Double/Multiple The Bright Star catalog sometimes will remark on binary and multiple stars. Such comments may give the time it takes for the stars to orbit one another, their relative masses and spectral types, and so on. DSS RealSky The DSS (Digital Sky Survey) is an enormous dataset consisting of the scanned images from the Palomar Observatory Sky Survey and several other sky surveys, compressed to fit on 100 CD-ROMs. (RealSky is a version of the DSS which has been compressed still further, allowing it to be stored on fewer CDs at the cost of losing some image quality.) Like RealSky, DSS is available from the Astronomical Society of the Pacific, but at considerable cost. But you can download a DSS image for any desired area from at least three Web servers. See http://www.projectpluto.com for a current list. Once the images have been downloaded, you can display them in Guide, using the Add DSS Image option. Dual-mode Cepheid Dual-mode Cepheid stars are like normal Cepheid stars, except that their variations consist of two cycles, one on top of the other. Usually the more rapid cycle takes about 71% as long to complete as the slower cycle, and the slower cycle runs from 2 to 7 days in length. Durchmusterung DM The DM, or Durchmusterung, catalog was compiled in the late 1800's and contains magnitude estimates made by eyeball and guesswork. Later catalogs used photoelectric gear, getting more precise values, but the DM catalog is still in use for historical reasons. A DM specification for a star consists of two numbers. The first defines a declination to within a degree; the second defines the star's order among those in that one-degree zone in order of right ascension. The DM is also divided into three sections, the BD or Bonner Dorchmusterung, compiled for declinations of about -20 degrees and north; the CD or Cordoba Dorch- musterung, compiled for declinations between about -20 and -50 degrees; and the CPD, or CP, Cape Photographic Durchmusterung, for stars south of -50 degrees. DWB Reference: Dickel, H.R., Wendker,H. and Bierlitz, J.H.: 1969, Astronomy and Astrophysics 1,270. The Cygnus X Region V. Catalogue and Distances of Optically Visible HII Regions. Dynamic Parallax Dynamic parallax is a method used to measure the distance to binary stars. It's not too difficult to measure the angular separation between the two stars, and it's also easy to measure how long it takes them to orbit one another. If you can also get some idea as to how massive they are (usually from the spectra of the stars), you can then do the math to figure out how far apart they are physically... and from that, determining their distance from Earth becomes a straightforward problem. This method is important because we have very few ways to determine the distance to a star. The most common method, geometric parallax, only works accurately out to perhaps 100 light-years. This method gives us a little more distance and a way to check our results. Dynamical Time TD Dynamical Time, or TD (from the French abbreviation), is the time system used in most astronomical calculations. The problem with using Universal Time (UT) is that it matches the Earth's rotation, which is not entirely regular; it speeds up and slows down erratically, and sometimes a "leap second" has to be inserted at the end of a month in order to correct for this. Dynamical Time, on the other hand, is a uniform time system based on atomic clocks; it is a successor to "Ephemeris Time", an earlier system based on planetary motions that served the same function (though not as precisely). The difference TD-UT, also known as Delta-T, is currently about a minute; it can't be well-predicted into the future because of the irregular changes in the earth's rotation (and hence in UT) mentioned earlier. It is shown in Guide when you ask for Quick Info. Eccentricity Eccentric Most objects that circle the Sun have orbits only slightly distorted from a circle, and therefore are always at roughly the same distance from the Sun all the time. The Earth is in this category: in January, we're but a mere 3.3% closer to the Sun than we are in December. The measurement of this departure from a circular orbit is eccentricity. An eccentricity of 0 means the orbit is a circle. An eccentricity close to 1 means a very elliptical orbit, where the object swings in close to the Sun, then swings far out (as most comets do). eclipsing binaries eclipsing binary Algol-type There are a few binary stars where, in the course of orbiting one another, one star will block the light of the other as seen from Earth. These are called eclipsing binaries, or Algol-type stars. The most notable example is the star Beta Persei, also known as Algol (from Al Ghoul, Arabic for the Demon Star). This is usually a magnitude 2.1 star, made up of a large, dim star and a small, bright star. Every 2.867315 days, the large star eclipses the bright star. For about two hours, Algol's magnitude drops to 3.4. You'll notice that