Telrad Ctrl-F6 The Telrad is a simple telescope aiming device; it uses an LED, lens, and beam-splitter to cause three concentric circles to appear in the sky at an apparently infinite distance. The circles are 4 degrees, 2 degrees, and .5 degree across, and are aligned with the telescope axis. The measurements dialog provides an option to toggle the display of a Telrad at the screen center. In the DOS version, you can toggle display of the Telrad with the Ctrl-F6 hotkey. terminator The terminator of a planet is the line between sunrise and sunset. The term can be applied to any planet, but you will usually see it mentioned in reference to the Moon. It is useful there because objects are best seen when they are near the terminator. If they are well in the sunlit side of the moon, then they cast no shadows and are difficult to see in the glare. If they are on the dark side, then of course, there is no light to observe them. When you "click for more info" about the moon, you'll be given some information such as the colongitude and selenographic position of the sun that can be useful in figuring out where the terminator is. text alignment menu This menu allows you to reset the alignment that will be used for text added to the overlay. ticks Ticks are the cross-hatches indicating intervals of right ascension and declination that are common to many star charts. You can adjust their spacing and turn them on and off within the measurements dialog. tides tidal tide Consider two points on the Moon, one the point closest to the Earth, the other the farthest from the Earth. The point closest to the Earth feels more gravitational force than the rest of the moon does. The farthest point feels less force. This difference, which tries to pull the moon into an egg shape, is a tidal force. Left to themselves, rocks put at these two points would drift apart. They don't do this because the moon has its own gravity, much more powerful than the tidal force. Similarly, the moon exerts a tidal force on us. The rocks in the earth are rigid and aren't affected much by this, but the oceans, being fluid, tend to "stretch" so that the water piles up a little at the points closest and farthest from the moon. As the earth turns, these points move around the globe and the ocean, at any given point, seems to rise and fall. If a moon gets very close to a planet, the tides will become more powerful than the satellite's own gravity, and the stress will rip it apart. This may have happened to form Saturn's rings, and definitely keeps the rings from lumping back up into a satellite. Titan Titan is the largest moon of Saturn. It is slightly larger than Mercury and about 50% larger than the Moon. Because it is so large and so cold, Titan is able to hold an atmosphere of methane and other compounds. This gives Titan a reddish-brown appearance, and also prevented the Voyager probes from actually seeing the surface. It has been widely speculated that Titan could have life on it, if the atmosphere retains enough heat. The atmosphere bears some resemblance to that of the Earth's just before the emergence of life. TLE A TLE (Two-Line Element) is a special, standard way of expressing a set of orbital elements for an Earth-orbiting satellite. It contains the "usual" elements, such as mean anomaly, longitude of the ascending node, and eccentricity, plus some which are specific to satellites, such as atmospheric drag terms. Files containing TLEs can be downloaded from many Internet sites; see http://www.projectpluto.com for links to such sites. Once you have downloaded a TLE file, you can use the TLE=(filename) option to view the satellites contained in it. Because artificial satellite orbits tend to change rapidly, you will need to update your TLE files frequently. A given orbit will be good for a few days to a few months, depending on the satellite and the level of precision you want. TLE=(filename) The TLE=(filename) option allows you to select a new TLE (Two-Line Element) file of orbital data for artificial satellites. By default, Guide draws satellites from a small file called BRIGHT.TLE. But the orbits of artificial satellites change rapidly, and you will eventually want new satellites. Also, BRIGHT.TLE contains only a few of the brighter satellites; you may wish to use a file containing data for geosynchronous satellites, or those in low-earth orbit, and so forth. The best sources for up-to-date elements are all on the Internet. See http://www.projectpluto.com for links to such sites. Toggle user datasets Alt-F9 The Toggle user datasets option in the Extras menu provides a way to control the display of user added datasets. Select this option, and Guide will list the currently installed datasets. When you click on one of these datasets, you will be provided with a standard data display dialog. You can also reach this option with the Alt-F9 hotkey. Toolbar Dialog By default, the toolbar contains a few buttons that cover some major features in Guide; but it is