J2000 B1950 Positions of objects in the sky are specified by a right ascension, declination and an epoch. The epoch is a date needed because RA and declination are measured relative to the Earth's pole and orbit, both of which shift with time. In the past, 1950 (or "B1950.0") was a commonly used epoch. More recently, almost all astronomical data has been updated to the 2000 (or "J2000.0") system, which is the system this program uses by default. The "J" means Julian epoch; the "B" refers to the older Besselian epoch. J2000 position at epoch Star positions given as "J2000 position at epoch" have been corrected for proper motion, but the coordinates have been left in the J2000 reference frame. Jalali (Persian) calendar The Jalali (Persian) calendar is the official calendar of Iran and of parts of surrounding areas such as Afghanistan and Central Asia. The year begins on the day of the vernal equinox; if this occurs before midday, Teheran local time, that day is 1 farvardin ("New Years Day"). Otherwise, the next day is 1 farvardin. The result is that most years are 365 days long, with almost a quarter having an extra day, much as in the Gregorian calendar; but the pattern is not as "orderly" as the Gregorian pattern. Usually, leap years are four years apart, but sometimes, they are five years apart. The months are: farvardin (frvrdyn) (31 days) ordibehesht (ardybhSt) (31 days) khordad (Krdad) (31 days) tir (tyr) (31 days) mordad (mrdad) (31 days) shahrivar (Shryvr) (31 days) mehr (mhr) (30 days) Aban (Aban) (30 days) Azar (AZr) (30 days) day (dy) (30 days) bahman (bhmn) (30 days) esfand (asfnd) (29 days; 30 in leap years) Some sources suggest that, instead of being based on the vernal equinox, the calendar uses a pattern of 683 leap years in a cycle of 2820 years, which closely matches the vernal equinox rule. jansky The jansky is a fundamental unit for radio astronomers, used in measuring the amount of radio energy detected from an object. Jet It's common for accretion disk objects to also show jets. Jets of outflowing matter form at right angles to the plane of the accretion disk. Often they move at a good percentage of the speed of light, in which case Doppler shift makes the jet approaching us bluer than normal, and that moving away from us redder than normal. The mechanism causing jets is still not well understood. It should be clear, though, that any matter flowing out from an object with an accretion disk would only be apparent at right angles to the disk. Otherwise, it would run into the disk and we would not find it easy to see. Johnson The Johnson magnitude system is a widely-used standard for devices measuring stellar magnitudes. It defines a series of photometric bands and the filters used to measure them. For stars from the Hipparcos and Tycho catalogs, Guide is able to provide a "Johnson V" (visual) magnitude; this comes either from observations made on the ground, or is computed from the BT magnitude and VT magnitude. Julian Julian calendar Gregorian The two calendars most frequently used by astronomers are the Julian and Gregorian calendars. The Julian calendar was created by Julius Caesar's resident expert in such matters, a Greek named Sosigenes. He set up the months as we now know them and added an extra day in February every fourth (leap) year. This provides where each year averages 365.25 days, which is pretty close to the actual length. However, there is an error of about three days every four centuries. By 1582, the calendar was about 10 days out of adjustment to the real world. Pope Gregory XIII took two steps to deal with this. First, he decreed that 4 Oct 1582 would be followed by 15 Oct 1582. Secondly, to keep this problem from recurring, he decreed that three out of every four century years (those ending in 00) would not be leap years. Thus, 1700, 1800, and 1900 were not leap years, even though they are divisible by 4, but 2000 will be a leap year. This is the calendar used in most of the world today. Regrettably, it took from 1582 to 1918 for the Julian calendar to completely die out, so you do have to make clear which one is being used for dates in that interval. (Now, except for some religious purposes, the Julian calendar is obsolete.) Julian Day JD The usual method of dealing with dates (expressing them in days, months, years, hours, minutes, and seconds) can be a little tricky to deal with mathematically. Try to figure out the difference, in days, between 4 Jul 1776 and 1 Sep 1939, and the problem will be evident. To avoid this problem, astronomers often express a time in terms of Julian Day. (The "Julius" involved is not Julius Caesar, and this system is unrelated to the Julian calendar.) JD 0.0 corresponds to 1 Jan -4712; any other JD is the number of days since then. Thus, 1 Jan 2000 is the same day as JD 2451545. Decimal fractions correspond to fractions of a day. This program uses the JD system internally, and it is used almost