UNDER CONSTRUCTION (constantly) UNDER CONSTRUCTION (constantly)
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abberation: the apparent angular displacement of a celestial object from its geometric position, caused by the motion of the observer with respect to the object and the speed of light.
age of moon: the angle between the Sun and the Moon measured at the Earth.
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calendar: a system for accounting for the days in a span of time (usually a year). Several calendar systems are in use, the Julian and Gregorian calendars being two of the most common calendars in western societies.
coordinated Universal Time (UTC): the time scale available from broadcast time signals. It differs from the Internal Atomic Time (T AI) by a whole number of seconds. It is maintained within 0.9 seconds of Universal TIme (UT1) by the insertion of leap seconds (added at the ends of June and December).
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Declination: in the equitorial coordinate system, the angle measured perpendicular to the reference line of the equator; north is positive, south is negative. (see also: Variation vs Declination: A Story of Scientific Stubborness)
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ecliptic: the plane containing the orbit of the Earth about the Sun.
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first point of Aries: the position on the celestial sphere of the vernal equinox.
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parfocal: Two optical devices which are at the same focus when interchanged. The simplest example of parfocal is when two eyepieces are set to the same focus. Given: 9mm and 26mm eyepieces. Insert the 9mm all the way into the eyepiece holder so that it is flush with the holder and then focus to the eyepiece. Drop in the 26mm eyepiece holder. If you allow it to be also flush with the holder, the image will not be in focus. Losen the retaining screw in the holder and slowly pull the 26mm eyepiece out of the holder until the image is in focus. Mark the barrel of the 26mm at this point (with pen/pencil or tape) and you have parfocused the two eyepieces. Drop the 9mm flush into the eyepiece holder and focus on an object. Exchange it with the 26mm dropped into the eyepiece holder to the mark on the barrel and you will be in focus without having to refocus each time you switch eyepieces.
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variation: The angular difference between True North and Magnetic North (see: Variation and Declination: A Story of Scientific Stubborness)
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weather year: The time from 01 July of one year through 30 June of the following year.
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