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.