|Zenith: straight up!
Meridian: N/S line going through the zenith
Altitude: height above the horizon
Zenith angle: 90-Altitude
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 path-length through the atmosphere.
For z<60, Airmass=secant(Zenith angle).
|Define coordinates by the
projection of the Earth's pole and equator onto the
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
(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)
dRA(deg)=(RA1-RA2)*cos(Dec) (if RA is measured in degrees)
dRA(deg)=15*(RA1-RA2)*cos(Dec) (if RA is measured in hours)
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 handy-dandy 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 noon-to-noon) 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