perigee
apogee
As an object orbits the Earth, its distance will vary
between two extremes. At its closest point to the Earth,
the object is said to be at perigee. At its greatest
distance, it is said to be at apogee. When you "click
for more info" on the Moon, Guide will list the times
when the Moon will reach perigee and apogee. It will also
give the Moon's distance at those times.
There are similar terms for objects orbiting things other
than the Earth. An object circling the Sun has a
perihelion and aphelion, for example.
perihelion
aphelion
As an object goes around the Sun, its distance will
vary between two extremes. At its closest point to the
Sun, the object is said to be at perihelion. At its
greatest distance from the Sun, the object is said to be
at aphelion. The "helion" comes from Helios, the god
of the Sun.
Similar terms, by the way, appear for objects that
orbit things other than the Sun. An object circling the
Earth has a perigee and apogee, the "gee" referring to
Geos, or the Earth. An object circling Jupiter has a
perijove and an apojove.
Perihelion distance
This line gives the asteroid's distance from the Sun
at perihelion, its minimum distance from the Sun. Since
some asteroids have very highly eccentric orbits, this
may be much less than the average distance from the Sun.
Period of orbit
This line shows how long it takes the asteroid to make
one orbit around the sun. Most asteroids take from three
to twelve years to do this, since most asteroids are
between Mars and Jupiter. A few have orbits of close
to one year in length, and cross the Earth's orbit. A
few are much farther from the Sun; 5145 Pholus, for
example, takes 94 years to circle the Sun, and 2060
Chiron takes 51 years.
PGC
LEDA
Principal Galaxy Catalog
The PGC (Principal Galaxy Catalog)/LEDA catalog
contains over 100,000 galaxies, providing detailed
information as to positions, sizes, listings in other
catalogs, etc.
You can find an object by its PGC/LEDA number by using
the Go to PGC option in the Go to Galaxy menu in the
Go To menu.
The PGC/LEDA was compiled by G. Paturel and L.
Bottinelli of the Universite of Lyon and L. Gouguenheim
of the Observatoire de Paris.
phase angle
The phase angle of a solar-system object is the angle
between the Sun and the observer as seen from that object
(the angle "Sun-object-Earth"). It is 0 degrees when the
object is fully illuminated, 90 degrees when the object
is half-illuminated (like the moon at first quarter and
last quarter), and 180 degrees when the object is between
us and the sun (like the moon at new moon.)
The phase angle is useful in computing the magnitude
of a solar system object. Obviously, an object will be
brightest when fully illuminated (phase angle = 0),
dimmer when half illuminated, and darkest when between
us and the sun.
Objects such as the Moon, Mercury, and Venus can have
phase angles covering the full range of 0 to 180 degrees.
Mars has a maximum phase angle of about 45 degrees, meaning
that it is always almost fully illuminated.
photographic magnitude
The magnitude of a star as measured on a photograph
may be considerably different than that measured by the
eye. This happens because most emulsions react more
strongly to red light than does the human eye, which
tends to be more sensitive to blue light. Hence, this
program must often differentiate between photographic
magnitude and visual magnitude.
Photometric band
When precise measurements of the magnitudes of objects
were first made, a difference was noted between visually
determined magnitudes and the photographic magnitudes recorded
by the mainly blue-sensitive early photographic plates.
Later the distinction was made between V (visual magnitudes),
B magnitudes ("blue", measured through a blue filter), and
R (measured through a red filter). These are photometric
bands.
A little later, measurements were made in the near
infrared, and the system had to be expanded to include
near-infrared and, later, far-infrared data. Such
measurements are complicated by the fact that the Earth's
atmosphere tends to absorb infrared, except at a few
wavelengths where we have some "windows". Measurements
made at the following wavelengths were assigned the
following letters:
B 440 nanometers (blue, just barely visible to humans)
V 550 nm (corresponds closely to a real, "visual" magnitude)
R 680 nm (red, just barely visible)
I 825 nm (just barely too "red" to be visible)
J 1250 nm
K 2200 nm
L 3400 nm
M 5000 nm
N 10200 nm
Piscids
Draconids
Orionids
Taurids
Pegasids
Leonids
Monocerids
Sigma Hydrids
Geminids
Leo Minorids
Delta Arietids
Ursids
Showers in October, November, and December
Oct 7 Piscids: Radiant--near Aries. 15 per hour at 28 km/sec.
Oct 9 Draconids: Radiant--near Hercules. Spectacular when comet
Giacobinni-Zinner passes near Earth. 200 per hour when comet is
close is not uncommon, 1000 per hour sometimes.