expected that users will want to adjust this to include only those icons of interest to them. This can be done using the Toolbar Dialog, accessed through the Settings menu. This dialog contains lists all buttons that can be put into the toolbar. Those in use are marked with an asterisk. You can double- click an item to toggle it; or, you can select a group of buttons and turn them all on or all off, using the two buttons provided for that purpose. Also, there is a checkbox given to shut off and turn on the toolbar. Finally, you can click "OK" to indicate that you want to keep the changes you've made, or "Cancel" to cancel those changes. topocentric Topocentric means "measured from the Earth's surface." For example, when you get information on an object in Guide (such as its position, distance, etc.), this data is usually measured from the Earth's surface, from the position described by the latitude, longitude and height above sea level you entered into the Settings menu. Usually, the difference between a topocentric and a geocentric (measured from the Earth's center) position is not very great; objects tend to be far away, and your position on Earth doesn't matter very much. But a nearby object, such as the Moon, can vary in position by two degrees depending on where you are on the Earth, and an observatory in Australia recently missed an asteroid that passed close to the Earth because they used geocentric, not topocentric, positions. transit time When you click on an object, this program calculates when that object will appear highest in the sky. This instant is called the transit time. transition variable Some variable stars that are in the process of evolving from one type to another are classified in this program as transition variable stars. They are either just starting or ending to vary, changing types, or simply haven't been studied enough to tell what type they really are. Trojan Jupiter's gravity tends to push asteroids into certain preferred orbits and places. Two such places form equilateral triangles with the Sun and Jupiter, such that one node travels in front of Jupiter, the other behind it. An asteroid in either node will take the same time to orbit the Sun as does Jupiter (about 11.86 years), and will stay in roughly that location. Some asteroids have fallen into these nodes. The first to be discovered was 588 Achilles. By convention, objects found in these nodes have been named after characters in the Iliad; hence, the term Trojan asteroids. In reality, these asteroids have been named after both Trojan and Greek characters. Those in one node are named after Greeks; those in the other, after Trojans. Leading Trojan asteroids (named for Greeks) (Jupiter and asteroids go clockwise around Jupiter Sun the Sun) Following Trojan asteroids (named for Trojans) @c 132,360,12 Sun @c 12,360,5 Jupiter @c 72,255,1 @c 74,454,1 @c 76,251,1 @c 78,456,1 @c 70,253,1 @c 68,458,1 @c 66,259,1 @c 64,462,1 @c 72,260,1 @c 72,460,1 @k 14 Dim gray @c 132,360,120 Jupiter's orbit Tycho The Tycho data was collected as part of the overall mission of the Hipparcos satellite; it represents a catalog of position, parallax, proper motion, and magnitude data collected for over 1 000 000 stars. In most cases, its precision is much greater than all earlier catalogs. About the only case in which the Tycho data would be ignored would be if Hipparcos data is available instead. The Tycho data is essentially a survey of all stars that were bright enough to be measured by the detector, and is essentially complete to about magnitude 10.5, with somewhat incomplete coverage to magnitude 11 or 11.5. Tycho Input Catalog When the Hubble Guide Star Catalog was put together, it contained stars from magnitudes 9 to 15. Since the brighter stars weren't needed to point the telescope, they were left out. Later on, the bright stars were added in from the Tycho Input Catalog, a separate source. type The Hubble Guide Star Catalog, or GSC, classifies each object as a star, non-star, galaxy, blend of two objects, or as an artifact (such as a scratch mark on a photographic plate). Since the same object sometimes appears on several different, slightly overlapping plates, it is not uncommon for an object that looks like a star on one plate to appear as a non-star on another. Guide will list these types for you when you ask for "more info" on a GSC star. Type I supernova A type I supernova is one of the two types of supernovae. This particular type shows little or no sign of hydrogen in its spectrum. After it peaks, it drops by about .1 magnitude a day for 20 to 30 days; then the rate of decrease slows down to about .014 magnitude/day. Type I supernovae are divided into three types: Ia, Ib, and Ic. Ia supernovae occur when a white dwarf is in close orbit with another star, and is pulling matter from its companion and growing in mass. Eventually, it gets to 1.4 times the mass of the Sun. This is the most massive a white dwarf can be; beyond this point, it must collapse