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RA/Dec or Az/Alt?

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#1 CRAZYeye29325

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Posted 23 July 2021 - 09:53 PM

Please forgive me while I butcher this terminology. 

 

Is there a preference, standard or majority for the forum???

 

I was looking at Stellarium and waiting for the moon to rise so I could determine where a specific direction was as it relates to my surroundings. Meaning, on Stellarium the moon was above the horizon, but I could not see it due to the trees across from my house. I waited about 20 minutes and saw it over the trees at the apex of the barbershop's roof. Checked an app and found it was:

At 9:35 Eastern time:

Az/Alt 126° 45'  /  +07 46'

RA/Dec  20hr 19min  /  -24° 32'

 

Which is the more common reference?  I like Az/Alt, makes more since to me.

 

Thanks, Phil.

 



#2 CassGuy47

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Posted 23 July 2021 - 10:41 PM

There is no right or wrong, however most serious observers tend to use equatorial mounts, so they are more accustomed to working with Right Ascension and Declination.  Star charts and setting circles also use the RA/Dec. coordinate system, so it's a more useful system for most observers.  RA/Dec. can actually be regarded as a projection of our earth-based longitude/latitude system on the celestial sphere.

 

I will admit that I'm old school.  When I began observing in the 1960's we didn't have computer programs that provided RA/Dec and Alt-Az coordinates.  Back then, you used star charts and setting circles as aids to finding objects in the night sky.  Alt-Az can definitely be more intuitive at your local observing location.  These days, with both options available via software, it depends upon what you're more accustomed to using.  The RA/Dec system is ingrained in me, so for me, Alt-Az is a bit like learning a 2nd language.


Edited by CassGuy47, 24 July 2021 - 08:04 AM.

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#3 therealdmt

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Posted 23 July 2021 - 11:29 PM

Well, I have to respectfully disagree with CassGuy47 that most serious observers tend to use EQ mounts. For one, I’d say that  a lot of serious observers have more than one telescope and more than one mount, and many of those have more than one type of telescope and more than one type of mount.

 

In particular, a lot of serious observers have Dobsonian telescopes, which is a kind of alt-az mount. Many other serious observers like to have a small to mid-sized refractor on an alt-az mount. 
 

As to which system to use, if you are using an EQ mount, the R.A./Dec system matches up with that. If you are using an alt-az mount, the alt-az system works well. Also, I’d say that for most beginners the alt-az system would be more intuitive, especially if they have an alt-az mount. 
 

In the end, use whichever system you feel most comfortable with and which matches your equipment and preferred observing style.

 

Me, i honestly don’t use either! I know my constellations pretty well and how they move, etc., and I think about things in relation to the constellations and prominent stars, plus the (rough) compass points on the horizon, the meridian (an imaginary line from north to south across the sky), the path of the ecliptic (and so also the zodiacal constellations), the galactic plane and the constellations along it, groupings of constellations (for example, the group of constellations centering on Orion) and also prominent asterisms such as The Summer Triangle.

 

If I’m trying to get to a new or unfamiliar target, rather than looking up its coordinates (in any system), I look up it’s location on a star chart and figure out a star hop to it visually starting from a known or easily identifiable (bright) star or grouping of stars. When out under the sky, in practice I’m including elements of both alt-az and R.A./Dec. in my thinking, but I don’t really think of it in that way and I essentially never look up exact coordinates or think in terms of any exact coordinates.

 

My general advice would be to figure out where the ecliptic plane is — it’s like a curving railroad track going from east to west across the sky, and all the planets, the Moon and the Sun basically move along it. Additionally, all the famous constellations of the zodiac (you know, "What’s your sign?") lie along it and it’ll be there all year ‘round. Being able to visualize the ecliptic when I looked up at the night sky was a major breakthrough for me in comparison to just knowing a somewhat random handful of prominent constellations. Also get to know how the Big Dipper’s "pointer stars" point to Polaris, the North Star. If facing Polaris, you’ll be facing the direction of north. The other directions follow from that (i.e., if facing north, east is off your right shoulder, etc.), and a lot more information actually. It all starts to fit together, but it’ll take some time/experience. Figuring out where the Moon rises like you did is a good start — that same area is basically the start of the ecliptic 


Edited by therealdmt, 24 July 2021 - 12:33 AM.

