constellation borders In 1930, the International Astronomical Union (IAU) officially defined 88 constellations and gave them borders. These borders are similar to the rectangular borders in some Western U.S. states that run exactly north/south or east/west. The constellation borders run along lines of declination and right ascension. (At least, they did in 1875. Since then, precession has added a very small rotation to this.) The display of these borders is controlled in the Overlay menu. constellation labels This program can label each constellation on the screen with its three-letter abbreviation. These labels are stored as overlays, and their display can be adjusted in the Overlay menu. You also can click on the labels themselves to get a full constellation name, and can also then click for "more info" on the constellation. constellation lines Most people find constellation lines a helpful aid to learning their way around the sky. For each constell- ation, a set of lines is defined that either matches what the constellation is supposed to represent (such as the Big and Little Dippers or the Southern Cross) or simply helps in remembering which stars are where. Keep in mind that there are no standards defining how these lines are set up, and the set in this program may not exactly match a set you've seen elsewhere. The display of these lines is controlled in the Overlay menu. contact binary In a contact binary star, the two stars orbiting one another are close enough together for matter to flow between them. This will at least make the star an elliptical variable, and probably an eclipsing binary as well. Because of the ellipticity, it can be hard to tell when eclipses begin or end. Periods are less than one day. The variation is usually less than .8 magnitude. COPERNICUS The COPERNICUS satellite made observations of the spectra of a large number of stars. In particular, it measured spectra in ultraviolet, a feat much easier to do from orbit than from ground level. (The ozone layer tends to absorb ultraviolet light; COPERNICUS, of course, was well above the ozone.) corrector Most telescopes are designed to produce a well-focused image at the center of the field of view. Usually, stars become blurrier as you move away from the center of the field. This isn't usually much of a problem, since you are usually only interested in an object at the center of the field. But if you want to take a survey photo of a large area of the sky, you need a corrector to improve the image over the entire field of view. This can be one ("singlet") small lens, placed near the focus of the telescope. However, if you use a two-lens ("doublet") or more lenses, you can get better correction over a larger field. CPM common proper motion Suppose you see two stars close together and think they might be binary. On the other hand, maybe one is very close and the other far away, and they just look close together. How can you tell if they are really binary, or just faking it? One good clue is to measure their proper motion. If they have the same, or common, proper motion (CPM), then you know that at the very least they are going in the same direction and are probably a real binary star. Critical List The Minor Planet Center maintains a Critical List of asteroids that are in particular need of having positions measured via astrometry. Objects on this list often have not been measured for a few years, and the precision of their orbits is beginning to degrade; given a few positional measurements, though, the orbital elements can be recomputed and greatly improved. Further information is available at http://cfa-www.harvard.edu/cfa/ps/mpc.html CTB G GDC GGD Gum Haro HeRa HN Lynga Mi MiC MWC Shajn Min LkHa VMT H LHa NS Re Pars SNR CG McD The catalog you just clicked on is one of several seldom-used catalogs that are mentioned in the Atlas of Galactic Nebulae. If you are interrested in the exact reference, please contact Project Pluto. Ctrl-B The Ctrl-B hotkey causes Guide to ask you to enter a Bayer or Flamsteed designation, using the usual abbreviations; or a common name. For example: alp umi 40 eri Bet Per Capella Sirius (The use or non-use of capitals is ignored.) Guide will then redraw the chart, centered on that object. This is provided as an alternative to the Go to Bayer/Flamsteed menu option; both accomplish the same task of finding a numbered or lettered star, but one method emphasizes the keyboard, while the other uses the mouse. Ctrl-E In the DOS version, you can use the Ctrl-E hotkey to reset the default epoch from J2000 to a different value. Hit Ctrl-E, or click on the epoch shown in the legend area, and Guide will prompt you to enter the new default epoch. Unless otherwise mentioned, all positions shown in Guide (or entered at the keyboard) will be in this default epoch. (There are exceptions, but they are all clearly labelled.) CTRL-F4 "Normally", the cursor keys