Escape velocity If an object leaves Earth's atmosphere at about 11 km/sec (7 miles/second, or about 25000 miles/hour), it will escape the earth's gravity; below this speed, it will eventually come back to earth. This speed is called the escape velocity of the earth. When you click for "more info" on a planet, the escape velocity is listed for that planet. It can be useful to know that an object orbiting a planet near its surface moves at about 71% of escape velocity; for example, an object in low-earth orbit travels at about 8 km/second. ESO ESO/Uppsala The ESO/Uppsala catalog is based on plates taken at the European Southern Observatory, and shows most of the galaxies in the southern half of the sky. ESO objects are labelled with two numbers and, sometimes, a letter. You can find an object by its ESO number by using the Go to ESO/Uppsala option in the Go to Galaxy menu in the Go To menu. ESO/Uppsala identifier For galaxies in the southern half of the sky, most of the data given comes from the ESO (European Southern Observatory) extension of the Uppsala General Catalog. Each galaxy in the ESO/Uppsala extension has an identifier, listed when you "click for more info" on such a galaxy. Europa Europa is the smallest of the four large satellites of Jupiter found by Galileo. It is covered in a shield of ice from pole to pole, broken occasionally by cracks. It is thought possible by some that life might form in the ocean under the ice, given some internal source of heat. Exp. Length This item tells you for how long the plate that this Guide Star Catalog star was found on was exposed. Explanation of solar eclipse data The solar eclipse data shown in these tables is directly copied from Fred Espenak's "Catalog of Solar Eclipses". The full catalog is available at: http://sunearth.gsfc.nasa.gov/eclipse/SEcat/SEcatalog.html When you create a list of solar eclipses using the option in the Tables Menu, the following data is given for each eclipse: The date and time, in UT, of greatest eclipse. The Gregorian calendar is used for dates after 1584 October 15. The Julian calendar is used before that date. The type of each eclipse is given by a capital letter: A=Annular; T=Total; P=Partial; H=Hybrid (the eclipse is annular at some points along the path, total at others.) In several cases, an extra lowercase letter is given. Those letters mean the following: "m" = Middle eclipse of Saros series. "n" = No northern limit. "s" = No southern limit. "+" = No northern limit and no center line. "-" = No southern limit and no center line. "b" = Saros series begins (first eclipse in series). "e" = Saros series ends (last eclipse in series). Next, the Saros number is given. Solar eclipses tend to occur in series spaced about 18 years apart; several such series are running at any given time. A given Saros series will run about 13 centuries; each series has a Saros number. Next, the "Gamma" value is given. "Gamma" is the distance of the shadow cone axis from the center of Earth (units of equatorial radii) at the instant of greatest eclipse. A Gamma of zero means that the shadow axis at mid-eclipse runs right through the center of the earth; one of +.997 means that it runs near the edge of the earth, near the north pole; and a Gamma of -.997 means that it runs near the edge of the earth close to the south pole. (The value is .997 to reflect the fact that the earth is slightly flattened, by about .3%.) If the value falls outside the range -.997 to +.997, then it is very likely to be a partial eclipse; within this range, it is likely to be a total or annular eclipse. Next, the "magnitude" of the eclipse is given. This does not have the standard astronomical meaning of "brightness"; instead, it refers to the portion of the sun covered by the moon. When the magnitude is greater than or equal to 1, the sun is totally eclipsed. When the magnitude is nearly zero, the sun is only very partially eclipsed. To be exact, the magnitude is defined as follows. It is the length of the line segment running through the center of the sun and moon, measured from the solar limb nearest the center of the moon to the lunar limb nearest the center of the sun; divided by the apparent diameter of the sun. (In a partial eclipse, this reduces to just "the fraction of the apparent diameter of the sun covered by the moon.") Next, the latitude and longitude where maximum eclipse would be seen is given, along with the altitude of the sun at this point. (In the case of a partial eclipse, that altitude will always be zero. In other words, if the eclipse is a partial one, then the greatest eclipse will be seen someplace where the event occurs on the horizon, and in a polar area.) Next, the width of the eclipse path is given for all non-partial eclipses, in kilometers, at the time of greatest eclipse. Finally, the duration of the eclipse is given for all non-partial eclipses, at the location of greatest eclipse. (The eclipse will be