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.