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difficulty with using this system is that RAs and declinations are in constant change over time. The position of an object expressed in 1950 coordinates may be .7 degrees different from its 2000 coordinates. The reason for this is that declination and RA are measured from the Earth, and the Earth doesn't stand still. The axis it rotates on slowly describes a circle in the sky, completed once every 25,800 years. This is why Polaris won't always be and hasn't always been the "North Star". This slow motion of the Pole is called precession. It's similar to what happens with a top; you've probably noticed that while the top spins rapidly, it also has a slower, "wobbling" motion. This motion makes a RA and declination alone are slightly ambiguous; you also need to know the year for which that position is valid. That year is called the epoch. Most catalogs are in a "standard" epoch. Standard epochs are separated by 50 years; some catalogs are still in the B1950.0 epoch, while most have been switched to J2000.0. If you read about the position of an object in a book or magazine, make sure you also get its epoch. This is especially important for dim objects. If you mistakenly looked for an object as dim as Pluto, say, in a J2000.0 position when it was really given as B1950.0, you would have no hope of finding it. (The difference is usually around half a degree.) By default, Guide shows you positions and accepts positions in J2000.0. If you wish to change this, use the Settings... Formats dialog (described on page 31.) You can set the epoch in which grids, ticks, hatches, and/or side labels are shown separately; there is a menu item in the Spacings menu that lets you do this. A brief note: You may wonder what the "B" before "1950.0" and the "J" before "2000.0" mean. The answer is: in terms of finding something, not much. In 1950, epochs were measured from the start of the Besselian year, which is 365.2421988 days long. In 1984, the International Astronomical Union decided to switch to the start of Julian years, which are exactly 365.25 days long. The actual difference in the sky is always well under .1 second of arc, and you can usually ignore these prefixes with very little harm. APPENDIX C: ALTITUDE AND AZIMUTH EXPLAINED You will note that all objects, when clicked on, show times of rising and setting, plus their "alt" and "azim". This refers to the object's "altitude" and "azimuth", which tell you where in the sky to look for an object. Altitude and azimuth are not reckoned from either the Earth's poles or the celestial poles. They are reckoned from the observer's position: the point straight overhead has an altitude of +90 degrees; that straight underneath, an altitude of -90 degrees. Points on the
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