Oct 20 Orionids: Radiant--near Taurus. 30 per hour, fast (67 km/sec)
often in colors with long trails. Duration--8 days
Nov. 5 Taurids: Radiant--near Pleiades. 10 per hour with many
fireballs. Debris from comet Encke. Duration--45 days.
Nov. 12 Pegasids: Radiant--Near Square. From Oct. 10 to late
Nov., 10 per hour, used to be spectacular.
Nov 17 Leonids: Radiant--near Sickle. Most spectacular of modern
showers. 1966 saw 500,000 per hour-- 140 per second. Comet
Temple--Tuttle is parent. 20 per hour between 33 year shows,
fastest known at 71 km/sec. Duration--4 days.
Dec. 10 Monocerids: Radiant-- near Gemini. 12 per hour.
Dec. 11 Sigma Hydrids: Radiant--near Head. 12 per hour, fast.
Dec. 14 Geminids: Radiant--near Castor. 60 per hour, many bright,
white but few trails. Icarus, the Earth-crossing asteroid seems to
be the parent. Duration--6 days.
Dec. 14 Leo Minorids: 10 per hour, somewhat faint. Discovered by
amateurs in 1971.
Dec. 20 Delta Arietids: 12 per hour, must view in early evening,
before radiant sets.
Dec. 22 Ursids: Radiant--Little Dipper Bowl. Medium speed, 20
per hour, many with bright trails. Duration--2 days
PK
Perek-Kohoutek
The PK (Perek-Kohoutek) catalog of planetary nebulae
gives positions, sizes, and magnitudes for 1,455
objects. All planetary nebulae are shown as circles with
four lines extending outward.
planet
There are nine known planets orbiting the Sun. You
are standing on one of them. The remaining eight,
Mercury, Venus, Mars, Jupiter, Saturn, Uranus,
Neptune and Pluto, are displayable in this program.
Normally, planets are shown in purple using symbols
created in antiquity. If you zoom in far enough on a
planet, however, it will change to a disk or crescent.
(Some objects, like the Sun and Moon, are almost always
large enough to show a disk or crescent.) If the object
is Jupiter, you will see its four largest satellites at
this point.
You can click on planets just as you can click on
stars. This gets you the planets' name and some basic
information, which can be expanded upon by clicking the
"Click for more info" line.
planet display dialog
The planet display dialog appears when you right-click on a planet
or natural satellite; then click "Display", leading to a data display
dialog; and then click "Options", to get access to functions that
specifically involve planets. For Mars, for example, the dialog would
look much like this.
!
[ ]-- Planets ----------------[X]
| |
| [X] Full precision |
| [ ] Label by name |
| |
| Mars: |
| [X] Show features |
| [X] Label features |
| [ ] Lat/lon grid |
| __30_ x __30_ degrees |
| |
| ( ) Shaded |
| ( ) Line figure |
| (o) Bitmap #1 |
| ( ) Bitmap #2 |
| ( ) Bitmap #3 |
| |
| Contrast: [ * ] |
| Brightness: [ * ] |
| |
| [ OK ] [ Cancel ] |
`-------------------------------'
!
The top two check-box options, and the 'contrast' and 'brightness'
slider bar options, apply to all planets uniformly. Tell Guide that
planets are to be labelled by name (instead of by their symbols), for
example, and that decision will apply to all other planets, too.
The remaining options apply only to the currently selected planet.
The "show feature" checkbox will cause country boundaries to be shown
on Earth, craters on the moon, sunspots on the sun, and a variety of
features on some other planets. Include the "Label feature" checkbox,
and these features will be labelled by name.
Each planet or satellite can have from zero to three "bitmaps",
used to provide detailed images when you zoom in on that object.
(Earth has three maps, for example. Venus has two: a "cloud map" and
a "radar map" from Magellan data. Many small satellites have no maps
at all.) You can select which bitmap appears; or tell Guide to show
the object as a shaded sphere; or as an outlined object. These last
two do not look as good as a "bitmapped" option, but can be drawn much
faster (a particular concern on older computers.)
planetary nebula
Planetary nebulae occur when a star blows a wind of
large amounts of gas in its vicinity. The central star
then produces enough light to keep this gas hot enough so
that it glows. An example is the Ring Nebula (M-57),
where the central star's wind expanded in such a manner
as to make a doughnut of gas around the star. Another
example is the Helix Nebula. These are called planetary
nebulae because some look a little like a planet in a
low-power telescope.
plate
The Hubble Guide Star Catalog, or GSC, was created
by scanning in photographic plates of the night sky. Each
plate has a unique ID code; when you request "more info"
on a GSC star, you will be given the plate(s) on which
that object appears.
Because the plates overlap, sometimes the same object
will be found on more than one plate. In such cases,
Guide will show the data from each plate.