this process, unlike most variable stars, doesn't involve any real change in power output from the star. It just looks that way to us on Earth, because some light has been cut off from view. Another kind of eclipsing binary is exemplified by the star Beta Lyrae. Beta Lyrae consists of two stars so close together that their gravity stretches them out into egg shapes. As a result, the light varies depending on how much of the eggs are visible. Here, the variation is continuous, rather than the relatively abrupt eclipses of Algol. Ecliptic Alt-F1 The ecliptic is the plane of the Earth's orbit, as well as (roughly) the plane in which most objects in the solar system travel. It (roughly) follows the constellations of the zodiac. By default, Guide displays the ecliptic; you can toggle it on or off in the Measurements dialog, or (in the DOS version) with the Alt-F1 hotkey. Ecliptic coordinates Ecliptic coordinate ecliptic latitude ecliptic longitude Alt-, Ecliptic coordinates are an alternative way to specify positions in the sky. The ecliptic itself is defined by the plane of the earth's orbit; thus, points with a zero ecliptic latitude are in that plane (which corresponds pretty well to the plane of motion of most objects in the solar system). The zero point, or "prime meridian", for ecliptic longitude is one of the two points where the ecliptic intersects the plane of the Earth's equator. (This point is also known as the "first point of Aries", since at one time, it lay in that constellation. It is also the zero point for right ascension.) Ecliptic coordinates are rarely used, but they are sometimes convenient for describing positions of solar system objects. The orbital elements for solar system objects, for example, are almost always expressed in this system. You can have Guide show the ecliptic coordinates of the cursor in the legend; this is turned on or off through the legend dialog. If you want to select a position in ecliptic coordinates, click on the ecliptic position shown in the legend, or hit the Alt-, hotkey, and Guide will prompt you to enter the new position. Edit comet data If you wish to change the data for a comet or enter a new one from its orbital elements, you should click on the Edit comet data option in the Extras menu. This will bring up a list of some recent comets, along with "new comet" and "new asteroid" options. Click on any of these, and Guide will show an orbital elements dialog box for the currently stored elements (for an existing object) or default data for a new object. elliptical variable An elliptical variable is a kind of variable and binary star. It varies because the stars are so close together that their mutual gravity stretches them into egg shapes. As they orbit each other, we see the eggs from different angles, and therefore see different amounts of light. The amount of stretching can't be too great, or the stars would have ripped each other apart long ago. This means the changes in brightness can't be more than a few tenths of a magnitude either. Sometimes the stars eclipse each other as well, making bigger changes in brightness. This kind of star is called a Beta Lyrae type variable. elongation As planets circle the Sun, their apparent distance from the Sun in the sky varies. This distance is called the elongation from the Sun; when it is small, the object is close to the Sun and difficult to see in the glare. When it is close to 180 degrees, the object will rise at sunset and set at sunrise, so you can see it all night, far from the Sun's glare. emission line When you heat up a gas and look at the spectrum of light it emits, you'll find that it tends to emit light at certain frequencies, or wavelengths. Different gases will emit at different frequencies; these are called emission lines, and by analyzing them, you can determine what gases you're looking at. emission nebula An emission nebula is a nebula which is close to a star (or several stars) emitting short-wave (blue or ultraviolet) radiation. This radiation ionizes and/or excites the atoms in the nebula. These atoms then get rid of their energy by emitting a photon, much as excited gas atoms in a neon light will emit radiation of a certain color. This is quite different from a reflection nebula, where the light from the nebula is simply reflected light from the central star. You can tell the difference by looking at the spectrum of the nebula and comparing it to that of the stars providing the initial energy. If it's a reflection nebula, then the spectra will match; if it's an emission nebula, it will show emission lines of its own. emulsion Photography (both of the celestial and "normal" sort) relies on chemicals that will change on being exposed to light. The mix of materials applied to a film or glass plate is called an emulsion. Most celestial objects do not emit a lot of light, so the emulsions used for photographing must provide good sensitivity over long exposure times. Quite a bit of human ingenuity