time dialog Alt-T The Time Dialog box provides all control needed to reset the time, time zone, and calendars used by Guide. The date and time currently set appear at the top of the box. Buttons above and below the time let you make changes of tens of days, single days, thousands of years, centuries, and so on, all the way down to changes of one second. You can also click on the day, year, hour, minute, or second to change these values directly. The calendar lets you reset the day of the month (or move to the preceding or following month) in one mouse click. Clicking on the "Time Zone" button brings up the time zone dialog. Of course, this button also tells you to what the zone is presently set. Clicking on the "JD" button will prompt you to enter the desired Julian Day. Again, the button also serves the purpose of telling you what JD is currently set. The next button toggles between the Julian and Gregorian calendars. Clicking on "Current Time" tells Guide to read your computer's built-in clock and reset the time accordingly. The array of twelve buttons at the bottom allows one to reset the month in one mouse click. You can reach this dialog box by hitting Alt-T; or by clicking on the time shown in the legend; or by using the menu items "Setting" and "Time". time zone dialog The time zone dialog box allows one to reset the default time zone. Time zones are listed explicitly, using the common three letter abbreviations, for the four US time zones and their daylight savings time (Summer Time) versions, and Greenwich Time, or Universal Time, is also listed. If you want to use a different local time, clicking on the "Enter time diff" option will let you enter a difference, in hours, between Greenwich and local time. For example, EST would be entered as -5; Tokyo time as 9; and so on. user object dialog F5 The user object dialog provides a way to maintain your own list of objects. It also allows you to load this list onto a Sky Commander. To use this option, find the object (or point in the sky) of interest and center on it. Bring up this dialog, and click on "Add Object". You'll be prompted for the object name; enter this, and the object will be added to the list. Once you have added objects, controls will be available to delete and "go to" (recenter the chart on) them. You can access this option with the F5 hotkey. [ Hitting [, or clicking on the "Faster" button in the animation dialog, lets you increase the rate of animation. ] Hitting ], or clicking on the "Slower" button in the animation dialog, lets you decrease the rate of animation. a sin i The spectrum of a binary star will show each line split, representing the different stars orbiting each other. The amount of split, plus the time it takes the stars to orbit one another, tells you the (minimum) distance between them. This quantity is called a sin i. 'a' is the distance between the two stars, measured in astronomical units. The 'sin i' is the sine of the angle between the orbit of the stars and a line from the stars to the earth; it basically means that a sin i is the minimum possible distance between the two stars, and we don't know the maximum. A1.0 A2.0 Ax.0 The USNO (US Naval Observatory) has created an enormous catalogs of stars, showing almost 500 million objects. The first version, A1.0, consumes a set of ten CD-ROMs; the updated version, A2.0, consumes eleven CD-ROMs, and is the current record-holder for most detailed dataset. The two have strong similarities, and are referred to as Ax.0 in situations where their differences are unimportant. Getting a set of the CD-ROMs for Ax.0 is not easy; the USNO didn't make many, and reserves them for especially deserving applicants. You can, however, download Ax.0 data for a small region through http://www.lowell.edu (Note that this URL may change!) You can view downloaded Ax.0 data in Guide. If you have the CDs, you can extract data from them and view it in Guide through the Get Ax.0 data option in the Extras menu. The USNO does make single-CD "Selected Ax.0", or SAx.0, versions available freely; check http://www.usno.navy.mil/pmm for current availability information. The SAx.0 data can also be viewed in Guide, in the same manner as Ax.0 data. You can also create a list of Ax.0 stars with the Create Star List option, and can click on them for "more info", just like the "usual" GSC and Hipparcos stars. AAVSO American Association of Variable Star Observers The AAVSO (American Association of Variable Star Observers) is an organization gathering and distributing data regarding variable stars. Observing variables is a fairly common amateur activity, and the data gathered can be extremely useful; there are a large number of variables whose behavior is still not well understood. It is an area of astronomy where you can