can be used to move the mouse. This can be especially useful if your system lacks a mouse; you can then still click on objects and such using the keyboard ("insert" is used for the left mouse button, "delete" for the right mouse button). In the DOS versions of Guide, you can use this option to toggle from this behavior to one where the cursor keys instead pan the field of view, in the direction of the cursor keys. CTRL-F8 The Ctrl-F8 hotkey recenters the CCD frame on the current cursor position. The frame will also be turned on, if it was not on already. CTRL-L Legend on/off This item lets you turn the legend on or off. When on, the legend takes up room in the lower left corner of the chart. Some users may not be very happy about losing part of their field of view to the legend, thus the need for this menu item. When you toggle the legend ON, the Legend dialog will be shown. This menu lets you select which items are shown in the legend (for example, captions, time, RA/dec, latitude/longitude, and so on.) You can toggle the legend at any point in Guide with the CTRL-L hotkey. Ctrl-U In DOS, the Ctrl-U hotkey can be used to set the display of labels showing the magnitudes of stars in the chart. For example, if you wanted to make a chart labelling the magnitudes of stars down to mag 12, you would hit Ctrl-U and enter 12. To get rid of the labels, you would hit Ctrl-U again. In the Windows version, this option is found in the Star Display dialog. CTRL-X Enter Longitude The Enter Longitude option lets you reset your geographic longitude. (You can get this from USGS maps and from most others.) You enter a longitude as a compass sign (E or W) followed by degrees, minutes, and seconds. For example, the longitude of Los Angeles could be entered as W 118 15 30 Sometimes longitudes are expressed as degrees and decimal minutes, or decimal degrees. You can use these instead: W 118 15.5 W 118.25833 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-X hotkey, or through the Location dialog in the Settings menu. CTRL-Z Enter Altitude The Enter Altitude option lets you reset your geographic altitude, in meters. For most purposes, this is not an important factor, but if you're looking for very precise timings of some events (to within a fraction of a second), it can be useful. You can reach this option at any time with the CTRL-Z hotkey, or through the Location dialog in the Settings menu. dark nebula A dark nebula is visible only in that it blocks the light of stars in the background. There are quite a few such objects, particularly in the plane of our galaxy. Indeed, without them, the sky would be a much brighter place. data display dialog The data display dialog provides controls over the way in which most on-screen data in Guide is shown. It appears when you use the Toggle User Datasets option in the Extras menu, or on the Toggle Overlays option in the Overlays menu. Also, when you right-click on many objects on the chart, the resulting dialog box often includes a "Display" button that will lead to this dialog. The dialog will generally look something like this: ! [ ]--- Quasars ----------------[X] | ( ) On [X] Labels | | ( ) Off | | (o) Auto | | ( ) Fixed | | | | Magnitude limit: __14.0_ | | | | Show at: __1_ - _180_ | | | | [ Options...] [ Color ] | | | | [ OK ] [ Cancel ] | |________________________________| ! You can turn a dataset On or Off, so that either everything is shown or nothing is shown. You can set 'Fixed' and have everything shown down to a magnitude limit, and that limit stays constant as you zoom in or out. 'Auto' does the same thing, except that Guide will adjust the magnitude limit as the field of view changes. And you can turn labels on or off and adjust the color used to display that class of object. In some cases, certain options will be grayed out. For example, turn a dataset on or off, and the magnitude limit doesn't matter anymore and is grayed out. Some user-added datasets (such as the "Millennium Star Atlas pages", "RealSky North plates", and so on) don't have any magnitudes, and the 'Auto' and 'Fixed' options are grayed; magnitude limits don't make much sense for, say, constellation borders either. Some datasets don't have labels, and that option is therefore grayed out. The "Show at A to B degrees" option is available for overlays and user-added datasets, and defaults to 0-180: that is, Guide will show this dataset no matter what field of view you have. But if you wish, you can use this option to tell Guide that a given dataset should only appear at certain fields of view. The "Options..." button is usually grayed out, but not always. It leads to a dialog box with options specific to the current data type. For example, for artificial satellites, it would lead to an option to reset the TLE filename. For comets, it would lead to options to reset the line of variation and to add MPC comets/asteroids. For planets, it would