Zenith: straight up! Meridian: N/S line going through the zenith Altitude: height above the horizon Zenith angle: 90Altitude Azimuth: where great circle connecting star and zenith touches horizon, measured N through E. Airmass or secz: another measure of altitude is Airmass, which measures pathlength through the atmosphere. For z<60, Airmass=secant(Zenith angle). 
Define coordinates by the
projection of the Earth's pole and equator onto the
celestial sphere. North celestial pole: projection of Earth's north pole Celestial equator: projection of Earth's equator Declination (delta): angular distance from the celestial equator (+=north, =south) Right Ascension (alpha): angular distance along circles parallel to the equator. Define zero point to be the vernal equinox, the point where the Sun's position crosses the celestial equator as it moves north. Right ascension increases going eastward. Dec is measured in degrees, minutes of arc, seconds of arc, or decimal degrees. RA is measured in either time (hr, min of time, sec of time), or in decimal degrees. So, the coordinates for M87 can be written as (a,d) = 12:30:49, +12:23:07
or (a,d) = 187.705, +12.39619 Measuring in Time: 24hrs=360 degrees, so 1hr=15 deg. 
Orientation: unless specified otherwise,
standard orientation is north up and east to the left. This
is flipped from terrestrial maps. Scale: you are looking at a curved surface projected onto a plane. Distortions abound! Common system: gnomic or tangent plane projection One big, common clanger: coordinate distances are not angular separations! For small separations (where tan(theta)~theta), we can say dDec(deg)=(Dec1  Dec2)
and dRA(deg)=(RA1RA2)*cos(Dec) (if RA is measured in degrees) or dRA(deg)=15*(RA1RA2)*cos(Dec) (if RA is measured in hours) then d=sqrt(dDec^2 + dRA^2). But for larger separations
this won't work.
Finally, solid angle is a measure of area on the sky, and has units of steradians (for big areas; 4pi=sphere) or square degrees or square arcseconds (for small areas). 
l: galactic longitude b: galactic latitude Galactic center: l=0, b=0 Direction of motion: l=90, b=0 The Earth's axis is tipped from the galactic plane by about 80 degrees or so, so the equatorial and galactic coordinate systems are nearly at right angles to one another. A handydandy visual calculator to transform coordinates, courtesy of level 5. And NED's Coordinate Converter 
The Earth is spinning on its
axis and orbiting the Sun. This means that a solar day
(defined as noontonoon) is different from a sidereal
day (defined as one Earth rotation). Mean Solar day: 24hrs Sidereal day: 23hrs, 56min This means that a fixed star rises 4 mins earlier each successive night, or two hours earlier each month. We define the Local Sidereal Time to be the RA which is currently transiting. Now see how LST, RA, and HA fit together: HA = LST  RA