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#4 CRAZYeye29325

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Posted 23 July 2021 - 11:52 PM

Ahhhh, depends on what type of mount you use! That makes since. I am considering degree circles later. 

 

What's are constellation?  Just kidding. OMG, I wiki'd those and there are 83(?) that are considered the standard for astronomy. I only know the little dipper and North star, got a little more to learn there. My head hurts already. 

 

Thanks for the help, Phil.


Edited by CRAZYeye29325, 23 July 2021 - 11:56 PM.

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#5 PNW

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Posted 24 July 2021 - 12:23 AM

I, too, have tree's and obstructions around my house. Alt-Az makes sense to me. I know what azimuths have obstructions at what altitude. I have heard that Stellarium will let you create your horizon by imputing the obstructions around your site. But that's the advanced class I know nothing about.


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#6 luxo II

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Posted 24 July 2021 - 02:14 AM

Use the coordinate system that suits your mount.

 

For visual observing an altaz mount is fine. For solar, lunar & planetary imaging it is fine.

 

Equatorial (RA, dec) only matters if you're using an equatorial mount - which IMHO is only needed for imaging DSO's.


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#7 therealdmt

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Posted 24 July 2021 - 02:52 AM

Ahhhh, depends on what type of mount you use! That makes since. I am considering degree circles later. 

 

What's are constellation?  Just kidding. OMG, I wiki'd those and there are 83(?) that are considered the standard for astronomy. I only know the little dipper and North star, got a little more to learn there. My head hurts already. 

 

Thanks for the help, Phil.

Well, a good number of those constellations will only be viewable in full from south of you. The farther south you go, the more of the southern constellations (such as you’d see from Australia, for example) you can see.  So like in southern Florida you could see some southern stars (and at least parts of the constellations they’re in) that you can’t see (or can barely see through the trees) at home. Accordingly, you definitely don’t have to worry about the full list of 88 constellations.

 

See, it’s getting easier already grin.gif

 

Just take it bit by bit and have fun enjoying learning the sky at your own pace — there’s certainly no race smile.gif You might start with learning the Big Dipper and the Summer Triangle. They’re not official constellations (the Big Dipper is the middle part of the constellation officially known as Ursa Major) but they’re easily recognizable reference points in the sky. The Sun (as well as the Moon and the planets) rises in the east, follows a curved path across the sky and sets in the west. That curved path across the sky is called the ecliptic — you’ll see that the planets are always on that same path, too.

 

Here’s an article on the Summer Triangle: 

https://earthsky.org...e-summer-seaso/


Edited by therealdmt, 24 July 2021 - 03:11 AM.

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#8 Tony Flanders

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Posted 24 July 2021 - 04:06 AM

I was looking at Stellarium and waiting for the moon to rise so I could determine where a specific direction was as it relates to my surroundings. Meaning, on Stellarium the moon was above the horizon, but I could not see it due to the trees across from my house. I waited about 20 minutes and saw it over the trees at the apex of the barbershop's roof. Checked an app and found it was:
At 9:35 Eastern time:
Az/Alt 126° 45'  /  +07 46'
RA/Dec  20hr 19min  /  -24° 32'
 
Which is the more common reference?  I like Az/Alt, makes more since to me.


If I understand your post correctly, you want to know whether a given object will be visible from your home at any given time, and if so, where to look for it. For that purpose, you want the alt-azimuth system, which describes directions in terms of compass bearing (azimuth) and height above the horizon (altitude).