produce useful data without a large telescope and an immense grant; in fact, observations made with anything from binoculars on up can be used. Those interested in further information should contact: American Association of Variable Star Observers 25 Birch St. Cambridge MA 02138 Internet: firstname.lastname@example.org Tel. (617) 354 0484 FAX (617) 354 0665 Abell The Abell catalog of clusters of galaxies is one of two such catalogs available in this program. (The other is the Zwicky catalog.) It contains 5,250 objects covering the entire sky, including the southern half. You can find an object by its Abell number by using the Go to Abell Cluster option in the Go to Galaxy menu in the Go To menu. This shouldn't be confused with the Abell catalog of planetary nebulae; that catalog is an entirely separate thing. aberration The earth's motion introduces a very slight distortion in the apparent positions of celestial objects. Readers of some science fiction stories will recognize the effect; in a spaceship moving at a large percentage of the speed of light, the stars would seem to "creep ahead" and cluster around the forward direction. The Earth moves at a much smaller speed, of course, but stars and planets do cluster very slightly toward the direction of our motion around the Sun. This effect is called aberration; it never amounts to more than about 20 arcseconds, enough to be noticeable with precise instruments. Along with nutation, the effect is included in converting a mean position at current epoch into an apparent position at current epoch. Above is the computed position for J1991.25 The Hipparcos satellite gathered data for several years, with 2 April 1991 being close to the center of the observations. So the catalog therefore gives positions for the date J1991.25, also known as 13:30 UT on 2 Apr 1991. This is the standard epoch of observation for the catalog. The catalog data is most precise for this date, and becomes gradually less precise for dates in the past and future. Note that ALL THE POSITIONS ARE STILL IN THE J2000 COORDINATE SYSTEM. Essentially, the Hipparcos and Tycho catalogs list the J2000 position for stars as they appeared on 2 Apr 1991, at 13:30 UT. absolute magnitude This term has two different meanings: one for stars, one for asteroids. The absolute magnitude of a star is the magnitude it would have if it were ten parsecs away from us. This measurement lets us see bright objects and dim objects as corrected for their different distances. While it is often a figure of great interest, it can be difficult to determine. The absolute magnitude of an asteroid is the magnitude it would have if it were one AU from the Sun, as seen by an observer on the sun. It is usually given the symbol 'H'. It is used along with the slope parameter to compute the asteroid magnitude for any position and distance, and also allows Guide to compute an assumed asteroid diameter. Acceleration in RA Acceleration in dec The Hipparcos satellite examined a large number of binary stars in an effort to analyze their orbits. In many cases, this was unsuccessful; the satellite could tell that the stars were not moving in straight lines (and were therefore presumably pulling on one another), but the data wasn't precise enough to compute an orbit. In such cases, the only really useful data that was derived was an acceleration in RA and an acceleration in dec, usually given in units of milliarcseconds per year per year. For example, an acceleration of -13 mas/yr/yr means that, each year, the proper motion of that star decreases by -13 milliarcseconds/year along that axis. accretion disk Many stars and quasars have accretion disks. These form when the object has lots of dust and gas orbiting it. As the dust particles and gas molecules collide, they lose energy to one another, so they wind up in lower orbits. Eventually, after a fair number of collisions, the matter hits the star itself. This entire process turns the kinetic energy of the gas and dust into heat energy; in other words, the disk is hot, and disks can sometimes be spotted by the infrared energy they emit. The disk shape results from the fact that the matter falling in will almost never head straight for the object at the center. One direction of orbiting will be favored by most of the matter, and anything going the wrong way will get hit by gas and dust until it does go the right way. Often the energy poured on the central object will raise jets, which will flow out from the object at right angles to the disk. ACT catalog The ACT catalog was produced by the USNO, and combines data from the Tycho and Astrographic (AC) catalogs to get very precise proper motions. One problem with the Tycho catalog is that the data was collected over a short time (the satellite was only used