Question about Right Ascension

Hello,

I am trying to learn to use Dec. and R.A. to locate objects. The manual
for my telescope (meade 4500 with equatorial mount) defines R.A. as 24
lines along the celestial equator, numbered 0-23 and separated by 15
degrees. It says "...R.A... is measured in hours, minutes, and seconds
from an arbitrarily defined 'zero' line of R.A. passing through the
constellation Pegasus."

The manual also says the celestial objects move from East to West along
"their line of Right Ascension."

My question then is, if the stars are moving along the line of R.A.,
then wouldn't the value for R.A. depend on where you were observing from
and the local time?

Or is 'zero' always passing through Pegasus, and the values for R.A.
move along with the celestial objects that map to them?

I think I must have misread/understood something somewhere...


Thank You,

Josh

"Josh Gregorio" wrote in message
...
Hello,

I am trying to learn to use Dec. and R.A. to locate objects. The manual
for my telescope (meade 4500 with equatorial mount) defines R.A. as 24
lines along the celestial equator, numbered 0-23 and separated by 15
degrees. It says "...R.A... is measured in hours, minutes, and seconds
from an arbitrarily defined 'zero' line of R.A. passing through the
constellation Pegasus."
The point is in a sense 'arbitrary', but does have a 'reason' for it's
selection. It is the point which the Sun passes, on the spring equinox
(Mar21st).

The manual also says the celestial objects move from East to West along
"their line of Right Ascension."

My question then is, if the stars are moving along the line of R.A.,
then wouldn't the value for R.A. depend on where you were observing from
and the local time?
No. If (for instance), the object is half way 'round' the sky from the
origin 'point' (180 degrees), then it's RA would be 12:00:00. The RA value
defines an angle round the sky from the origin point, and since the whole
sky is seen to move together (remember the 'movement' of the sky, is not
real - the only objects with visible 'motions' in the sky, are the planets,
and (over a number of years), some closer stars - the 'motion', is just the
result of the Earth turning with you on it).

Or is 'zero' always passing through Pegasus, and the values for R.A.
move along with the celestial objects that map to them?

I think I must have misread/understood something somewhere...
The 'key' is to remember that the sky doesn't move. Think of it as if you
were standing on a record player, inside a spherical room. The room has
coordinates drawn on it, just like the lines of latitude/longitude on maps
of the Earth. Assume there is an 'arbitrary' base point (the door!...). As
the record player turns, you have to look in different directions to see an
object, but relative to the room, it's coordinates remain fixed.
Now the translation between RA/Dec, and the local Alt/Az, then depends on
time (which defines how much the 'record player' has turned), but the RA/Dec
remain fixed.

Best Wishes