shorter elsewhere, for total eclipses; or longer elsewhere, for annular eclipses.) Exposure start The Hubble Guide Star Catalog, or GSC, was created by scanning in photographic plates of the night sky. The time when the plate was taken is among the information given when you ask for "more info" on a GSC star; this time is the exposure start. Extras menu Alt-X The Extras menu contains a set of "advanced" options that you will probably not use at first. It contains the following menu items: RealSky image Add DSS image Clear RealSky images Shut off images Get Ax.0 data Edit Comet data Add MPC comets/asteroids Use MPCORB Enter Time Fixed levels Line of variation Toggle user datasets Make PostScript file Make .BMP File (Windows only) Find conjunction Show eclipse (or occultation, or transit) In DOS, you can always reach this menu with the Alt-X hotkey. F1 Getting help for a menu item You can get some help information for any menu item in Guide by moving the mouse cursor over the item in question and hitting F1. Guide will then show some help data specific to that menu item. If you hit F1 while in the chart area, Guide will provide some introductory "how to use Help" information. F10 This option is intended solely for a few Panasonic printers with slightly faulty Epson emulation. A few of these printers "stretch" their output by 1/8, resulting in printouts that are distorted. For those printers, use this option and enter 1.125. That will introduce an opposite "stretching", resulting in correct output. This option can be reached at any time with the F10 key. F11 CTRL-F1 Slew scope This option should send commands to the telescope that will slew it to the current screen center. It will produce a series of comments ("port opened", "set up", and so on) as it does this. It appears in the main menu when you have selected a port in the scope control dialog, and have chosen a scope system that supports slewing. This option can be reached at any time via the F11 key, or (useful for those with keyboards that have no F11 key) the CTRL-F1 key. The counterpart of this is the F12 key, which moves Guide to where the telescope is. F12 CTRL-F2 Slew Guide This option should send commands to the telescope that will find out where the telescope is pointed, and recenter Guide at that point. It will produce a series of comments ("port opened", "set up", and so on) as it does this. Once you have selected a telescope port in the scope control dialog box, this option will appear in the main menu. It can be reached at any time via the F12 key, or (useful for those with keyboards that have no F12 key) the CTRL-F2 key. The counterpart of this is the F11 key, which moves the telescope to Guide's current chart center. F3 You can reset Guide's time to the present moment (as measured by the computer's built in clock) with the F3 key, or by clicking on "Current Time" in the time dialog. F7 Hitting the F7 key results in toggling the previous chart. This lets you flip back and forth between two views. It differs from the ALT-B key, which backs up a view at a time, up to five views. F8 Create star list This option lets you generate a list of positions for GSC and Tycho stars in the area currently on the screen. The list is displayed in Guide's help system; you can then print the list or save it to a file as you would any help topic. When you select this option, you are prompted to enter a limiting magnitude, which lets you filter out stars too dim to be of interest. An example of the resulting output: ! GSC # RA (J2000) dec mag spec PPM HIP HD SAO 5549 935 13h45m52.351s -11 13' 16.04" 7.66 F8 227806 67169 119852 158151 5549 1127 13h45m35.709s -11 28' 59.79" 9.79 K0 227796 119803 5549 371 13h44m45.604s -10 48' 07.38" 9.99 A5 227772 119669 158136 5549 192 13h44m26.834s -10 23' 52.48" 10.59 0 044B 5549 827 13h44m28.435s -10 34' 53.87" 11.79 0 044B 5549 1155 13h44m15.506s -11 26' 10.00" 8.09 3 044B ! Each line of the output shows data for one star. The GSC identifier is the first item in each line; next are the RA and declination for the star. The format for the position is the format you currently have selected in the Settings Menu. Next, the magnitude of the star is given. If the star came from the Tycho or Hipparcos catalogs, you then get the spectral type and some cross-references to four catalogs. If the data comes from the GSC, then there is no spectral type data available. Instead, an object type is shown (usually 0 for a star or 3 for a non-star) and the identifying number of the plate on which that GSC star was imaged. Field rotation rate When you ask for Quick Info, one of the many items shown will be the field rotation rate for the center of the screen, at your currently set position on the earth and date and time. If you attempt to do CCD or photographic imaging from an alt/az mount, you can run into the problem that the image rotates with time. You can get around this by adding an extra "field