universally when astronomical calculations are made. It is possible that the time of an astronomical event will be given to you as a JD, in which case you can enter it inside the Time dialog. Julian epoch Besselian epoch The "J" (or sometimes "B") before an epoch introduces a minor twist, negligible by all save people intent on fanatic-grade accuracy. The "J" indicates a Julian epoch, as distinguished from the older "B" or Besselian epoch. Julian epochs measure the year as 365.25 days exactly, the length of a year in the old Julian calendar; Besselian epochs have 365.2421988 days, matching a "real" (tropical) year. This makes for a small difference in the epoch date, and therefore in actual positions. If there is no letter given, assume 1950 and earlier epochs to be Besselian, and later to be Julian. Alternatively, ignore the difference unless you need an accuracy of better than about .1 arcsecond. Jupiter Jupiter, the fifth planet from the Sun, is appropriately named for the king of the gods. This planet has eleven times the diameter of our own. If you took the surface of the Earth and stretched it out on Jupiter, it would look like the area India covers on the Earth. Jupiter is five times as far from the Sun as we are, causing it to take twelve years to go around the Sun and providing it with 1/25 as much sunlight for a given area. The planet's atmosphere is about 80% hydrogen, 20% helium, plus slight traces of methane and ammonia. The nature of its interior is still mostly unknown. It has fairly violent weather; one hurricane, the Great Red Spot, was seen by Galileo in 1610 and is still there now. Jupiter has four major moons, each roughly the size of our own, found by Galileo: Io, Europa, Ganymede, and Callisto. If you zoom in on Jupiter in this program, the four moons will appear. There are at least a dozen other moons, all of much lesser size. You can see the big four with binoculars, and easily with a small scope (which is all Galileo had back then.) Most of what we know about Jupiter and its satellites was found by four probes, Pioneer 10 and 11 and Voyagers 1 and 2. Each returned many pictures and measurements of these objects. In 1995, the Galileo probe should carry out further observations. Jupiter satellite events Occ Tra Ecl Sha When you request "more info" on Jupiter, among other data, a list of the events of Jupiter's moons is shown. As Jupiter's four largest moons orbit Jupiter, they cast shadows are themselves eclipsed by Jupiter, and pass in front of and behind Jupiter. All of these events can be observed with small telescopes. The list shows events for the next 7 days. For each event, the number of the moon is given (I=Io, II=Europa, III=Ganymede, IV=Callisto), followed by the type of the event: Sha = Shadow cast by the satellite on Jupiter Ecl = Moon is eclipsed by Jupiter Tra = Moon crosses the disk of Jupiter Occ = Moon is blocked from view by Jupiter Also, each line tells you if this is the beginning or end of the event, and its time and date. Jupiter System I Jupiter System II Jupiter System III Measuring longitude values on Jupiter is made difficult by the fact that the planet rotates more rapidly near the equator than it does at the poles. So three systems are used. Jupiter System I is used for features within about 10 degrees of Jupiter's equator, where a full rotation takes about 9 hours, 50.5 minutes. Jupiter System II is used for features north and south of this zone (such as the Great Red Spot), where a rotation takes about 9 hours, 55.677 minutes. There is another system, Jupiter System III, which is based on the rotation of Jupiter's interior; it is used for radio observations, and is not particularly useful for visual observers. This rotation time of 9 hours, 55.495 minutes probably reflects the rate at which the solid core of Jupiter rotates, far below the cloud layers. Keyzer Pieter Dirckszoon Keyzer was a Dutch explorer of the 1590s. During some trips around Africa to the Far East, he had an opportunity to be one of the first Europeans to observe the southern sky, and came up with twelve constellations that have survived to the present: Apus the Bird of Paradise, Chamaeleon the Chameleon, Dorado the Goldfish (a large fish found in tropical oceans), Grus the Crane, Hydrus the Sea Monster, Indus the (American) Indian, Musca the Fly, Pavo the Peacock, Phoenix the Phoenix, Triangulum Australe the Southern Triangle, Tucana the Toucan, and Volans the Flying Fish. Keyzer may have borrowed these groups from the local peoples. He managed to get them into Bayer's 1603 atlas, thereby ensuring their continued use. kiloparsec A kiloparsec (kpc) is, logically enough, 1000 parsecs, or roughly 3,260 light-years. It is a convenient unit for measuring distances to globular clusters. (For comparison, the center of our galaxy is about ten kpc from Earth. Our galaxy is about 30 kpc in diameter, and the nearest spiral galaxy to our own, M-31 in