The "plates", by the way, are glass sheets coated
with photographic emulsion. They are the equivalents of
film in more normal cameras.
Pluto
Pluto is the dimmest and (usually) most distant
planet. At its great distance from the Sun (30 AU right
now), the Sun would appear 1/900 as bright as it does to
us. This distance and lack of light make Pluto a
magnitude 14.8 object.
Pluto was found in 1930 as the result of an extensive
search conducted by Percival Lowell, W. H. Pickering,
and Clyde Tombaugh. The search hoped to find a planet big
enough to explain peculiarities in the motions of Neptune
and Uranus. The process involved taking two photos of the
same part of the sky on different nights and examining
them with a "blink comparator" to see if any objects had
moved during that time.
In 1978, photos of Pluto were found to show a small
moon, Charon. Subsequent calculations show that Pluto
is about 2400 km in diameter, Charon about 800 km. (The
Earth is about 12,000 km across.) Pluto and Charon are
about 16,000 km apart and orbit one another every 6.39
days. They take about 248 years to orbit the Sun. They
are, incidentally, much too small to be the planet the
discoverers were looking for, and some astronomers are
still searching for a Planet X that has the mass to make
the observed alterations in planetary positions.
Pluto's orbit is much more elliptical than that of
most planets. It ranges in distance from the Sun from 30
AU to 50 AU. Right now, it is in the close ("summertime")
location, and is closer to the Sun than Neptune.
pmtext
This line shows the proper motion, in seconds of arc
per year, for the star you just clicked on.
Polarization
Some stars emit polarized light, that is, light that
vibrates in only one direction, or more in one direction
than another. The Bright Star catalog sometimes will
comment on this in the Polarization section of the
Remarks.
Portrait/Landscape
This option toggles between printouts with portrait
or landscape orientation. For portrait printouts, Guide
will print out the area of the sky shown in the chart
region, plus a little on the top and bottom edges. In
the latter case, Guide will print the chart region plus
a little from the left and right edges.
Position angle
A galaxy's tilt (the angle between its long axis and
a line from its center headed north) is its position
angle. This value is what enables this program to draw
the galaxy as a tilted ellipse.
For a double star, the position angle is the angle
between a line drawn from the brighter star to the dimmer,
and a line drawn north from the brighter star.
In Guide, you can determine the position angle (and
distance) between two points by clicking on the first point
with the right mouse button, and holding down that button
while you drag the mouse to the second point, and finally
releasing the mouse button. Guide will pop up a small dialog
box with the distance/position angle data.
If you want the distance/position angle between two specific
objects, you can right-click on them both, then hit Insert
(in Windows) or Ctrl-Insert (in DOS).
posn err
The positions of objects in the Hubble Guide Star
Catalog, or GSC, are generally accurate to better than
one second of arc. An error measurement is given for each
position, and shown in the remarks for the object as the
"posn err" (positional error).
POSS
Palomar Observatory Sky Survey
The POSS, or Palomar Observatory Sky Survey, was
originally done in the 1950s; it was an effort to take
survey photos of the entire sky as visible from Palomar.
Each plate is about six degrees on a side; the entire
survey consists of 1037 plates, covering the region from
the north celestial pole down to declination -45. An
effort is underway (and is mostly complete) to repeat the
survey. The POSS is the source of data used for most of
the GSC in the northern half of the sky.
PPM
The PPM, or Position and Proper Motion catalog,
is a successor to the SAO catalog. The SAO catalog
was compiled from data mostly gathered in the early part
of this century, and its positional accuracy is not
very good (usually about one arcsecond or worse).
Also, the proper motions were often calculated from
only two observations. If one observation was in error,
there is no easy way to spot it.
The PPM includes data from more recent surveying
efforts, such as the FOKAT-S survey of the southern
sky. Also, by now, most of these objects have been
examined more than once, so any large errors showed up
when compared to other measurements. The result is
higher quality data. If both SAO and PPM data are given
for an object, it is usually best to use the PPM data.
precession
Declination is measured from the celestial equator.
The celestial equator depends on where the Earth's axis
of rotation points in space. This axis is not fixed. Due
to the gravity of the Sun and moon, it slowly traces out
a circle in the sky, much like a top. This motion is
called precession. Over a period of 25,800 years, the
earth "wobbles" around this circle.
The result of this motion is that a declination (or
a right ascension) measured at one time won't match that
measured at another time. This is why positions in the
sky must be given with the time for which they are valid.
This time is known as an epoch.
Preliminary designation
When an asteroid is first found, it is given a
preliminary designation, a sort of temporary name for
use until its orbit is figured out precisely. Once the
orbit has been firmly established, the asteroid is given
a number and (sometimes) a name.