derotator" motor. (Though this is mostly theory; such motors appear to be quite rare in reality.) In some parts of the sky, you can take advantage of the fact that the rotation rate is zero when the object is directly east or west (azimuth is 90 or 270 degrees). But in general, you'll want to know this figure when imaging from an alt/az mount. File Menu Alt-F The File Menu provides controls to: Save a Mark (store your position and settings to a file) Go to Mark (recover position and settings from a file) Delete a Mark Print Printer Setup Exit Filter The GSC was scanned from a collection of photographic plates. Each plate was taken with a particular combination of filter, emulsion, and telescope. There are ten different combinations, labelled SERC-J1, SERC-J2, SERC-V, PAL-V1, PAL-V4, PAL-V5, PAL-J, PAL-E, GPO-V, and BBO-B. When you ask for information about a GSC star, the plates it appeared on are listed, and so is the filter used for that plate. Find Conjunction This option, in the Extras menu, provides a way to find the time and location of the conjunction of two objects. To do this, you must right-click on the first object in question, click OK, and then right-click on the second object and click OK. Once you've done this, the "Find Conjunction" option will no longer be grayed out. Click on it, and Guide will find the nearest conjunction and will display it for you. If one object is the Moon, then you can click this option repeatedly to find successive conjunctions. Unfortunately, you can't do that for other cases, such as conjunctions of Jupiter with Saturn (yet). If you click on two non-moving objects, Guide will figure out that it cannot show a conjunction, and this option will remain grayed out. find distance and position angle You can "drag" a line to determine the distance and position angle between two points. First, click with the right mouse button on the first point. Holding that button down, move the mouse to the second point; a line will drawn between the two. When you let go of the mouse button, the distance and position angle between the two points will be shown. Fixed levels By default, Guide draws charts with fixed levels, in which the size of a chart progresses in 20 steps, from 180 degrees to 90 degrees, and so on down to one arcsecond. You can't readily get, for example, a 3-degree field of view, because Level 7 has a two-degree field of view and Level 6 has a five-degree field of view. But if you click on the Fixed Levels option in the Extras Menu, this restriction is relaxed. Drag open a box with the mouse, and it opens smoothly, instead of in steps. You can then turn Fixed Levels back on when you decide you want specific fields of view. You can also access this option through the Ctrl-G hotkey. FK Com The FK Com type of variable star is a rapidly rotating giant of spectral type G or K (like the Sun or a little cooler, but much larger). Their surfaces are nonuniform in brightness, and as they spin, we see different amounts of light. They may be a later version in the evolution of contact binary stars. FK5 Fundamental Katalog 5 The FK5, or Fifth Fundamental Catalog, contains 1,535 stars distributed in a roughly even manner across the entire sky. Its purpose is to provide a reference standard for determining the positions of other stars. Flamsteed Almost all stars visible to the unaided eye (that is to say, those brighter than magnitude 6) have a Flamsteed number. These stars are referred to by the Flamsteed number and constellation, such as 61 Cygni or 40 Eridani. When you see a star with a number on it in this program, that's its Flamsteed number. You can turn these numbers off inside the Star Display dialog. You can find a particular Flamsteed star through the Go to Bayer/Flamsteed menu under the Go To menu. flare Occasionally, there will be a brief, high temperature outburst in the solar atmosphere (chronosphere). These appear as bright areas on the sun, called flares. They are usually associated with sunspots, and can cause magnetic and radio disturbances on the earth. A few stars, such as UV Ceti, have flares of their own of such violence that the star will shoot up in brightness by several magnitudes, usually within a few seconds. This type of variable star is called a flare star. (The Sun's flares aren't quite in this league.) Flattening equatorial radius polar radius A rotating planet will tend to bulge out slightly at its equator, resulting in an "oblate spheroid". For example, the earth has an equatorial radius of 6378.14 km, but a polar radius of 6356.75 km. This difference of 1 part in 298.257, or .003353, is the flattening of the earth. Jupiter and Saturn each rotate more than twice as rapidly as we do and have much lower densities; the combination gives them much greater flattening, of 0.06487 and 0.09796. Fluctuating X-ray Fluctuating X-ray stars are a form of X-ray variable where the levels of X-ray and visible radiation fluctuates over a