into a neutron star, releasing its energy in a supernova explosion while it does so. Because the mass limit (called Chandrasekhar's limit) is so exact, these supernovae are always of about the same brightness, so they make a good means of determining the distance to distant galaxies. Type Ib and Ic supernovae are probably type II supernovae where the parent star has lost its outer layer of hydrogen (for Ib) or hydrogen and helium (for a Ic). Type II supernova A type II supernova is one of the two types of supernovae. Unlike a type I supernova, a type II will show mostly hydrogen and helium in its spectrum. This type also shows wider variations in how they fade away, but usually, after 40 to 100 days, they are fading by .1 magnitude per day. Type II supernovae occur in very massive stars, at least ten times the mass of the Sun. Such stars start out by fusing hydrogen into helium. Later, they start forcing helium nuclei together into carbon, oxygen, and still heavier nuclei, until they produce iron. The iron cannot be fused to provide further energy to maintain the core temperature; eventually, the core collapses under the weight of the upper layers, rebounds, and throws off the upper layers of the star. UCAS galaxy catalog The UCAS galaxy catalog is a special catalog of galaxies between RA 10h58m and 12h48m, declination S 21 45' to N 10 48' (J2000 coordinates). The objects were examined from a series of UK Schmidt IIIa-J plates (originals, not copies) taken for an asteroid survey conducted by Bobby Bus. 26 of the best plates were chosen to scan for galaxies. Brian Skiff did this by examining them on a light-table with a 6x loupe containing a compass and millimeter reticle. He got morphological types for all the galaxies appearing in some catalogue (via the Dixon et al overlays for the POSS-I) and others that were reasonably large/bright, such that they would have appeared in Zwicky's catalogue had it extended south of the equator, plus whatever else caught his attention. He measured for size and position angle all the NGC/IC galaxies plus some other of the largest non-NGC/IC galaxies. UCAS galaxy size The sizes in the UCAS galaxy catalog include somewhat more of the outlying areas of a galaxy than is usual. Catalogs such as the RC3 measure the size of a galaxy out to the point where its surface brightness drops to 25 magnitudes per square arcsecond (i.e., if you took a chunk of the sky from here that was one arcsecond on a side, and somehow isolated it, it would show up as a 25th magnitude object.) The UCAS measurements, on the other hand, didn't stop until the surface brightness dropped to 26 or 27 magnitudes per square arcsecond. So galaxy sizes from UCAS tend to be a bit larger than those listed in other catalogs. In general, you should use RC3 sizes in preference to UCAS sizes. UGC Uppsala General Catalog The UGC, or Uppsala General Catalog, contains galaxies too faint to appear in the NGC catalog. It was compiled by astronomers in Uppsala, Sweden, and has detailed information on the position, size, and other characteristics of almost 13,000 galaxies north of about declination S 2 degrees. You can find an object by its UGC number by using the Go to UGC option in the Go to Galaxy menu in the Go To menu. ultraviolet UV The human eye can see a range of visible light from red to blue. Light just outside the blue limit is called ultraviolet, or UV, radiation. Its wavelength ranges roughly from 5 to 400 nanometers. It's difficult to observe celestial objects in the ultraviolet, because the ozone layer absorbs almost all of it. In general, this is just as well, since the sun emits enough UV to kill us all. Most UV observing has been done from satellites, such as Copernicus, IUE, and EUVE (Extreme Ultraviolet Explorer). UV is mostly emitted by extremely hot objects; things such as Markarian galaxies and cataclysmic variables, for example. Ultraviolet FeII The spectrum of a star shows lines corresponding to elements heated to high temperatures. One such line, in the ultraviolet part of the spectrum (between visible light and X-rays), is due to iron. The symbol for iron is Fe, and many stars show this ultraviolet FeII line. (A given element usually produces a lot of different lines; they are indicated by Roman numerals which indicate the ionic state of the element. The type of spectrum changes with the number of electrons in the last, partly filled, shell. NaI (neutral sodium) has a hydrogen-like spectrum; the spectrum of NaII (sodium once ionized) is very much like that of Ne. FeII means iron once ionized; OIII means oxygen twice ionized) Uranus Uranus, the seventh planet from the Sun, was found by William Herschel in 1781, using a small homemade telescope. At times, the planet gets just bright enough to see with an unaided eye under good conditions. Physically, Uranus is much like Jupiter, Saturn and Neptune: a large planet composed of mostly hydrogen, methane and other gases. It is 19 times farther from the Sun than