Andromeda, is about 900 kpc away.) km kilometer One km (kilometer) is equal to 1000 meters, or .6214 miles. There are 149597870 kilometers in one AU. Kuiper belt In 1951, the astronomer Gerard Kuiper suggested that there might be billions of small, icy objects just beyond the orbits of Neptune and Pluto. These objects would be spread out over so large a volume that they wouldn't tend to collect to form actual planets. They would also give a possible explanation as to why there are so many short-period comets. This idea had to remain in the realm of theory for four decades; these objects were too dim to be detected until 1992. Since then, several dozen of these Kuiper belt (or "transneptunian") objects have been found. They tend to brightnesses of about magnitude 23. La Caille In the 1750s, the astronomer La Caille filled in some of the gaps then existing between constellations with some of his own. Most are scientific apparatus of the era (Antlia the Air Pump, Telescopium the Telescope, Fornax the Chemical Furnace). He also created Sculptor and assorted tools, such as Caelum the Chisel, Pictor the Easel, and Norma the Level, supposed to be used by Sculptor in his work. There are a total of 14 La Caille constellations. Language menu Alt-F5 The Language Menu, within the Settings Menu, allows you to switch Guide's user interface to English, French, German, Spanish, Italian, Japanese, Dutch, or Russian. All menus and dialog boxes, and much of the text used elsewhere in Guide, will be shown in the new language. (The "help" data currently exists only in English, German, Italian, and Dutch, so it obviously can't be shown properly in the other languages yet.) As of this writing, some versions are incomplete translations; some text will still appear in English. More complete versions will be available at http://www.projectpluto.com as they are created. Also, more languages may be posted if translators are found. It should also be noted that the Japanese version cannot work in the DOS software, due to font limitations. Also, the Windows software can have problems showing Japanese or Russian if you don't have suitable fonts installed. latitude longitude Latitude and longitude are numbers used to specify points on the Earth's surface. For example, Project Pluto's corporate headquarters is located at latitude N 43.01 degrees, longitude W 69.70 degrees; those two numbers specify its position to within a mile or so. (With enough digits, the location of individual atoms could be specified.) You can find your latitude and longitude on a USGS map, or, with some training and a good watch, you can calculate it from observing stars. This program uses latitude and longitude when figuring out such information as rise and set times or the altitude and azimuth of an object. Also, if you want to find solar eclipses, you must provide the lat/lon of the place where the eclipse is visible. (A solar eclipse visible in one place won't necessarily be visible elsewhere.) To enter a new lat/lon, use the Location dialog in the Settings menu. Level Size F6 The Level Size option lets you reset the field of view shown at a particular level. For example, by default, the field of view at zoom level 9 is 30 arcminutes, or .5 degree. Suppose you would like to reduce this to 24 arcminutes, or .4 degree. You would go to level 9, and click on this item. You are asked to "Enter new screen size:" Enter 24', or .4 (degrees), or 1440". The screen will be redrawn at the new size. You can also access this feature with the F6 hotkey at any point in the program. Libration In general, we see only one face of the moon. In reality, the moon appears to rock slightly from side to side and from top to bottom. This tilting is called libration, and it allows us to see about 59% of the moon's surface, instead of the 50% we would otherwise see. There are three causes of libration. Most significant is the fact that the moon spins at a fairly constant rate, but orbits us at a somewhat variable rate because its orbit is not a perfect circle. This is called the optical libration, because it is caused by our viewpoint and not by any real changes in the moon's spin. Also, as the earth turns, the angle at which an observer sees the moon changes slightly. This effect can be as great as about one degree, and isn't quite as important as the optical effect. This is called the diurnal (daily) libration. Finally, there is a real, but very minor, rocking motion that amounts to a few arcminutes. This effect is called the physical libration. light curve If you plot the brightness of a variable star over time on a graph, you have produced a light curve. The shape and dimensions of a light curve can tell you what kind of variable star you're looking at and what sort of processes are going on in it. light-year A light-year is the distance light travels in a year. Note that this is a unit of _distance_, not time. (The 'year' part confuses some people.) It