Positional Astronomy:
Coordinate systems: the second equatorial or "RA-dec." system

{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.}

diagram

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:
Aries symbol
(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.



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