we are, and therefore tends to stay at about -350 degrees Fahrenheit (-200 Centigrade). It is unusual in that its poles are almost in the plane of its orbit. If this were true for the Earth, the Sun would be straight overhead in June as seen from the North Pole; it would slowly spiral down to the horizon, making a circle around the sky once a day, finally setting in September; the Sun would go farther and farther below the horizon, finally starting back up in December; it would rise again in March, and spiral its way back up until June again. You can see that this process of having a 'day' that lasts all year might lead to some extreme weather conditions. It takes Uranus 84 years to circle the Sun, so first one hemisphere gets most of the sunlight for 42 years while the other half freezes; then the hemispheres reverse for the next 42 years. Uranus has several satellites, all named for Shake- spearean characters, none much larger than 600 mi (1000 km). The planet and its satellites were examined in some detail by Voyager 2 in 1986. Use MPCORB By default, Guide uses an immense built-in data of orbital elements for computing highly accurate positions of asteroids. Because this data is precomputed, it can account for most planetary perturbations and be very fast, and show accurate positions over a long time span. However, you can use the Use MPCORB option in the Extras menu to cause Guide to instead use data provided by the Minor Planet Center. Their data file, MPCORB.DAT, is available via anonymous ftp at: ! ftp://cfa-ftp.harvard.edu/pub/MPCORB ! The MPC provides the database in two forms: as "MPCORBCR.DAT" and "MPCORB.DAT". Both contain exactly the same data, but the former is in PC format (with a carriage return/line feed at the end of every line of data), the latter in Unix format (a line feed but no CR at the end of each line). Guide will recognize either file. MPC also makes both files available in .ZIP format, reducing the download size considerably. The files are updated daily. There are two important disadvantages to use of MPCORB. First, the data exists at only one epoch, always within 100 days of the present. If you try to compute positions far away from that epoch, the quality of the results will gradually deteriorate. The Guide CD provides orbital elements at 50-day intervals over a span of several decades, so the epoch of the data is never off by more than 25 days. Thus, for objects that were well-determined in the original Guide data, MPCORB positions will be _less_ accurate. It's best to avoid MPCORB, for example, in computing asteroid occultations, which always involve well-determined objects. (In defense of MPC, it should be pointed out that if Guide had a built-in orbital integrator, this problem would go away and MPCORB would be more accurate in all cases.) Another disadvantage to MPCORB will be immediately apparent: it's slow. For the built-in data, Guide has some precomputed information it can use to immediately determine that 99% of the asteroids will not appear in a given chunk of the sky. That precomputed data doesn't exist for MPCORB. user added dataset The user added dataset capability makes it fairly simple to add your own datasets to Guide. Many examples now exist; if you click on the Toggle User Datasets option in the Extras Menu, you will get a full list and can turn them on or off and adjust their display. The examples include datasets such as catalogs of quasars, active galactic nuclei, radio catalogs, and so on. These can be of considerable use in their own right, even if you do not have datasets of your own that you want to add to Guide. For full details on adding your own datasets, see the manual. If you've turned a dataset on and now you want to find a particular object in that dataset, you should use the Go to .TDF object option in the Go To menu. Using Help Guide has an extensive help system to explain both the operation of the program and some of the astronomical information it provides you. The information is in a linked hypertext form; if you click on a boxed item shown in light blue, the display will switch to show information about that topic. For example, clicking on "Guide" above will lead to a list of phone numbers, addresses, and so forth. Some of the more useful help features are: Using the Glossary section Printing help information Writing help information to an ASCII file Getting a list of hotkeys Getting help for a menu item Using the Glossary section , You can enter the Glossary section of Guide by hitting the comma (,) key at any point. You can also enter it by clicking on the "glossary" item at the bottom of any help screen. The glossary is divided into several pages; when in the Glossary section, controls for selecting a page appear at the bottom of the screen. USNO The USNO (United States Naval Observatory) pursues a variety of research tasks, many involving astrometry. In this regard, it is the creator of several important star catalogs, such as the SA1.0 and A1.0.