is equal to 5.88 trillion miles, or 9.46 trillion kilometers. It's a standard unit for measuring distances to objects outside our solar system. limb angle The moon's libration is often expressed as a total amount and a limb angle. The limb angle tells you which part of the moon is tilted toward you. It is measured from lunar north, at 0 degrees, counter- clockwise: 0 Mare Humboldtianum Sinus Roris Mare Marginis 90 270 Mare Australe 180 For example, if the limb angle is about 220 degrees, Mare Australe is tilted toward the earth and is in a better than usual position for viewing. The total amount of tilt can range up to about 8 degrees. As a result, about 59% of the moon can be seen at one time or another. @c 220,240,120 Lunar disk @m 327,220 Mare Crisium @l 328,209 @l 325,199 @l 314,190 @l 306,199 @l 308,209 @l 316,220 @l 327,220 @m 334,224 Mare Undarum @l 335,230 @l 329,228 @l 331,224 @l 334,224 @m 338,220 Mare Marginalis @l 337,220 @l 333,203 @m 334,203 @m 108,199 Oceanus Procellarum (limb) @l 112,240 @l 124,260 @l 151,271 @l 148,281 @l 141,281 @l 154,299 @l 161,299 @l 167,294 @l 170,298 @l 168,301 @l 171,308 @l 183,305 @l 183,299 @l 202,299 @l 211,294 @l 206,260 @l 189,246 @l 204,246 Sinus Medii @l 232,240 @l 242,224 @l 238,209 @l 225,203 @l 218,209 @l 220,220 @l 226,224 @l 204,234 @l 204,224 @l 214,218 @l 204,209 @l 234,184 @l 255,209 @l 259,240 mare nectaris @l 266,240 @l 266,260 @l 279,260 @l 276,281 @l 283,284 @l 292,281 @l 295,266 @l 291,260 @l 284,260 @l 287,236 @l 297,240 @l 304,230 @l 306,232 @l 300,244 @l 299,260 @l 313,266 @l 303,277 @l 304,286 @l 317,281 Mare Foecunditatus @l 326,240 @l 310,222 @l 296,222 @l 295,220 @l 305,220 @l 292,199 paulus somnii (s edge) @l 284,199 @l 286,207 @l 273,201 @l 276,199 @l 276,199 @l 270,177 @l 290,177 @l 279,163 lacus somniorum @l 287,159 @l 269,149 @l 266,140 @l 266,140 @l 243,135 @l 235,138 Mare Frigoris @l 182,137 @l 157,139 Sinus Roris (S edge) @m 158,170 Sinus Iridum @l 168,163 @l 174,153 @l 185,154 @l 194,149 @l 220,151 @l 223,160 @l 238,160 @l 233,167 @l 256,172 @l 257,168 @l 250,160 @l 271,162 @l 258,149 @l 230,149 @l 211,140 @l 196,143 @l 185,141 @l 173,150 @l 159,151 @l 150,167 @l 153,170 @l 158,170 Line of variation Ctrl-F11 If you are trying to recover or identify a long-lost asteroid or comet (or one whose orbit is simply not well-determined), it's very likely that the object won't be detected very close to its predicted position. Small errors in the old orbit pile up over time, to the point where (in extreme cases) the object could be just about anywhere. In less extreme cases, the object may be a few arcminutes or a few degrees away from the prediction, but it probably still lies along a certain line called the line of variation, or LOV. Knowing where that line is can be useful in limiting the region to be searched, and Guide provides the ability to display that line for all asteroids and comets. This is a somewhat specialized function, but if you are trying to recover an object, it can be very valuable. To turn it on, click on the Line of Variation option in the Extras Menu. Guide will ask for the length of the line in days; a starting value of one day is usually a good idea. (This corresponds to a guess that the object may be one day "ahead of prediction" or "behind prediction".) Guide will display a one-day LOV for all asteroids and comets on the screen. To turn it off, return to this menu option and enter an LOV length of zero days. You can also access this option through the Ctrl-F11 hotkey, or by right-clicking on an asteroid or comet; then selecting "Display", then "Options". Local Events only The Local Events only option appears in the Extras menu, when in eclipse mode. It provides a way to limit Guide to finding only events that will be visible from a particular position. Suppose you have a chart of a solar eclipse path on the screen, generated using the Show Eclipse function. If you click on the Local Events Only option, a check-mark will be added next to it. Zoom in on the part of the world in question, and hit Next. Instead of just going to the next eclipse, Guide will keep searching until it finds an event visible from that position. One drawback of this can be that it may be a long time before the next eclipse comes your way; Guide may therefore take a long time to find it. This is especially true when the Partial Events option is turned off; in this case, Guide will be hunting for a total or annular event, and these can be quite rare indeed! Local Group Our own galaxy, the nearby Andromeda galaxy, and a few other much smaller galaxies form a small cluster called the Local Group. long. ascending node longitude of the ascending node Among the numbers making up the orbital elements of an object is the longitude of the ascending node. This number is part of the definition of how the orbit is oriented in space relative to the Earth's orbit. It is usually represented by an uppercase Omega.