Question that you might want to think about

[Rakesh Nath]

Hello all

So here is a question for y'all to think about.

Lets consider a relatively fast moving body, say the sun. Now can anyone
tell me, without consulting the software, I mean this is for you to
think about, when would it be that the RA and DEC coordiantes of this
fast moving body changes slower than the Alt az does.

Firstly does the RA and DEC change at all during the day for any body.
This is an interesting question. You might want to think about what the
physical significance of each entity is.

Happy thinking!

--
Thanks and Regards
Rakesh Nath
"It is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring."
Carl Sagan

[praveen kulkarni]

@Rakesh Nath

thanks for this... I haven't scratched my head this much over a problem recently. Nice fodder for the brain. Violating your rule, I consulted stellarium and switched on the Grids. The penny did not drop. 

(I have zero experience at observational astronomy)

Tried googling for intuitive definitions of these concepts. I am still not clear about them.  Has it got to do with the celestial sphere being pretty much rock-solid-fixed except for precessional effects?

When you said "fast moving" objects, did you mean real speed OR  the apparently faster moving bodies in earth sky, like comets and planets compared to the seemingly-unmoving stars? 

If you meant real motion, 

Then I'd guess that farther stars will have constant RA and Dec and hence will change slower than Alt/Azim values. Correct?

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[Naveen Nanjundappa]

Nice Question, I make it a point to explain about RA and Dec in the star parties, here are a few lines about them.

Alt/Az :

this is measured with reference to horizon and north direction, hence requires -  Location of observation, Time/date to be specified along with Alt/Az, in this coordinate system the alt/az values of the object continously changes as time pass. Thus, not a comfortable to share the location of any object to distant observers or time zones.

RA/Dec: 
This is actually with reference to the sky RA from First point of aries, and Dec from celestial equator. hence these co-ordinates "remain" constant. irrespective of the location, time, or date. their positional coordinates ( RA/Dec) will always be same. 

we refresh the RA/Dec values over few years as the first point of aries(POA) drift towards west. we have year 2000 reference (POA) coordinates now. 

Thanks,

Naveen 

On Sat, Feb 18, 2012 at 7:25 AM, Rakesh Nath <rak...@gmail.com> wrote:

[Rakesh Nath]

Think about it this way.

When the Sun or the Moon is moving across the sky how do you expect the RA-DEC to change. Does it change? why and how does it change?
Now look at the Alt-az, as naveen mentioned the alt az is aligned with the horizon but the RA-DEC is aligned to the first point of Aries, which is obviously arbitrary.

On 02/19/2012 03:09 PM, praveen kulkarni wrote:

@Rakesh Nath

thanks for this... I haven't scratched my head this much over a problem recently. Nice fodder for the brain. Violating your rule, I consulted stellarium and switched on the Grids. The penny did not drop. 

(I have zero experience at observational astronomy)

Tried googling for intuitive definitions of these concepts. I am still not clear about them.  Has it got to do with the celestial sphere being pretty much rock-solid-fixed except for precessional effects?

When you said "fast moving" objects, did you mean real speed OR  the apparently faster moving bodies in earth sky, like comets and planets compared to the seemingly-unmoving stars? 

If you meant real motion, 

Then I'd guess that farther stars will have constant RA and Dec and hence will change slower than Alt/Azim values. Correct?

[keerthi kiran]

The RA and DEC of sun and moon do change! RA and DEC of SUN and MOON are NOT constant!!!

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[Rakesh Nath]

The general assumption always has been that the RA-DEC is this constant system of coordinates which are the same world over, which is true, but the idea being that if you have a celestial coordinate system which never changes like the latitude and longitude it would be easier to fix objects and track them. But the challenge is to figure out how they change, right?

So what physical idea about the coordinate system do we notice when we see that RA and DEC of the sun changes. Now we know that Alt-az changes implies that the sun and the moon move from east to west and n-s through the day, so what does the RA and dec tell you? It mostly is related to the same idea, RA is from east to west and DEC is from north south.DEc change is easily explainable with the tilt of the earth, but what about RA? why does RA change?

On 02/20/2012 10:45 AM, keerthi kiran wrote:

The RA and DEC of sun and moon do change! RA and DEC of SUN and MOON are NOT constant!!!

On Mon, Feb 20, 2012 at 9:47 PM, Rakesh Nath <rak...@gmail.com> wrote:

Think about it this way.

When the Sun or the Moon is moving across the sky how do you expect the RA-DEC to change. Does it change? why and how does it change?
Now look at the Alt-az, as naveen mentioned the alt az is aligned with the horizon but the RA-DEC is aligned to the first point of Aries, which is obviously arbitrary.

[Sunil G.R.]

On Sat, Feb 18, 2012 at 7:25 AM, Rakesh Nath <rak...@gmail.com> wrote:

when would it be that the RA and DEC coordiantes of this
fast moving body changes slower than the Alt az does.

I think for the pole-star, RA/DEC changes faster than the Alt/Az.
What i meant by 'Pole-star' is not the one visible to the naked eye. It may not be the perfect pole-star. But there could a star or galaxy which is PERFECT pole-star or DSO for which Alt/Az does not change for hundreds of years!
But RA/DEC changes based on the wobbling nature of Earth's axis :-)

Regards,
Sunil.

[Harshad RJ]

On Tue, Feb 21, 2012 at 11:26 AM, Sunil G.R. <super...@gmail.com> wrote:

On Sat, Feb 18, 2012 at 7:25 AM, Rakesh Nath <rak...@gmail.com> wrote:

when would it be that the RA and DEC coordiantes of this
fast moving body changes slower than the Alt az does.

I think for the pole-star, RA/DEC changes faster than the Alt/Az.

That's a good catch :)
But...
 

What i meant by 'Pole-star' is not the one visible to the naked eye. It may not be the perfect pole-star. But there could a star or galaxy which is PERFECT pole-star or DSO for which Alt/Az does not change for hundreds of years!

Note that RA / Dec can be expressed in different celestial reference frames. There is the International celestial reference frame (ICRF) which is defined with respect to very far away objects (beyond our galaxy) and doesn't change much even after 1000 years. It coincides with the mean equinox reference frame at J2000 epoch. It is the recommended frame that should be used, but people still use the older convention of mean-equinox-of-date frame .

The PERFECT pole-star will remain stationary in the ICRF frame of J2000 for a really long time, except for the proper motion of the star itself.

However, a pole star in mean equinox frame will remain within a few arc-minutes of the pole for only about three to four years, if I remember the calculations correctly.

--

Harshad RJ
http://lavadip.com