number of constellations according to the IAU
88
the ecliptic
the path the sun follows in the sky, crossing the equator at 23.5deg
zenith
point directly overhead
meridian
line running from north to south through the zenith
azimuth
direction along the horizon clockwise from north in degrees
altitude
height above the horizon in degrees
angular size
the angle an object appears to span in your field of view
angular distance
the angle that appears to separate 2 objects
angular size / 360deg =
= physical size / 2pi * distance
circumpolar
stars near the north celestial pole that never dip below the horizon
precession
gradual axial wobble that causes changes in earth's axis over timescales of 26,000 years
Right Ascension (RA)
much like lines of longitude on the earth's surface. measured going east from the vernal equinox in seconds, minutes, and hours
Declination (dec)
much like lines of latitude on earth's surface. measured north (+) or south (-) of the celestial equator in degrees, arcminutes, and arcseconds.
the position of Vega during the vernal equinox
Celestial Position:
Dec = +38deg44'
RA = 18h35m
waxing
moon phases from new to full
waning
moon phases from full to new
crescent
phases before / after new moon
gibbous
phases before / after full moon
stellar parallax
apparent shift in the position of nearby stars from different parts of earth's orbit
retrograde motion
planetary motion relative to stars appears to move backwards then forwards periodically from the perspective of earth.