{Note: If your browser does not distinguish between "a,b" and " α, β " (the Greek letters "alpha, beta") then I am afraid you will not be able to make much sense of the equations on this page.}
Coordinates in the first equatorial system (HA and
declination)
still depend on the time of observation.
Now we change the zero-point for our coordinates.
We choose a fixed point on the celestial equator,
called the vernal equinox, or the First Point of Aries.
The symbol for this is the astrological symbol for Aries:
(The function of this point will become clearer later on.)
The declination (δ) of object X is measured in the same
way as before.
The Right Ascension or RA (α) of object X
is the angle along the celestial equator measured eastwards
from the vernal equinox to the meridian of X.
Like HA, RA is measured in hours 0-24h, but it goes in the opposite direction.
Comparison of these celestial coordinate systems with the terrestrial system:
|
terrestrial |
alt-az |
HA-dec. |
RA-dec. |
|
equator |
horizon |
celestial equator |
celestial equator |
|
North Pole |
zenith |
North Celestial Pole |
North Celestial Pole |
|
South Pole |
nadir |
South Celestial Pole |
South Celestial Pole |
|
latitude |
altitude |
declination |
declination |
|
co-latitude |
zenith distance |
North Polar Distance |
North Polar Distance |
|
parallel of latitude |
parallel of altitude |
parallel of declination |
parallel of declination |
|
meridian of longitude |
vertical circle |
meridian |
meridian |
|
Greenwich Meridian |
Principal Vertical |
celestial meridian |
vernal equinox |
|
longitude |
azimuth |
Hour Angle |
Right Ascension |
Exercise:
The four stars at the corners of the “Great Square of Pegasus” are:
|
star |
R.A. |
declination |
|
α And |
00h 08m |
+29°05' |
|
β Peg |
23h 04m |
+28° 05' |
|
α Peg |
23h 05m |
+15° 12' |
|
γ Peg |
00h 13m |
+15° 11' |
Calculate the lengths of the two diagonals of the “Square”.
Click here for the answer.
The Right Ascension and declination of a star
do not normally change over short periods of time;
but the Hour Angle changes constantly with time.
Consequently we have to find a way of defining the time.