Printer setup
Alt-C
The Printer Setup menu lets you set up the four
pieces of information the DOS version of Guide needs
in order to print: the kind of printer (HP LaserJet,
Epson LQ, PostScript, etc.); whether you want landscape
or portrait oriented printouts; the resolution at which
you wish to print; and where the output is to go (the
printer, a file on your hard drive, or a serial port).
It also provides the ability to add a printout to a
queue and to flush the queue.
You can reach this menu at any time with the ALT-C key
in the DOS version of Guide, or from the File menu in
either DOS or Windows.
Help is available for:
Select printer
Select printer output
Select resolution
Add to Print Queue
Flush Print Queue
Portrait/Landscape
Black stars on white/white on black
Margins menu
Printing help information
If you click on the "Print" option at the bottom of a
help screen, Guide will print whatever help topic is
currently showing. You can also use this to print any
ephemeris information you generate.
Projection menu
stereographic
orthographic
gnomonic
equidistant
The Projection menu, inside the Settings Menu, allows
you to reset the projection to stereographic (the default),
orthographic, gnomonic, or equidistant.
The stereographic projection preserves the shapes of objects, and is
about the closest thing to an "ideal" chart projection. It is usually
used for 180 degree "all-sky" views in astronomy magazines, with the
zenith at the center of the chart. Any other projection would show a
lot of distortion over so large an area; the stereographic does result
in scale changes, but the constellations still look correct.
On charts drawn with the gnomonic projection, great-circle lines
appear as straight lines. The price for this convenience is bad
distortion at larger fields of view. This capability was requested by
meteor observers; on charts made with this projection, the meteor
trails are straight lines emitting from the radiant.
The orthographic projection is really intended to show "earth-from-
space" views. If you set this projection and zoom out to level 2 (90
degrees), you will see what is meant by this remark.
The equidistant projection is usually only used for maps of the earth.
In this projection, distances and angles measured from the center of the
projection are correct. For this reason, short-wave radio users are
fond of maps made with this projection; the distance and bearing to any
other part of the world can be easily measured. This is also the only
projection of the four that can cover an entire globe (celestial or
terrestrial). But again, the price for these benefits is terrible
distortion at large fields of view.
prominence
Sometimes, pressures inside the Sun will cause what
looks like a volcanic eruption, and a huge streamer of
gas will fly up from the surface. This is called a
prominence. Sometimes the energy of the outburst is
such that some gas will leave the Sun forever.
The gas in a prominence is so hot that much of it is
ionized (electrically charged), and bends over in loops
as it moves through the Sun's intense magnetic field.
You've probably seen pictures taken of flares near the
edge of the Sun.
proper motion
The stars are far enough away so that, unlike the
planets, they appear not to move. However, if you
make very precise measurements over the years, you
will find that some stars slowly move across the sky.
This is called proper motion. It is measured in
arcseconds per year.
Generally, the larger a star's proper motion, the
closer it is to us. Of course, some nearby stars are
moving almost straight toward or away from us, and these
don't show much proper motion.
The amount of shift is always small. The star with the
greatest proper motion, Barnard's Star (a red dwarf that
is, after Alpha Centauri, the next closest star to the
Earth), moves about 10 arcseconds per year.
proper motion total
Proper motion is usually measured in units of arc-
seconds per year. It can be broken down into components
in right ascension and declination, and the total
speed can also be expressed. This program shows all
three figures.
PV Tel
PV Tel type variable stars are blue-hot supergiant
stars, rich in helium. They pulsate with periods of 2
hours to one day, by about .1 magnitude.
Quadrantids
Delta Cancrids
Coma Berenicids
Delta Leonids
Corona-Australids
Camelopardalids
March Geminids
Showers in January, February, and March
Jan. 4 Quadrantids: Radiant--Bootes. Very short lived shower,
less that one day. Variable rate, but generally around 60 per hour.
Speed 41 km/sec and bluish color.
Jan. 16 Delta Cancrids: Radiant--just west of Beehive. Minor
shower, rate about 4 per hour. Very swift.
Jan. 18 Coma Berenicids: Radiant--near Coma star cluster. Only
one or two per hour, but among fastest meteors known--65 km/sec.
Feb. 26 Delta Leonids: Radiant--midway in Leo's back. Feb. 5 to
Mar. 19 with peak in late Feb. 5 per hour at 24 km/sec.
Mar. 16 Corona-Australids: Radiant--16 hr 20 min, -48 deg. 5 to 7
per hour from Mar. 14 to Mar. 18.
Mar. 22 Camelopardalids: No definite peak, with only one per hour.
Slowest meteors at 7 km/second.
Mar. 22 March Geminids: Discovered in 1973 and confirmed in 1975.
Rate generally about 40 per hour. Seem to be very slow meteors.