{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.
Previous section: Coordinate
systems: the first equatorial or "HA-dec" system
Next section: Sidereal time
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