Technical data

Glossary 52
color index - difference between two magnitudes measured in two different spectral regions. For example, B-V which
is the difference between the B(blue) magnitude centered in the blue and the V(visible) magnitude, which corresponds
to yellow-orange.
coma - an optical defect that affects only image details away from the center of the field of view causing stars to appear
flared, like tiny comets pointing inward from the edge of the field.
comet - objects of small mass that orbit the Sun typically in highly elliptical orbits.
conjunction - time at which two objects appear closest together in the sky.
constellation - an arrangement of stars on the celestial sphere that resemble something familiar such as animals, people
or objects. They are usually named after mythological beings, animals, ancient gods and heroes.
declination (DEC) - the angular distance North or South of the Celestial Equator and is measured in deg, min, sec..
ecliptic - apparent path of the Sun as projected on the Celestial Sphere.
ephemeris - tabulated data showing the positions or other calculated data of a celestial body for selected time intervals.
epoch - a fixed instant used as a reference point on a time scale i.e., B1900.0, B1950.0, J2000.0. This instant
corresponds to the position of the vernal equinox on the first instant of the cited calendar year to which all precessional
adjustments are applied.
equinoxes - the two points on the Celestial Sphere where the Ecliptic crosses the Celestial Equator.
focal length - the distance from the lens or mirror to the focal point.
focus - the point in an optical system where all the image components are brought together.
geocentric - coordinate system whose reference point is the center of the Earth.
great circle - the shortest distance between two points on a sphere.
greenwich mean time (G.M.T) - the mean solar time at Greenwich, England, which is located on the zero meridian.
heliocentric - coordinate system whose reference point is the center of the Sun.
hour-angle - the measurement from your meridian WESTWARD to the object in question.
julian date - time elapsed since 1 January 4713 B.C. Avoids the discontinuity in the day count at the beginning of
each month.
light year - the distance that light travels in a year, equivalent to; 6.324x10
4
AU, .307 parsecs, 9.46x10
15
meters
local mean time - specific use of mean solar time but adjusted for a particular location, it varies approximately 4
minutes per degree of longitude.
magnification - the process of increasing the apparent angular size of a celestial object.
minute of arc - 1/60 of one degree.
nadir - the point on the celestial sphere that is directly opposite the zenith and directly beneath the observer.
nebula - cloud of interstellar dust and gas.