lead to a rather complex planet display dialog. data inferred When you ask for "more info" about a star for which Hipparcos or Tycho data is available, Guide will first show that data, and will then show some quantities derived from them. For example, both catalogs have parallax and magnitude data; Guide can use the parallax to determine a distance to the star, and can then compute the luminosity and absolute magnitude. This data appears in a separate "data inferred" section. The idea is that this information doesn't actually appear directly in the catalog; it is just computed based on the catalog data. Data Shown CTRL-D This menu lets you determine what kinds of objects are shown on the chart. From this menu, you can turn Messier and NGC objects, nebulae, planets, variables and suspected variables, and other classes of celestial objects on or off, or control how they are shown. This menu can be reached at any time via the CTRL-D hotkey. date of perihelion A comet or other object moves around the sun in an ellipse, parabola or hyperbola. The point where it is closest to the Sun is its perihelion. Guide will list this date for comets. If the comet is periodic, it will return to another perihelion; Guide will list the date closest to the current one. declination dec RA right ascension RA and declination are the measurements astronomers use to specify locations in the sky. They are very similar to latitude and longitude on the Earth. The declination of a point in the sky, like the latitude on the Earth, is a number between -90 and 90 degrees. The RA (or right ascension) of a point on the sky is very similar to longitude, except that instead of being expressed in degrees, it is expressed in hours, minutes and seconds, and can range from 0h0m0s to 24h0m0s. Given an RA and a declination, any point in the sky can be specified precisely. For example, Sirius, the brightest star in the sky, is at RA 6h45m08.9s, dec -16.716 degrees. One difference between RA/dec and latitude/longitude is that the celestial equivalents of the poles, equator and prime meridian drift over time, so in giving an RA/dec, you usually specify its epoch, the time for which it is valid. In the past, it's been common to use epochs for the years 1900 and 1950; by now, most people have switched to 2000, also shown as J2000. The default epoch for the legends, grids and other markings in this program is J2000. Latitude and longitude on Earth are measured from the Equator and the Prime Meridian. RA and declination are measured from the vernal equinox and the celestial equator. degree The term degree here is meant as a unit for measuring the angle between objects in the sky. The angle between the horizon and the point straight overhead (the zenith) is 90 degrees. The angle between the "pointer stars" in the Big Dipper is about five degrees. The angle across your thumb at arm's length is about one degree, which can make a pretty useful measuring tool. The Moon and Sun are about half a degree across. For measuring small angles, the units arcminutes and arcseconds are used. There are sixty arcminutes to a degree, and sixty arcseconds to an arcminute. Delta Cep Beta Cep There are several types of Cepheid variable. The Delta Cep type resembles the star Delta Cepheus. These are young stars with very regular periods of a few days and have a certain set relationship between the amount they vary, their period of variation, and their actual intrinsic brightness. The Beta Cep type of variable have periods of .1 to .6 days, and vary by .1 to .3 magnitudes, less than the variations in Delta Cepheids. Most simply expand and contract radially, but a few have oddball "quivering" motions that give them more complex behavior (and more random-looking light curves.) Delta Sct The Delta Sct type of variable star is a low-mass, pulsating star. The pulsations aren't symmetric; the star quivers like a ball of Jello. This means the changes in light aren't very regular either. They vary from .003 to .9 magnitudes from minimum to maximum; usual changes are a few hundredths of a magnitude. They also expand and contract, just like normal Cepheid variables. There are usually two or more major pulsations at slightly different frequencies. Sometimes one set of pulsations will be just out of synch with another set, and the changes in brightness will go away for a while. Delta-T Delta-T is the difference between Dynamical Time (TD), the uniform time system based on atomic clocks, and Universal Time (UT), the slightly irregular system based on the earth's rotation. It is widely used in astronomical formulae. Guide uses observed values of Delta-T for dates from 1620 to 1994, and a formula to approximate values before 1620. (Our knowledge of the earth's rotation, and hence of UT, is limited for medieval and ancient dates.) For dates in the future, it uses a separate formula that makes some assumptions about how the