few hundredths of a second. Examples are Cygnus X-1, also known as V1357 Cyg, and V821 Ara. FOKAT-S FOKAT-S is a star catalog for the southern sky, assembled from photographs taken around 1984 in a Soviet- Bolivian cooperative effort. The entire region was photographed four times, which helped to reduce errors significantly. Many of the positions and proper motions listed in the PPM come from this survey. FU Orionis FU Orionis type variable stars will gradually brighten by about six magnitudes over several months, then either stay almost constant of slowly drop down by one or two mags. They are probably a stage in the development of T Tauri type stars. All presently known FU Ori variables are coupled with reflection nebulae. full screen tab Hitting the Tab key toggles between full screen mode and "menu visible" mode in the DOS version. Galactic latitude Galactic longitude galactic coordinate galactic equator Alt-F3 Alt-. Galactic coordinates are an alternative way to specify positions in the sky. The galactic equator runs roughly in the plane of our galaxy; thus, points with a zero galactic latitude are in the galaxy, and as the latitude departs from zero, one gets away from the Milky Way. The zero point, or "prime meridian", for galactic longitude is in the direction to the center of the galaxy, in Sagittarius. Galactic lat/lon is usually given in decimal degrees. Many objects tend to concentrate in the plane of the galaxy (certain types of nebulae, for example). The convenient thing about galactic coordinates is that you can quickly tell how close to the plane of the galaxy a given object is. This is not a truly immense convenience, which is why galactic coordinates don't get a lot of use. By default, Guide displays the galactic equator; you can turn it off in the Measurements dialog, or with the Alt-F3 hotkey. You can also have Guide show the galactic 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 galactic coordinates, click on the galactic position shown in the legend, or hit Alt-. (Alt-period), and Guide will prompt you to enter the new position. galaxy galaxies Galaxies are collections of from thousands to trillions of stars. Our own galaxy, the Milky Way, is a spiral of about a hundred billion stars stretching a hundred thousand light-years across. Galaxies come in several forms. Our own is a spiral; it has a dense core from which two arms point out and spiral around the center like a pinwheel. Others are elliptical galaxies, which vary from being almost round balls of stars to being stretched out into almost cigar shapes. Others are irregular galaxies; as the name implies, these have no particular shape. Galaxies are shown in this program as ovals followed by a Messier, NGC, IC, or PGC number. You can find a galaxy by using the Go to Galaxy option in the Go To menu. You can choose from any of several catalogs or use a "common name" such as "Maffei I" or "Whirlpool". (You can also find it by Messier, NGC, or IC number.) galaxy inclination The galaxy inclination measures the angle at which a spiral galaxy is tilted from us. A galaxy with a 90 degree galaxy inclination is seen edge on. One with a 0 degree galaxy inclination is seen face on. Galaxy RV A Galaxy RV (radial velocity) is a measure of its velocity toward (or, more frequently) away from us. It is usually determined by measuring its "redshift", the reddening of light caused by the Doppler effect. When you examine the light from a galaxy, you'll notice that its spectrum shows a shift to redder (longer) wavelengths. By measuring the amount of shift, one can determine the radial velocity of the object. The velocity can be determined by looking at emissions from neutral hydrogen or from the shift in optical light. The observations will give a velocity relative to the earth. They are then corrected to give a velocity relative to the sun (a heliocentric velocity), and can then be corrected to give velocity relative to the GSR (the galactic standard of rest), or to the 3K background, the microwave emissions from the Big Bang. Gamma Cas The Gamma Cas type of variable star rotates so rapidly that centrifugal force can push matter at the equator into a ring or disk. When this happens, the star may fade by up to 1.5 magnitudes. These are usually Be stars. gamma ray Gamma rays are emitted by the most energetic and usually cataclysmic of cosmic events. Gamma radiation is really just light with a very short wavelength; it is produced by some astronomical objects, such as quasars. It's not easily studied from the ground, but the Compton gamma-ray observatory has, from orbit, studied many objects at gamma-ray wavelengths. In particular, "gamma-ray bursters" have grabbed a lot of interest. These objects release huge amounts of gamma radiation for a brief time (usually a few seconds), and then vanish from view. There are many theories as to their cause, including such exotic possibilities as colliding neutron stars.