 

Note, however, that an object's alt-az coordinates are constantly changing. By the time you have typed the query into your device and looked at the results, they are already out of date -- though perhaps not by enough to matter over a period of a few minutes.

 

The whole point of the RA/Dec (equatorial) system is that the coordinates of stars and deep-sky objects don't vary over time -- at least not on the scale of a human lifetime. The RA and Dec of the Moon does change as it moves through the "fixed" stars, but quite slowly.

 

So neither system is more common than the other, better than the other, or anything like that. They just serve different purposes.


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#9 Illinois

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Posted 24 July 2021 - 06:18 AM

I don’t use computer or electric clock drive. I use Az Alt all my life because I know the sky, red dot finder and good star maps.  If I want look at M81/M82 then I use red dot finder point the area where I know M81 and M82 then I use low power around 40 power. I can see two faint galaxies and I use low and medium power to look at it.  Use low power, red dot finder and good star maps. 


Edited by Illinois, 24 July 2021 - 06:19 AM.

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#10 Ulmer Spatz

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Posted 24 July 2021 - 06:28 AM

Here's a little thought experiment replacing the sky with Earth and a star with your home town:

 

Imagine yourself on a space station looking back at Earth. You want to see your hometown with a telescope and look up your town's geographical coordinates. Those coordinates are equivalent to right ascension and declination. They are tied to your home town, never change and rotate along with your hometown as the Earth spins on its axis.

 

But now you want to know where to aim your telescope to see your hometown. Since the Earth rotates, it depends on what time you want to take a look. For that particular time, you find that you have to point your scope to a certain altitude and azimuth. A bit later, those values will be different as your home town moves with the Earth's rotation.


Edited by Ulmer Spatz, 24 July 2021 - 06:33 AM.

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#11 kathyastro

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Posted 24 July 2021 - 06:40 AM

Which coordinate system to use depends not only on your equipment, but also on your purpose. 

 

If you are talking about whether or not a target will be visible over a treeline, for example, the relevant coordinates are alt/az, because it relates to a specific site and its physical attributes.  Alt/az coordinates are site-specific.

 

If you are identifying a target to someone on the other side of the world, you would use RA/dec coordinates, because they are independent of location.

 

In both cases, the equipment in use is irrelevant.


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#12 Wisconsin Steve

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Posted 24 July 2021 - 07:16 AM

I have an old Unitron 114 and really like the alt/az mount - it reminds me of an Etch a Sketch from when I was a kid and is very intuitive!

 

Steve


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#13 CRAZYeye29325

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Posted 24 July 2021 - 07:29 AM

Well, a good number of those constellations will only be viewable in full from south of you. The farther south you go, the more of the southern constellations (such as you’d see from Australia, for example) you can see.

(Light bulb emoji) duh me! Curious, so it would still be more than half that number, correct?

 

(One could think of themselves as an intellectual, until you move into an area of thought you've never explored!) (As my wife said of herself, 'I do pretty good considering how little I know!' That statement almost knocked me to the floor.  That pretty much applies to all of us.)

 

Thanks!


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#14 clearwaterdave

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Posted 24 July 2021 - 08:07 AM

You all ready know at least 12 of the constellations.,( what sign are you ).,See.,This isn't so hard.,cool.gif .,I have the Orion DeepMap 600 which is a full sky map that folds up like a road map .,It's a great tool for seeing how things are arranged in the sky.,

As the name implies it has 600+ targets shown including all of the Messier objects.,Well worth the price of admission.,imo.,Best2u.,

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#15 CRAZYeye29325

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Posted 24 July 2021 - 08:25 AM

Here's a little thought experiment replacing the sky with Earth and a star with your home town:

 

Imagine yourself on a space station looking back at Earth. You want to see your hometown with a telescope and look up your town's geographical coordinates. Those coordinates are equivalent to right ascension and declination. They are tied to your home town, never change and rotate along with your hometown as the Earth spins on its axis.