has gone into making emulsions that will record fainter objects in less time. Enter caption You can click on this menu item to change the caption that can be shown in the legend at lower left. Notice that this just lets you change the text. Turning the display of the caption on or off is done with the Caption on/off menu option; clearing it is done with the Clear Caption menu option. Enter Latitude CTRL-Y The Enter Latitude option lets you reset your geographic latitude. (You can get this from USGS maps and from most others.) You enter a latitude as a compass sign (N or S) followed by degrees, minutes, and seconds. For example, the latitude of Bowdoinham, Maine could be entered as N 44 1 30 Sometimes longitudes are expressed as degrees and decimal minutes, or decimal degrees. You can use these instead: N 44 1.5 N 44.025 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-Y hotkey, or through the Location dialog in the Settings menu. Enter RA/DEC ALT-E You can use this item to go to any desired RA and declination. When you click on it, you will be asked for a RA; you can enter this in hours, minutes and decimal seconds, as hours and decimal minutes, or as decimal hours. The program will recognize either 4.4h or 4h24m or 4h24.0m or 4h24m0.0s as the same RA. Next, you are asked for a declination. Once again, the program is reasonably good about understanding different formats: it will recognize 63.3 or N63.3 or +63d18' or 63d18m0.0" as the same declination. Once you have entered a position in a given format, Guide will use that format to display all latitude and longitude values. This item can be reached at any time via the ALT-E hotkey. Also, if the legend is shown, you can click on the RA/dec shown (if that's turned on) to reach this option; or you can use the 'Enter RA/Dec' option in the Go To menu. Enter Time Ctrl-F9 In general, most users of Guide find that the Time Dialog is the simplest way to reset the date and time Guide uses for its display and calculations. Some people, though, prefer to type in the time directly and avoid use of the mouse. The Enter Time menu option in the Extras menu is intended for that purpose. Click on Enter Time, and you'll be prompted to enter the new date and time. Here are some examples of valid dates or times: 13/6/1987 to reset the date to 13 Jun 1987 13/6 to reset the date to 13 Jun of the current year 13 to reset the date to the 13th of the current month 16:45:02 to set the time to 16:45:02 of the current day 16:45 to set the time to 16:45 of the current day 16 to set the time to 16:00 of the current day 13/6 16:45:02 to set the time to 16:45:02 on 13 Jun, current year You can also use any of the "date" formats followed by any of the "time" formats. Also, the following are valid: +27.3 to advance the current date by 27.3 days -10.4h to back up the current date/time by 10.4 hours +2356m to advance the current date/time by 2356 minutes -63.1s to back up the current date/time by 63.1 seconds -100y to back up 100 years from the current date/time J2450540.321 to set the current date/time to JD 2450540.321 You can also reach this option with the Ctrl-F9 hotkey. ephemeris An ephemeris is a list of positions for an object, usually a planet, asteroid, or comet. (In Guide, ephemeris creation is limited to those objects). An ephemeris can also list distances of an object from the Sun and/or Earth, as well as magnitudes. (Guide will usually include that information.) For example, this is an ephemeris for the comet 1993v (McNaught-Russell). It shows data at two-day intervals from 28 Feb 94 to 24 Mar 94, for midnight UT, as seen from Project Pluto headquarters in Bowdoinham, Maine. ! Date RA declination r delta mag ---- -- ----------- - ----- --- 28 Feb 94 03h17m36.45s S34 29' 30.3" 1.034 0.853 7.5 2 Mar 94 03h21m39.21s S32 56' 37.9" 1.016 0.824 7.3 4 Mar 94 03h25m52.16s S31 16' 22.4" 0.998 0.794 7.0 6 Mar 94 03h30m15.18s S29 27' 49.2" 0.981 0.765 6.7 8 Mar 94 03h34m48.14s S27 29' 57.6" 0.964 0.735 6.4 10 Mar 94 03h39m30.91s S25 21' 40.5" 0.949 0.706 6.2 12 Mar 94 03h44m23.36s S23 01' 44.2" 0.935 0.678 5.9 14 Mar 94 03h49m25.37s S20 28' 49.8" 0.922 0.650 5.6 16 Mar 94 03h54m36.86s S17 41' 34.6" 0.911 0.622 5.3 18 Mar 94 03h59m57.79s S14 38' 35.7" 0.900 0.596 5.1 20 Mar 94 04h05m28.24s S11 18' 35.7" 0.891 0.571 4.8 22 Mar 94 04h11m08.37s S 7 40' 30.0" 0.884 0.548 4.6 24 Mar 94 04h16m58.51s S 3 43' 38.6" 0.877 0.527 4.4 ! Ephemeris Items The Ephemeris Items option in the Make Ephemeris dialog gives you full control over what data is shown in an ephemeris. You can, for example, create an ephemeris containing only data such as an objects' alt/az, distance, and magnitude. A check-box is given for each possible item in an ephemeris. There are a few options in this dialog that could use some explanation. The "elongation" refers to elongation from the Sun, of course. The "phase angle" is often given in MPC ephemerides; it refers to the angle Sun-target-Earth (target at the