argument of perihelion - angle along the orbit of a planet or other
solar system object as measured from the ascending node (analogous to
right ascension and longitude).

ascending node - the plane of a planet's orbit (or other solar system
object) is usually tilted with respect to the plane defined by the
solar system (the ecliptic).  These 2 great circles intersect at 2
points.  As the planet travels around its orbit, at one of the
intersection points it will pass from below the ecliptic plane to above
it (i.e. nortward).  This point is called the ascending node.

astronomical twilight - defined as the time when the sun is 12-18
degrees below the horizon.  When the sun moves from 19 to 18 degrees
below the horizon, astronomers can begin to detect an increase in the
brightness of the sky, although to the naked eye the sky still looks
dark.

AU - astronomical unit - the average Earth-sun distance, equal to 149.5
million km or 93 million miles.

celestial sphere - an imaginary sphere centered on the Earth, or
arbitrary large radius on the surface of which the stars are considered
to be fixed.

coma - a large cloud of dust and gas which escapes from the nucleus of
an active comet.  

declination - celestial coordinate which is equivalent to the latitute
of an object on Earth.  This is an angular measure of how far above the
celestial equator (which is an imaginary great circle in the sky drawn
concentric to the Earth's equator) an object lies.

eccentricity -  the measure of the degree of elongation of an ellipse.
For example, a circle has an eccentricy of 0, and a parabola (an open 
figure) an eccentricity of 1.

ephemeris - a table of positions in right ascension (celestial coordinate 
corresponding to longitude on earth) and declination (celestial coordinate 
correspoinding to latitude on Earth), as a function of time for a moving
celestial object.

geocentric distance - the distance from the Earth.

heliocentric distance - the distance from the Sun.

inclination - angle between the plane of the object's orbit and the
ecliptic.

light year - distance that light can travel in 1 year.  Light travels at a
speed of 300,000 km / sec, so this distance is equal to 9.46 x 10^12 km.

magnitude - brightness measured on a logarithmic scale, based on the ancient
practice of noting that the brightest stars in the sky were of "first 
importance" or "first magnitude", the next brightest being "second magnitude"
etc.  The human eye is a logarithmic detector, and in 1854 Pogson formalized
this scale and defined a difference of 5 magnitudes to be exactly a factor
of 100 in brightness.  The scale is calibrated to the bright star Vega which
is defined to have a magnitude of 0.  For reference, the faintest naked-eye
magnitude visible from a dark site is +6.  Cmet C/1996 B2 Hyakutake reached
magnitude 0 in late March 1996.

nautical twilight - defined as the time when the sun is 6-12 degrees below
the horizon.  Sky begins to get noticeably bright.

non-gravitational forces - forces which alter the path of a comet which are
not caused by gravitational interaction with the sun and planets.  Typically
this refers to uneven outgassing from vents on the comet which serve to act
like rocket nozzles.

nucleus - km-sized "dirty snowball" composed of dust (refractory material) and
primarily water-ice which gives rise to all of the features observers associate
with comets.  As the nucleus approaches the sun the temperatures rise
sufficiently to cause the <b>sublimation</b> of the ices.  As the gas leaves
the nucleus, it is able to drag some of the dust with in in the low gravity.
The sunlight reflected off the dust is what we see as the yellowish coma and
tail of the comet, and interaction of the solar radiation with the gases gives
us the characteristic blue appearance of the plasma tail.

parallax - apparent motion of a nearby object as projected against more
distant background objects due to the motion of the observer.

parsec [pc] - a unit of astronomical distance equal to 3.26 light
years, or 206265 AU, which equals 3.08 x 10^13 km.

perihelion - point in the elliptical orbit of a comet about the sun in which it
passes closest to the sun.

refractory - material which is gaseous only at relatively high temperatures,
e.g. metals, minerals, rocks.

resolution - the degree to which fine details in an image are separated or
resolved.

right ascension - (RA) celestial coordinate which is equivalent to the
longitude of an object on Earth.  The starting point for longitude on
Earth is at Greenwich England, whereas in the celestial coordinate
system it is at a point in the sky called the vernal equinox.  The two
great circles defined by the celestial equator and the ecliptic, which
is defined as the plane of the solar system, cross each other at 2
points:  at the vernal equinox (first day of spring) and the autumnal
equinox (first day of autumn).  The vernal equinox is the starting
point for the RA coordinate system.

solar elongation - the angle between the lines of sight to the Sun and to
the celestial body in question

solar conjunction - a condition where the solar elongation is zero, or 
practially speaking, so small that the celestial object cannot be seen in 
the glare of the sun.

spectroscopy - the study of spectra.  This is a means of looking at the
intensity or brightness of an object as a function of wavelength or color.

sublimation - phase transformation from solid to gas.  

volatile - material which is gaseous at fairly low temperatures.  This is
a relative term, usually applied to gases in planetary atmospheres, and to
common ices (e.g. water, carbon dioxide, carbon monoxide, methane, 
ammonia etc.)