 

But now you want to know where to aim your telescope to see your hometown. Since the Earth rotates, it depends on what time you want to take a look. For that particular time, you find that you have to point your scope to a certain altitude and azimuth. A bit later, those values will be different as your home town moves with the Earth's rotation.

 

Processing...



#16 therealdmt

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Posted 24 July 2021 - 09:32 AM

(Light bulb emoji) duh me! Curious, so it would still be more than half that number, correct?

 

(One could think of themselves as an intellectual, until you move into an area of thought you've never explored!) (As my wife said of herself, 'I do pretty good considering how little I know!' That statement almost knocked me to the floor.  That pretty much applies to all of us.)

 

Thanks!

Roughly, you can forget about around a third of the total number of constellations for now due to them always being either completely below your horizon or, for some more, never getting very high and so typically being behind trees and houses, etc. And even if you can get to a place with a clear horizon to the south (say, over the water, or from a hill with a flat view to the south), the quality of views right down by the horizon tends to suck because you have to look through so much atmosphere, pollutants and often sources of artificial light. Some constellations, like the famous southern constellation Centaurus that contains the nearest star system (Alpha Centauri), will be on the border line, where you can see the top of the constellation but will never get to see the whole thing because the lower part is too far south.

 

Meanwhile, all constellations aren’t created equal — some are small, some are large, some have lots of interesting objects (or just one great object) in them that can be seen with a small telescope, others are empty for a small telescope. But beyond that, a few of them are quite dim — you don’t have to start out memorizing every dim, non-description constellation. Start with learning the big bright bold ones and you’ll naturally build from there as you start searching for some of the "deep sky objects" (DSOs) you’ll read about (DSOs are things out in space beyond the solar system that are not just stars or simple multi-star systems. They are things like star clusters, nebulae and galaxies).

 

Besides the Summer Triangle I gave a link to [an article on] earlier, another great asterism to learn that can help you learn the sky is the Big Dipper. It’s easy to find, it’s well visible most of the year for you, and its stars line up so as to point to a number of other important constellations. 
Here’s an article that gives a little intro on using the Big Dipper to point you to other constellations:

http://sjastronomy.c...r-sign-post.pdf

 

warning: the drawing covers a huge amount of sky and all the constellations shown might not be visible at the same time. Also, don’t worry about learning them all at once — as the year unfolds (and so the parts of the sky presented above you), the idea will start to make more sense


Edited by therealdmt, 24 July 2021 - 09:59 AM.

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#17 CRAZYeye29325

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Posted 24 July 2021 - 09:52 AM

 

Here’s an article that gives a little intro on using the Big Dipper to point you to other constellations:

http://sjastronomy.c...r-sign-post.pdf

 

warning: the drawing covers a huge amount of sky and all the constellations shown might not be visible at the same time 

 

NICE!!!


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#18 belliott4488

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Posted 24 July 2021 - 03:17 PM

Please forgive me while I butcher this terminology. 

 

Is there a preference, standard or majority for the forum???

 

I was looking at Stellarium and waiting for the moon to rise so I could determine where a specific direction was as it relates to my surroundings. Meaning, on Stellarium the moon was above the horizon, but I could not see it due to the trees across from my house. I waited about 20 minutes and saw it over the trees at the apex of the barbershop's roof. Checked an app and found it was:

At 9:35 Eastern time:

Az/Alt 126° 45'  /  +07 46'

RA/Dec  20hr 19min  /  -24° 32'

 

Which is the more common reference?  I like Az/Alt, makes more since to me.

 

Thanks, Phil.

You can see the answer by going to Stellarium, turning on both the az/alt and RA/DEC coordinate grids, and then advancing the time minute by minute. What you'll see is that the az/alt grid stays fixed, whereas the RA/DEC grid moves with the sky.

 

That shows you the value of each system: az/alt is fixed relative to you and your location; RA/DEC is fixed relative to the stars. They're both useful and worth learning.


