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horizon have 0 degree altitudes. An object halfway up in the sky has an altitude of 45 degrees. Thus, looking at altitude tells you immediately if the object is even visible. A negative altitude means the object is below the horizon. A positive altitude less than, say, +10 degrees, may make the object so close to the horizon that trees or buildings or smog may make it invisible. Altitude gives you the "how far from the horizon"; you use azimuth to determine "which direction around the sky." An azimuth of zero degrees puts the object in the North. An azimuth of 90 degrees puts the object in the East. An azimuth of 180 degrees puts the object in the South, and one of 270 degrees puts the object in the west. Thus, if Guide tells you that an object is at altitude 30 degrees, azimuth 80 degrees, look a little North of due East, about a third of the way from the horizon to the zenith. The fact that altitude and azimuth are referred to things you can actually see (the horizon and the zenith) makes them very useful. Remember that Guide needs a correct time and lat/lon to provide correct alt/az values! The star you see straight overhead might not even be visible thousands of kilometers away; if you don't have the right lat/lon, Guide may well believe it is being run in Bowdoinham, Maine, and has never left Project Pluto. It is quite simple to find a position by alt/az coordinates; the process is described on page 13. APPENDIX D: TROUBLESHOOTING POSITIONS At some point in the use of Guide, you will not find an object at its expected position. This should not be too surprising. Guide has a huge array of settings, and if one or more is set incorrectly, positions will not be as expected. Check the following items: EPOCH. The current epoch is shown in the RA/dec Format dialog and, usually, in the legend. By default, it is set to J2000.0, the current system of choice. Most galaxy catalogs, and a few other sources, are still in B1950.0 coordinates. The difference can be of the order of half a degree. LATITUDE/LONGITUDE. An object viewed from Texas will not be found in the same position as seen from Brazil. The difference is usually small, but some objects come close enough to the Earth for it to matter. Your altitude above sea level, set in the Location dialog in the Settings menu, has a similar, but much smaller, effect. While in that dialog, you should also check your... HOME PLANET. (see page 34). If you are observing the Solar System from, say, Venus, objects will obviously not be in the same place as if they were observed from Canberra, Australia.
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