for a few years), which leads to very rough proper motion data. The USNO fixed this by using Tycho positions combined with AC positions; the AC positions all date from around 1900, so they provide a long baseline for computing proper motions. Guide uses Hipparcos data when possible, but that only covers about 118,000 stars. For the remaining stars, Guide uses Tycho positions combined with ACT proper motions wherever it can, for the greatest possible accuracy. Add a Trail Use the Add a Trail option to add a planetary (or asteroid or comet or artificial satellite) trail to the display. To use this, you should first set the current time, in the Time dialog, to that for the start of the trail. Right-click on the object for which you want a trail. (This option, and the somewhat similar Make Ephemeris option, won't work if you haven't clicked on a solar system object.) Finally, when you click on "Add a Trail", you'll be asked for the desired number of steps and the step size. If you've set the step size to 2 days/step, for example, and would like to cover a 90-day period, you would enter 45 at this point. Guide will calculate the position of the object for 90 days at 2-day intervals, and show the resulting trail. A button is also provided to set the color for the trail, and you can select the frequency at which index marks are added to the trail. Add DSS image If you have downloaded a DSS (Digital Sky Survey) image from the Internet, you can use the Add DSS image to view it in Guide, just as if it were an image from the RealSky CDs. To do this, click on the Add DSS image option. In Windows, this will bring up a file dialog; in DOS, you'll be asked to type in the file name. Do so, and Guide will analyze the FITS file header and will add it to the list of images to be drawn. When you click on "Clear RealSky Images", it will be dropped from the list. Of course, you can then just click on Add DSS Image to restore it to the list. Add MPC comets/asteroids Ctrl-F12 Gareth Williams, at the Minor Planet Center, has very kindly provided orbital elements for comets and asteroids in the native format of Guide at the MPC's Web site. You can get data for currently visible comets, Critical List minor planets, and distant minor planets (for example, Kuiper belt and Centaur-type objects. You can download these files from http://cfa-www.harvard.edu/cfa/ps/Ephemerides/Soft02.html in either ASCII text or HTML format (Guide can use both, so it doesn't matter which you use.) Once you have downloaded the data you'd like to use, click on Add MPC comets/asteroids in the Extras menu. Guide will ask for the name of the file you've downloaded; provide it, and Guide will add new objects and update old objects using that data. You can also access this option through the Ctrl-F12 hotkey. Also, if you subscribe to the Minor Planet Electronic Circulars (MPECs), you can specify a file containing one or more Circulars. Guide will comb through it for orbital data, and add the orbits to its database. ADS The Aitken Double Star, or ADS, catalog, lists most prominent double, or binary, stars. The catalog includes information about the position of the stars relative to each other, their magnitudes, and their angular separation. The WDS catalog often contains this information. AGK The AGK (Astronomisches Gesellschaft Katalog) is an older catalog covering stars in the northern half of the sky. AGK numbers are given for reference purposes. Albedo An object's albedo is a measurement of how much light it reflects. An object with a high albedo reflects most of the light hitting it; Venus, for example, has clouds that reflect 70% of the sunlight that hits it, which is one reason it's so bright. An albedo of 0 means no light is reflected (totally black); an albedo of 1 means all light is reflected (perfectly polished). Most asteroids have low albedos, around 10% or so. 437 Rhodia has the highest measured asteroid albedo; it 56% of the light hitting it. From up close, it would probably look like chalk. Most asteroids are much more dull than this. Alpha CVn The Alpha CVn class of variable stars show small changes in brightness caused by the star's rotation. One side of the star is slightly different from the other, and as the star spins, the brightness changes by from .01 to .1 magnitude. These are mostly hotter stars than the Sun (spectral types B8 through A7), with strong magnetic fields. It is thought the fields may help make different parts of the star's surface of different compositions and brightness, in a manner similar to how the Sun's magnetic field makes sunspots. The spectrum of this kind of star shows a lot of silicon, strontium, chromium and rare earth elements. Alpha Cyg The Alpha Cyg type of variable star changes in brightness because the star physically