is not well known for times
before about 1500 AD,  since we don't know just what the
Earth's rotation was doing back then.

   On the Earth, the latitude and longitude of a place
are numbers that don't change.  That's because they are
measured from the Equator and Prime Meridian,  which don't
   Regrettably,  right ascension and declination are
measured from the vernal equinox and celestial equator,
and these do move over time because of the Earth's
precession.  So when giving an RA/dec,  you need to give
the time for which it's valid,  which is called the epoch.
Positions are usually given in 50-year epoch intervals.
By now,  most astronomers have switched over from 1950 to
epoch 2000,  or "J2000.0",  positions,  with a few
positions given in 1975 coordinates.
   The "J" in front of an epoch date refers to the more
modern Julian epoch;  the "B" refers to the older
Besselian epoch.

epoch of elements 
   When the orbital elements of an object are given,  the
epoch of elements is usually given as well.  This is
because the numerical values of the elements change slowly
over time.  An object in the solar system is always
subject to the gravity of other planets,  not just the
Sun,  and this makes the orbit (and hence the elements)
   The epoch of elements is the date and time for which
the elements accurately describe the position and velocity
of the object.  For a few months around that date,  you
can use those elements to get a good position (usually
within arcseconds).  Beyond that time,  the gravitational
effects of the planets will put the object in a very
different orbit than that described by the elements.

Equat. radius @ 1 bar 
   Defining the radius of a "gas giant" planet (Jupiter,  Saturn,
Uranus,  and Neptune) presents a problem,  because we don't see the
solid surfaces of these planets;  all we see are outer layers of the
atmospheres.  So instead,  the radius is measured from the center of
the planet to the point where the atmospheric pressure drops to one
bar (basically,  that of sea-level air pressure on Earth).  This
is then given as the "equatorial radius at one bar".

Equation of time 
   One reason sundials fail to keep very good time is
that the Sun does not appear to move at a constant rate
across the sky.  This is partly because the earth's orbit
is elliptical,  and partly because the earth's axis is a
little tilted.
   The equation of time is the difference between "mean
solar time" (the time a sundial would keep if the sun's
motion was uniform) and "apparent solar time" (the time
described by the actual movement of the sun).  It is
shown in Guide when you click for "quick info".
   The equation of time varies throughout the year (and
slightly from year to year).  At present,  it reaches
extremes of about -14 minutes in February,  and about +16
minutes in November.