rotational speed will vary. It shows the value used for Delta-T in the Quick Info section. DG Reference: Dorschner, J. and Grtler, J.: 1964, Astronomische Nachrichten 287, 257. Verzeichnis von Reflektionsnebeln. diffuse nebula Diffuse nebulae are large masses of gas in interstellar space. In some cases, the gas is heated up by stars inside to the point where it glows and forms a bright diffuse nebula; in other cases, the gas simply blocks the light that would normally come from background stars, forming a dark diffuse nebula. Diffuse nebulae are most common near the core of our galaxy, in the direction of the constellation Sagittarius. Dim Stars - You can hit the - key at any point in Guide to make the stars a little dimmer, and to lower the limiting magnitude for all objects. You can use it repeatedly until all stars have been dimmed into oblivion, though you'll probably want to stop before that point. Each use of this option subtracts .5 from the current limiting magnitude. You can also add a button for this option to the toolbar, by using the Toolbar Dialog. Direct to screen Usually, when Guide draws a new chart, it erases the screen and you watch as the chart is drawn. This is "Direct to Screen" mode. In Windows, you can turn this mode off in the Display Menu. Do so, and when charts are drawn, the old one will remain while a new chart is being drawn; the new chart will appear to instantly replace the old one. This option exists for two reasons. First, some people prefer it to seeing the charts drawn. Second, on some systems, it can be a little faster; this is especially true on slower video cards. Discovered at This line shows where the asteroid was discovered. Discoverer's ID The various observers of double stars have been given designations, ranging up to three letters. A double can be specified with the discoverer's ID and a number, such as HJ 2574, the 2574th double catalogued by John Herschel. Discoverer's name This line shows the name of the discoverer of the asteroid. Discovery date This line shows the date on which the asteroid was found and recognized to be an asteroid. Sometimes it turns out that the asteroid was photographed at an earlier date, but went unrecognized; such cases are called prediscoveries. (For example, Pluto, discovered in 1930, was later found on old plates dating back to 1914.) display menu Alt-D The Display menu offers the following options for determining what markings are shown on your chart: Star Display dialog (set the sizes for stars and how they are labelled) Data Shown (Windows only; in DOS, this is part of the Main Menu) Legend on/off CCD Frame on/off Inversion dialog (flip the chart north/south and east/ west; rotate the chart; put the zenith at the top, or ecliptic or galactic north) Measurements dialog (ticks, grids, etc.) All markings used to indicate distances and coordinates Screen fonts (Windows only) Printer fonts (Windows only) Direct to screen (Windows only) Background Dialog Isophotes distance at closest approach time of closest approach If you click for "more info" about an asteroid, and the object is within 1 AU, Guide will assume that the object is about to make a close approach to your home planet (or has done so recently), and will calculate the time when the object was closest, and how far away it was from you at the time. This data is shown as the distance at closest approach and the time of closest approach. Doppler shift Doppler effect The Doppler shift is most familiar from the sounds vehicles make. An approaching train, for example, makes a higher-pitched sound than one moving away from you. The sound waves are "pushed together" in the first case, and "stretched apart" in the second. Light waves do much the same thing. An object approaching us rapidly appears shifted toward blue; one receding appears redder than usual. This effect, though small, can be measured and can be used to tell you how fast an object is moving toward or away from you. (It can't tell you anything about sideways motion.) It makes the spectrum of an object appear shifted relative to what one might "normally" expect. Most galaxies have fairly large red shifts, meaning that they seem to move very rapidly away from us. Hubble figured out that there is a direct relationship between the shift (and therefore the velocity) and the galaxy's distance from us. double star It is common to find stars in pairs or larger groups. Sometimes, the pairing is by coincidence: the objects simply appear near to one another as seen from Earth, even though one star may be much farther away than the other. Often, however, the stars are really physically bound to one another by gravity, as the Earth is to the moon; such objects are called binary stars. Double stars are listed in a variety of catalogs; if you click on Go to Double Star in the Go to Star menu, you will see about 130 catalogs listed, such as the ADS and Struve catalogs.