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#19 CRAZYeye29325

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Posted 24 July 2021 - 03:41 PM

You can see the answer by going to Stellarium, turning on both the az/alt and RA/DEC coordinate grids, and then advancing the time minute by minute. What you'll see is that the az/alt grid stays fixed, whereas the RA/DEC grid moves with the sky.

 

That shows you the value of each system: az/alt is fixed relative to you and your location; RA/DEC is fixed relative to the stars. They're both useful and worth learning.

I like that and still wrapping my head around this.

We see the stars spin, and point to a star, az/alt?

The stars see the earth spin, gives them the hour of the day, Ra/Dec?


Edited by CRAZYeye29325, 24 July 2021 - 03:42 PM.


#20 belliott4488

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Posted 24 July 2021 - 05:42 PM

I like that and still wrapping my head around this.

We see the stars spin, and point to a star, az/alt?

The stars see the earth spin, gives them the hour of the day, Ra/Dec?

Something like that! laugh.gif

 

Each star has RA & DEC coordinates that never change (at least, not in our lifetimes). Their az/alt position will change as you watch them move across the sky, however.

 

Someone floating in space could similarly watch you move West to East as the Earth rotates, but your lat/lon coordinates don't change.

 

Kind of the same thing ...


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#21 magnnum_paul

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Posted 02 August 2021 - 09:24 AM

Although I'm not so experienced, I have to say that I am not using any system when trying to see where things are but I'm doing what others have said: picking constellations and the patterns of stars in the sky to get the idea of the surroundings. To find my desired object i just use my red dot finder to get as close as possible to my target so that I can make a star hop (with a start chart, either physical or on a phone). I am using an equatorial mount and I find the process of setting it up and getting it polar aligned more in tune with the Earth so to speak, more connected to it. On the practical side of things, if you do watch a lot of objects and also share the view with others it does help to only have to adjust one knob for each object, instead of tuning two to keep the target in view.


Edited by magnnum_paul, 02 August 2021 - 09:24 AM.

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#22 CRAZYeye29325

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Posted 02 August 2021 - 10:43 AM

Although I'm not so experienced, I have to say that I am not using any system when trying to see where things are but I'm doing what others have said: picking constellations and the patterns of stars in the sky to get the idea of the surroundings. To find my desired object i just use my red dot finder to get as close as possible to my target so that I can make a star hop (with a start chart, either physical or on a phone). I am using an equatorial mount and I find the process of setting it up and getting it polar aligned more in tune with the Earth so to speak, more connected to it. On the practical side of things, if you do watch a lot of objects and also share the view with others it does help to only have to adjust one knob for each object, instead of tuning two to keep the target in view.

Amen to all that.



#23 ShaulaB

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Posted 02 August 2021 - 10:54 AM

It would be a good idea to learn the constellations and names of bright stars.

As you can see, some of the CN folks have been using telescopes for decades. It takes time to learn the sky and how to best use instruments. So no worries, just take your time. Try to learn something new to you every time you take out the telescope.
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#24 CRAZYeye29325

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Posted 03 August 2021 - 10:23 AM

It would be a good idea to learn the constellations and names of bright stars.
 

Ideas just keep getting better and better!

 

Thanks!


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#25 CRAZYeye29325

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Posted 12 August 2021 - 05:42 PM

I'm starting to see the advantages of RA/Dec,, but still figuring it out.

 

I'm thinking about putting degree circles on my scope and considering an outer 16" 360° azimuth circle and an inner 15" 24hr right ascension circle on the scope base. Printing them both on the same 18x24 sheet, counter-clockwise numbers, both zeros at the same point. Then laminating them. I read the last 5pgs of the degree circle thread and scanned some of the other pages.

 

https://www.cloudyni...degree-circles/

 

Would that be correct? Az and RA on the same sheet? CCW? Both 0's at the same spot?

 

Or is it bass-ackwards to do this at all, and do one or the other, or do them on separate sheets?

 

Thanks.




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