pulsates, in an uneven manner like a ball of Jello. This makes for very irregular changes in brightness. The changes have to stay small or the star would be really unstable and might split or blow up, and in fact, this kind of star usually varies by less than a tenth of a magnitude. ALT-0 This option sets the time to 0 hours UT (midnight) for the current day. This option can be reached at any time with the ALT-0 key. ALT-B This option allows you to back up to previous views, up to five views. It differs from the effect of the F7 key, which toggles between two charts. This option can be reached at any time with the ALT-B key. Alt-F4 The Alt-F4 hotkey allows you to exit Guide from any point in the menus, instead of requiring you to back up to the main menu. When you hit Alt-F4, Guide will check to make sure you really want to quit. Also, this lets you exit the program while in full screen mode. Alt-F8 In the DOS software, hitting Alt-F8 toggles between the usual, small cursor and a full-screen "cross" cursor. This was added because a few people find this large cursor easier to use, especially at night. ALT-L This option toggles to "flashlight" mode: black on a red screen. This can provide enough light to be useful in finding dropped objects, etc. when out in the field. This option can be reached at any time with the ALT-L key in the DOS version. Alt-M Set Video mode In the DOS versions of Guide, the default video mode is 640x480, 16 color; this is "standard" VGA. In the Settings menu, there is an option to reset the video mode to a higher resolution. Be aware that not all cards support all resolutions! If you find that GUIDE won't select the new video mode, try using WGUIDE instead; or if WGUIDE fails, try GUIDE. (It's not very common for _both_ GUIDE and WGUIDE to fail, although it does happen sometimes.) You can reach this option with the Alt-M hotkey. Alt-N Hitting the Alt-N hotkey causes Guide to recenter at the zenith with a 180-degree field of view, with the time reset from the PC clock. The resulting chart looks like the foldouts in the center of many astronomy magazines, showing a "horizon-to-horizon" view. For certain tasks while observing (such as following artificial satellites or telescope control), such a feature can be quite helpful. ALT-V This option gives one wider control over what is shown on the chart, and lets you do things that might usually be considered "ugly", or too slow, or otherwise unusual, such as displaying tenth-magnitude stars at level 2. When this option is turned on, the data shown on the screen is "frozen". You can zoom in and out to any level, and the data used will be the same. The program will cease to make assumptions about what makes a "reasonable" display. When the option is turned off, the program will resume making its usual decisions in these matters. This option can be toggled at any time with the ALT-V key. Alt-Z This item lets you switch between zenith up and celestial north up. The first puts the zenith at the top of the screen; the second puts the celestial north pole at the top. You usually want the first to match the screen to the real sky, and the second to match star atlases and charts. When you switch from one to another, the usual effect will be that what you see on the screen rotates around, sometimes upside down. You can reach this option at any time with the Alt-Z hotkey; or you can go into the Inversion dialog and select "zenith at top" or "north at top". Alternate names Dozens of catalogs of galaxies have been compiled for different purposes, and they tend to have a huge degree of overlap. It is common for objects to be listed in seven or more catalogs. Both the PGC and RC3 catalogs of galaxies contain extensive information as to alternate names an object may have been given in other catalogs. altitude azimuth You can use the altitude and azimuth of an object to find it in the sky. These figures tell you where an object is, as seen from a particular place at a particular time, in the sky. Altitude tells you how high the object is above the horizon. An altitude of 45 degrees puts the object halfway between the horizon and zenith (straight overhead); of 90 degrees, straight overhead; of zero degrees, on the horizon; and so on for values in between. An altitude of less than zero puts the object below the horizon, invisible to all lacking X-ray vision. Azimuth tells you in which direction to look. An azimuth of zero degrees puts the object to the north; of 90 degrees, to the east; of -90 or 270 degrees, to the west; of 180 or -180 degrees, to the south. Therefore, if you click on an object and are told that it is at altitude 10 degrees, azimuth 100 degrees, you should look for it a little south of due east, and about a ninth of the way up from the horizon to the zenith.