vertex). When this is near zero, the object is near to opposition; a phase angle of 180 degrees, on the other hand, suggests that the object is transiting the Sun. The "In Shadow" field only works for artificial satellites; it gives an "I" if the satellite is Illuminated, or an "S" if it is in Shadow. The "Lat/Lon" option also only works for satellites, and gives the point on the earth over which the satellite is currently situated (if you stood at this point, the satellite would be at the zenith.) The final three options provide ways to filter out unwanted observations (and exist only in the Windows software). Click on the "Sunlit (satellites)" option, and data will only be listed for a satellite if it is actually sunlit. Click on the "Above Horizon" option, and data for any object will only be listed if it is above your horizon. Click on the "Sun below altitude" box, and you can ensure that only nighttime data is given; an edit box allows you to tell Guide how far below the horizon the sun must be. The default is six degrees, corresponding to nautical twilight. The default is for all three of these options to be OFF, so you see every line in the ephemeris, even if the satellite isn't at all visible. Click on all three, and data for a satellite will only be shown if all three of the key criteria are satisfied: the satellite must be sunlit, it must be above the horizon, and it has to be dark outside. Ephemeris Time ET Usually, the times of astronomical events are given in Universal Time. UT is coordinated with the Earth's rotation. But the Earth's speed of rotation varies; part of this is because the tides raised by the Sun and Moon are slowly dragging down the Earth's spin. The result of all this is that one year in UT may not be as long as a different year. Sometimes "leap seconds" must be added at the end of a year. It's much easier to make astronomical calculations with a system in which a year is always a year, and a "day" is always 24x60x60 seconds. This system is Ephemeris Time. ET provides a uniform system lacking the semi-random fluctuations found in UT. For the last century or so, the difference between ET and UT (called "delta-T") has been under a minute. Delta-T is not well known for times before about 1500 AD, since we don't know just what the Earth's rotation was doing back then. epoch On the Earth, the latitude and longitude of a place are numbers that don't change. That's because they are measured from the Equator and Prime Meridian, which don't move. Regrettably, right ascension and declination are measured from the vernal equinox and celestial equator, and these do move over time because of the Earth's precession. So when giving an RA/dec, you need to give the time for which it's valid, which is called the epoch. Positions are usually given in 50-year epoch intervals. By now, most astronomers have switched over from 1950 to epoch 2000, or "J2000.0", positions, with a few positions given in 1975 coordinates. The "J" in front of an epoch date refers to the more modern Julian epoch; the "B" refers to the older Besselian epoch. epoch of elements When the orbital elements of an object are given, the epoch of elements is usually given as well. This is because the numerical values of the elements change slowly over time. An object in the solar system is always subject to the gravity of other planets, not just the Sun, and this makes the orbit (and hence the elements) change. The epoch of elements is the date and time for which the elements accurately describe the position and velocity of the object. For a few months around that date, you can use those elements to get a good position (usually within arcseconds). Beyond that time, the gravitational effects of the planets will put the object in a very different orbit than that described by the elements. Equat. radius @ 1 bar Defining the radius of a "gas giant" planet (Jupiter, Saturn, Uranus, and Neptune) presents a problem, because we don't see the solid surfaces of these planets; all we see are outer layers of the atmospheres. So instead, the radius is measured from the center of the planet to the point where the atmospheric pressure drops to one bar (basically, that of sea-level air pressure on Earth). This is then given as the "equatorial radius at one bar". Equation of time One reason sundials fail to keep very good time is that the Sun does not appear to move at a constant rate across the sky. This is partly because the earth's orbit is elliptical, and partly because the earth's axis is a little tilted. The equation of time is the difference between "mean solar time" (the time a sundial would keep if the sun's motion was uniform) and "apparent solar time" (the time described by the actual movement of the sun). It is shown in Guide when you click for "quick info". The equation of time varies throughout the year (and slightly from year to year). At present, it reaches extremes of about -14 minutes in February, and about +16 minutes in November.