Technical data

Glossary 53
nutation - a short period oscillation superimposed on the Earth's precession caused by the gravitational torques of the
Sun and Moon.
obliquity of the ecliptic - the inclination of the plane of the ecliptic with respect to the celestial equator, currently it is
approximately 23.5 degrees.
parsec - the distance from the Earth an object would have to be so that it's parallax would be one second of arc. One
parsec is equivalent to approximately 3.26 LY.
perihelion - the point on an object's orbit that is nearest the Sun.
photometry - measurement of the intensity of light.
planet - any one of the presently known large bodies that orbit the Sun. From the Greek word for "wanderer".
precession - the wobbling of the Earth's axis caused mainly by the gravitational pull of the Moon and to a lesser extent
the Sun, on the Earth's equatorial bulge. This causes the celestial equator to wobble, thus causing the precession of the
equinoxes
precession of the equinoxes - the slow westward motion of the equinoxes along the ecliptic caused by the precession
of the Earth's axis.
right ascension (RA) - the RA is the distance measured EASTWARD from the vernal equinox and is measured in
hours, mins, sec.
second of arc - 1/3600 of one degree.
sidereal time - the hour angle of the vernal equinox, which turns out to be equivalent to the RA on your meridian.
topocentric - coordinate system whose reference point is the observer's location on the Earth's surface.
universal time - equivalent to Greenwich Mean Time (GMT).
vernal equinox - the point on the celestial sphere where the Sun crosses the celestial equator going from South to
North, this point is also defined as the fundamental direction. The vernal equinox occurs on approximately March 21.
It's corresponding point on the opposite side of the celestial sphere is the Autumnal Equinox.
zenith - the point on the celestial sphere that is directly above the observer.